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@@ -82,93 +82,111 @@ Most of the hard work is done for the driver in the PCI layer. It simply
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has to request that the PCI layer set up the MSI capability for this
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device.
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-4.2.1 pci_enable_msi
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+4.2.1 pci_enable_msi_range
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-int pci_enable_msi(struct pci_dev *dev)
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+int pci_enable_msi_range(struct pci_dev *dev, int minvec, int maxvec)
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-A successful call allocates ONE interrupt to the device, regardless
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-of how many MSIs the device supports. The device is switched from
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-pin-based interrupt mode to MSI mode. The dev->irq number is changed
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-to a new number which represents the message signaled interrupt;
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-consequently, this function should be called before the driver calls
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-request_irq(), because an MSI is delivered via a vector that is
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-different from the vector of a pin-based interrupt.
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+This function allows a device driver to request any number of MSI
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+interrupts within specified range from 'minvec' to 'maxvec'.
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-4.2.2 pci_enable_msi_block
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+If this function returns a positive number it indicates the number of
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+MSI interrupts that have been successfully allocated. In this case
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+the device is switched from pin-based interrupt mode to MSI mode and
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+updates dev->irq to be the lowest of the new interrupts assigned to it.
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+The other interrupts assigned to the device are in the range dev->irq
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+to dev->irq + returned value - 1. Device driver can use the returned
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+number of successfully allocated MSI interrupts to further allocate
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+and initialize device resources.
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-int pci_enable_msi_block(struct pci_dev *dev, int count)
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+If this function returns a negative number, it indicates an error and
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+the driver should not attempt to request any more MSI interrupts for
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+this device.
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-This variation on the above call allows a device driver to request multiple
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-MSIs. The MSI specification only allows interrupts to be allocated in
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-powers of two, up to a maximum of 2^5 (32).
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+This function should be called before the driver calls request_irq(),
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+because MSI interrupts are delivered via vectors that are different
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+from the vector of a pin-based interrupt.
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-If this function returns 0, it has succeeded in allocating at least as many
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-interrupts as the driver requested (it may have allocated more in order
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-to satisfy the power-of-two requirement). In this case, the function
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-enables MSI on this device and updates dev->irq to be the lowest of
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-the new interrupts assigned to it. The other interrupts assigned to
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-the device are in the range dev->irq to dev->irq + count - 1.
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+It is ideal if drivers can cope with a variable number of MSI interrupts;
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+there are many reasons why the platform may not be able to provide the
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+exact number that a driver asks for.
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-If this function returns a negative number, it indicates an error and
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-the driver should not attempt to request any more MSI interrupts for
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-this device. If this function returns a positive number, it is
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-less than 'count' and indicates the number of interrupts that could have
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-been allocated. In neither case is the irq value updated or the device
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-switched into MSI mode.
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-
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-The device driver must decide what action to take if
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-pci_enable_msi_block() returns a value less than the number requested.
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-For instance, the driver could still make use of fewer interrupts;
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-in this case the driver should call pci_enable_msi_block()
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-again. Note that it is not guaranteed to succeed, even when the
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-'count' has been reduced to the value returned from a previous call to
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-pci_enable_msi_block(). This is because there are multiple constraints
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-on the number of vectors that can be allocated; pci_enable_msi_block()
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-returns as soon as it finds any constraint that doesn't allow the
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-call to succeed.
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-
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-4.2.3 pci_enable_msi_block_auto
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-
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-int pci_enable_msi_block_auto(struct pci_dev *dev, unsigned int *count)
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-
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-This variation on pci_enable_msi() call allows a device driver to request
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-the maximum possible number of MSIs. The MSI specification only allows
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-interrupts to be allocated in powers of two, up to a maximum of 2^5 (32).
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-
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-If this function returns a positive number, it indicates that it has
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-succeeded and the returned value is the number of allocated interrupts. In
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-this case, the function enables MSI on this device and updates dev->irq to
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-be the lowest of the new interrupts assigned to it. The other interrupts
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-assigned to the device are in the range dev->irq to dev->irq + returned
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-value - 1.
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+There could be devices that can not operate with just any number of MSI
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+interrupts within a range. See chapter 4.3.1.3 to get the idea how to
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+handle such devices for MSI-X - the same logic applies to MSI.
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-If this function returns a negative number, it indicates an error and
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-the driver should not attempt to request any more MSI interrupts for
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-this device.
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+4.2.1.1 Maximum possible number of MSI interrupts
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+
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+The typical usage of MSI interrupts is to allocate as many vectors as
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+possible, likely up to the limit returned by pci_msi_vec_count() function:
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+
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+static int foo_driver_enable_msi(struct pci_dev *pdev, int nvec)
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+{
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+ return pci_enable_msi_range(pdev, 1, nvec);
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+}
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+
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+Note the value of 'minvec' parameter is 1. As 'minvec' is inclusive,
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+the value of 0 would be meaningless and could result in error.
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-If the device driver needs to know the number of interrupts the device
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-supports it can pass the pointer count where that number is stored. The
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-device driver must decide what action to take if pci_enable_msi_block_auto()
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-succeeds, but returns a value less than the number of interrupts supported.
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-If the device driver does not need to know the number of interrupts
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-supported, it can set the pointer count to NULL.
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+Some devices have a minimal limit on number of MSI interrupts.
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+In this case the function could look like this:
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-4.2.4 pci_disable_msi
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+static int foo_driver_enable_msi(struct pci_dev *pdev, int nvec)
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+{
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+ return pci_enable_msi_range(pdev, FOO_DRIVER_MINIMUM_NVEC, nvec);
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+}
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+
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+4.2.1.2 Exact number of MSI interrupts
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+
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+If a driver is unable or unwilling to deal with a variable number of MSI
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+interrupts it could request a particular number of interrupts by passing
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+that number to pci_enable_msi_range() function as both 'minvec' and 'maxvec'
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+parameters:
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+
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+static int foo_driver_enable_msi(struct pci_dev *pdev, int nvec)
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+{
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+ return pci_enable_msi_range(pdev, nvec, nvec);
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+}
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+
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+4.2.1.3 Single MSI mode
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+
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+The most notorious example of the request type described above is
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+enabling the single MSI mode for a device. It could be done by passing
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+two 1s as 'minvec' and 'maxvec':
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+
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+static int foo_driver_enable_single_msi(struct pci_dev *pdev)
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+{
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+ return pci_enable_msi_range(pdev, 1, 1);
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+}
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+
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+4.2.2 pci_disable_msi
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void pci_disable_msi(struct pci_dev *dev)
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-This function should be used to undo the effect of pci_enable_msi() or
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-pci_enable_msi_block() or pci_enable_msi_block_auto(). Calling it restores
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-dev->irq to the pin-based interrupt number and frees the previously
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-allocated message signaled interrupt(s). The interrupt may subsequently be
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-assigned to another device, so drivers should not cache the value of
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-dev->irq.
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+This function should be used to undo the effect of pci_enable_msi_range().
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+Calling it restores dev->irq to the pin-based interrupt number and frees
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+the previously allocated MSIs. The interrupts may subsequently be assigned
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+to another device, so drivers should not cache the value of dev->irq.
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Before calling this function, a device driver must always call free_irq()
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on any interrupt for which it previously called request_irq().
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Failure to do so results in a BUG_ON(), leaving the device with
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MSI enabled and thus leaking its vector.
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+4.2.3 pci_msi_vec_count
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+
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+int pci_msi_vec_count(struct pci_dev *dev)
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+
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+This function could be used to retrieve the number of MSI vectors the
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+device requested (via the Multiple Message Capable register). The MSI
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+specification only allows the returned value to be a power of two,
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+up to a maximum of 2^5 (32).
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+
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+If this function returns a negative number, it indicates the device is
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+not capable of sending MSIs.
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+
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+If this function returns a positive number, it indicates the maximum
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+number of MSI interrupt vectors that could be allocated.
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+
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4.3 Using MSI-X
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The MSI-X capability is much more flexible than the MSI capability.
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@@ -188,26 +206,31 @@ in each element of the array to indicate for which entries the kernel
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should assign interrupts; it is invalid to fill in two entries with the
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same number.
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-4.3.1 pci_enable_msix
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+4.3.1 pci_enable_msix_range
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-int pci_enable_msix(struct pci_dev *dev, struct msix_entry *entries, int nvec)
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+int pci_enable_msix_range(struct pci_dev *dev, struct msix_entry *entries,
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+ int minvec, int maxvec)
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-Calling this function asks the PCI subsystem to allocate 'nvec' MSIs.
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+Calling this function asks the PCI subsystem to allocate any number of
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+MSI-X interrupts within specified range from 'minvec' to 'maxvec'.
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The 'entries' argument is a pointer to an array of msix_entry structs
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-which should be at least 'nvec' entries in size. On success, the
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-device is switched into MSI-X mode and the function returns 0.
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-The 'vector' member in each entry is populated with the interrupt number;
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+which should be at least 'maxvec' entries in size.
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+
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+On success, the device is switched into MSI-X mode and the function
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+returns the number of MSI-X interrupts that have been successfully
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+allocated. In this case the 'vector' member in entries numbered from
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+0 to the returned value - 1 is populated with the interrupt number;
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the driver should then call request_irq() for each 'vector' that it
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decides to use. The device driver is responsible for keeping track of the
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interrupts assigned to the MSI-X vectors so it can free them again later.
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+Device driver can use the returned number of successfully allocated MSI-X
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+interrupts to further allocate and initialize device resources.
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If this function returns a negative number, it indicates an error and
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the driver should not attempt to allocate any more MSI-X interrupts for
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-this device. If it returns a positive number, it indicates the maximum
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-number of interrupt vectors that could have been allocated. See example
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-below.
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+this device.
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-This function, in contrast with pci_enable_msi(), does not adjust
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+This function, in contrast with pci_enable_msi_range(), does not adjust
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dev->irq. The device will not generate interrupts for this interrupt
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number once MSI-X is enabled.
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@@ -218,28 +241,103 @@ It is ideal if drivers can cope with a variable number of MSI-X interrupts;
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there are many reasons why the platform may not be able to provide the
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exact number that a driver asks for.
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-A request loop to achieve that might look like:
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+There could be devices that can not operate with just any number of MSI-X
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+interrupts within a range. E.g., an network adapter might need let's say
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+four vectors per each queue it provides. Therefore, a number of MSI-X
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+interrupts allocated should be a multiple of four. In this case interface
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+pci_enable_msix_range() can not be used alone to request MSI-X interrupts
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+(since it can allocate any number within the range, without any notion of
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+the multiple of four) and the device driver should master a custom logic
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+to request the required number of MSI-X interrupts.
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+
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+4.3.1.1 Maximum possible number of MSI-X interrupts
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+
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+The typical usage of MSI-X interrupts is to allocate as many vectors as
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+possible, likely up to the limit returned by pci_msix_vec_count() function:
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static int foo_driver_enable_msix(struct foo_adapter *adapter, int nvec)
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{
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- while (nvec >= FOO_DRIVER_MINIMUM_NVEC) {
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- rc = pci_enable_msix(adapter->pdev,
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- adapter->msix_entries, nvec);
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- if (rc > 0)
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- nvec = rc;
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- else
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- return rc;
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+ return pci_enable_msi_range(adapter->pdev, adapter->msix_entries,
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+ 1, nvec);
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+}
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+
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+Note the value of 'minvec' parameter is 1. As 'minvec' is inclusive,
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+the value of 0 would be meaningless and could result in error.
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+
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+Some devices have a minimal limit on number of MSI-X interrupts.
