drm/doc: Document ioctl errno value patterns

We're not super-consistent about these, but I think it's worth to
document at least the commmon patterns.

v2:
- Add a not about ENOTTY (it's just a confusing name, but used
exactly what it's meant for in DRM) (Chris).
- Unconfuse the text for ENODEV (Daniel)
- Move text undert the IOCTL heading (Chris).
- typos

Cc: Daniel Stone <daniel@fooishbar.org>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: "Zhang, Tina" <tina.zhang@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818174328.6386-1-daniel.vetter@ffwll.ch

Documentation/gpu/drm-uapi.rst

@@ -168,6 +168,61 @@ IOCTL Support on Device Nodes
 .. kernel-doc:: drivers/gpu/drm/drm_ioctl.c
    :doc: driver specific ioctls
 
+Recommended IOCTL Return Values
+-------------------------------
+
+In theory a driver's IOCTL callback is only allowed to return very few error
+codes. In practice it's good to abuse a few more. This section documents common
+practice within the DRM subsystem:
+
+ENOENT:
+        Strictly this should only be used when a file doesn't exist e.g. when
+        calling the open() syscall. We reuse that to signal any kind of object
+        lookup failure, e.g. for unknown GEM buffer object handles, unknown KMS
+        object handles and similar cases.
+
+ENOSPC:
+        Some drivers use this to differentiate "out of kernel memory" from "out
+        of VRAM". Sometimes also applies to other limited gpu resources used for
+        rendering (e.g. when you have a special limited compression buffer).
+        Sometimes resource allocation/reservation issues in command submission
+        IOCTLs are also signalled through EDEADLK.
+
+        Simply running out of kernel/system memory is signalled through ENOMEM.
+
+EPERM/EACCESS:
+        Returned for an operation that is valid, but needs more privileges.
+        E.g. root-only or much more common, DRM master-only operations return
+        this when when called by unpriviledged clients. There's no clear
+        difference between EACCESS and EPERM.
+
+ENODEV:
+        Feature (like PRIME, modesetting, GEM) is not supported by the driver.
+
+ENXIO:
+        Remote failure, either a hardware transaction (like i2c), but also used
+        when the exporting driver of a shared dma-buf or fence doesn't support a
+        feature needed.
+
+EINTR:
+        DRM drivers assume that userspace restarts all IOCTLs. Any DRM IOCTL can
+        return EINTR and in such a case should be restarted with the IOCTL
+        parameters left unchanged.
+
+EIO:
+        The GPU died and couldn't be resurrected through a reset. Modesetting
+        hardware failures are signalled through the "link status" connector
+        property.
+
+EINVAL:
+        Catch-all for anything that is an invalid argument combination which
+        cannot work.
+
+IOCTL also use other error codes like ETIME, EFAULT, EBUSY, ENOTTY but their
+usage is in line with the common meanings. The above list tries to just document
+DRM specific patterns. Note that ENOTTY has the slightly unintuitive meaning of
+"this IOCTL does not exist", and is used exactly as such in DRM.
+
 .. kernel-doc:: include/drm/drm_ioctl.h
    :internal: