ACPI / SBS: Add 5 us delay to fix SBS hangs on MacBook

Commit 7bc5a2bad0b8 'ACPI: Support _OSI("Darwin") correctly' caused
the MacBook firmware to expose the SBS, resulting in intermittent
hangs of several minutes on boot, and failure to detect or report
the battery.  Fix this by adding a 5 us delay to the start of each
SMBUS transaction.  This timing is the result of experimentation -
hangs were observed with 3 us but never with 5 us.

Fixes: 7bc5a2bad0b8 'ACPI: Support _OSI("Darwin") correctly'
Link: https://bugzilla.kernel.org/show_bug.cgi?id=94651
Signed-off-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Cc: 3.18+ <stable@vger.kernel.org> # 3.18+
[ rjw: Subject and changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/sbshc.c

@@ -14,6 +14,7 @@
 #include <linux/delay.h>
 #include <linux/module.h>
 #include <linux/interrupt.h>
+#include <linux/dmi.h>
 #include "sbshc.h"
 
 #define PREFIX "ACPI: "
@@ -87,6 +88,8 @@ enum acpi_smb_offset {
 	ACPI_SMB_ALARM_DATA = 0x26,	/* 2 bytes alarm data */
 };
 
+static bool macbook;
+
 static inline int smb_hc_read(struct acpi_smb_hc *hc, u8 address, u8 *data)
 {
 	return ec_read(hc->offset + address, data);
@@ -132,6 +135,8 @@ static int acpi_smbus_transaction(struct acpi_smb_hc *hc, u8 protocol,
 	}
 
 	mutex_lock(&hc->lock);
+	if (macbook)
+		udelay(5);
 	if (smb_hc_read(hc, ACPI_SMB_PROTOCOL, &temp))
 		goto end;
 	if (temp) {
@@ -257,12 +262,29 @@ extern int acpi_ec_add_query_handler(struct acpi_ec *ec, u8 query_bit,
 			      acpi_handle handle, acpi_ec_query_func func,
 			      void *data);
 
+static int macbook_dmi_match(const struct dmi_system_id *d)
+{
+	pr_debug("Detected MacBook, enabling workaround\n");
+	macbook = true;
+	return 0;
+}
+
+static struct dmi_system_id acpi_smbus_dmi_table[] = {
+	{ macbook_dmi_match, "Apple MacBook", {
+	  DMI_MATCH(DMI_BOARD_VENDOR, "Apple"),
+	  DMI_MATCH(DMI_PRODUCT_NAME, "MacBook") },
+	},
+	{ },
+};
+
 static int acpi_smbus_hc_add(struct acpi_device *device)
 {
 	int status;
 	unsigned long long val;
 	struct acpi_smb_hc *hc;
 
+	dmi_check_system(acpi_smbus_dmi_table);
+
 	if (!device)
 		return -EINVAL;