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+In this case the function could look like this:
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+
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+static int foo_driver_enable_msix(struct foo_adapter *adapter, int nvec)
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+{
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+ return pci_enable_msi_range(adapter->pdev, adapter->msix_entries,
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+ FOO_DRIVER_MINIMUM_NVEC, nvec);
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+}
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+
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+4.3.1.2 Exact number of MSI-X interrupts
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+
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+If a driver is unable or unwilling to deal with a variable number of MSI-X
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+interrupts it could request a particular number of interrupts by passing
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+that number to pci_enable_msix_range() function as both 'minvec' and 'maxvec'
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+parameters:
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+
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+static int foo_driver_enable_msix(struct foo_adapter *adapter, int nvec)
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+{
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+ return pci_enable_msi_range(adapter->pdev, adapter->msix_entries,
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+ nvec, nvec);
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+}
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+
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+4.3.1.3 Specific requirements to the number of MSI-X interrupts
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+
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+As noted above, there could be devices that can not operate with just any
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+number of MSI-X interrupts within a range. E.g., let's assume a device that
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+is only capable sending the number of MSI-X interrupts which is a power of
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+two. A routine that enables MSI-X mode for such device might look like this:
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+
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+/*
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+ * Assume 'minvec' and 'maxvec' are non-zero
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+ */
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+static int foo_driver_enable_msix(struct foo_adapter *adapter,
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+ int minvec, int maxvec)
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+{
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+ int rc;
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+
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+ minvec = roundup_pow_of_two(minvec);
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+ maxvec = rounddown_pow_of_two(maxvec);
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+
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+ if (minvec > maxvec)
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+ return -ERANGE;
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+
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+retry:
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+ rc = pci_enable_msix_range(adapter->pdev, adapter->msix_entries,
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+ maxvec, maxvec);
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+ /*
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+ * -ENOSPC is the only error code allowed to be analized
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+ */
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+ if (rc == -ENOSPC) {
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+ if (maxvec == 1)
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+ return -ENOSPC;
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+
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+ maxvec /= 2;
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+
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+ if (minvec > maxvec)
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+ return -ENOSPC;
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+
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+ goto retry;
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}
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- return -ENOSPC;
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+ return rc;
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}
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+Note how pci_enable_msix_range() return value is analized for a fallback -
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+any error code other than -ENOSPC indicates a fatal error and should not
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+be retried.
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+
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4.3.2 pci_disable_msix
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void pci_disable_msix(struct pci_dev *dev)
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-This function should be used to undo the effect of pci_enable_msix(). It frees
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-the previously allocated message signaled interrupts. The interrupts may
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+This function should be used to undo the effect of pci_enable_msix_range().
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+It frees the previously allocated MSI-X interrupts. The interrupts may
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subsequently be assigned to another device, so drivers should not cache
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the value of the 'vector' elements over a call to pci_disable_msix().
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@@ -255,18 +353,32 @@ MSI-X Table. This address is mapped by the PCI subsystem, and should not
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be accessed directly by the device driver. If the driver wishes to
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mask or unmask an interrupt, it should call disable_irq() / enable_irq().
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+4.3.4 pci_msix_vec_count
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+
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+int pci_msix_vec_count(struct pci_dev *dev)
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+
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+This function could be used to retrieve number of entries in the device
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+MSI-X table.
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+
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+If this function returns a negative number, it indicates the device is
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+not capable of sending MSI-Xs.
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+
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+If this function returns a positive number, it indicates the maximum
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+number of MSI-X interrupt vectors that could be allocated.
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+
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4.4 Handling devices implementing both MSI and MSI-X capabilities
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If a device implements both MSI and MSI-X capabilities, it can
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run in either MSI mode or MSI-X mode, but not both simultaneously.
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This is a requirement of the PCI spec, and it is enforced by the
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-PCI layer. Calling pci_enable_msi() when MSI-X is already enabled or
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-pci_enable_msix() when MSI is already enabled results in an error.
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-If a device driver wishes to switch between MSI and MSI-X at runtime,
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-it must first quiesce the device, then switch it back to pin-interrupt
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-mode, before calling pci_enable_msi() or pci_enable_msix() and resuming
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-operation. This is not expected to be a common operation but may be
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-useful for debugging or testing during development.
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+PCI layer. Calling pci_enable_msi_range() when MSI-X is already
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+enabled or pci_enable_msix_range() when MSI is already enabled
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+results in an error. If a device driver wishes to switch between MSI
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+and MSI-X at runtime, it must first quiesce the device, then switch
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+it back to pin-interrupt mode, before calling pci_enable_msi_range()
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+or pci_enable_msix_range() and resuming operation. This is not expected
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+to be a common operation but may be useful for debugging or testing
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+during development.
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4.5 Considerations when using MSIs
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@@ -381,5 +493,5 @@ or disabled (0). If 0 is found in any of the msi_bus files belonging
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to bridges between the PCI root and the device, MSIs are disabled.
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It is also worth checking the device driver to see whether it supports MSIs.
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-For example, it may contain calls to pci_enable_msi(), pci_enable_msix() or
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-pci_enable_msi_block().
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+For example, it may contain calls to pci_enable_msi_range() or
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+pci_enable_msix_range().
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