gsmi: Fix bug in append_to_eventlog sysfs handler

[ Upstream commit 655603de68469adaff16842ac17a5aec9c9ce89b ]

The sysfs handler should return the number of bytes consumed, which in the
case of a successful write is the entire buffer.  Also fix a bug where
param.data_len was being set to (count - (2 * sizeof(u32))) instead of just
(count - sizeof(u32)).  The latter is correct because we skip over the
leading u32 which is our param.type, but we were also incorrectly
subtracting sizeof(u32) on the line where we were actually setting
param.data_len:

	param.data_len = count - sizeof(u32);

This meant that for our example event.kernel_software_watchdog with total
length 10 bytes, param.data_len was just 2 prior to this change.

To test, successfully append an event to the log with gsmi sysfs.
This sample event is for a "Kernel Software Watchdog"

> xxd -g 1 event.kernel_software_watchdog
0000000: 01 00 00 00 ad de 06 00 00 00

> cat event.kernel_software_watchdog > /sys/firmware/gsmi/append_to_eventlog

> mosys eventlog list | tail -1
14 | 2012-06-25 10:14:14 | Kernl Event | Software Watchdog
Signed-off-by: NDuncan Laurie <dlaurie@chromium.org>
Reviewed-by: NVadim Bendebury <vbendeb@chromium.org>
Reviewed-by: NStefan Reinauer <reinauer@chromium.org>
Signed-off-by: NFurquan Shaikh <furquan@google.com>
Tested-by: NFurquan Shaikh <furquan@chromium.org>
Reviewed-by: NAaron Durbin <adurbin@chromium.org>
Reviewed-by: NJustin TerAvest <teravest@chromium.org>
[zwisler: updated changelog for 2nd bug fix and upstream]
Signed-off-by: NRoss Zwisler <zwisler@google.com>
Reviewed-by: NGuenter Roeck <groeck@chromium.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>

drivers/firmware/google/gsmi.c

@@ -480,11 +480,10 @@ static ssize_t eventlog_write(struct file *filp, struct kobject *kobj,
 	if (count < sizeof(u32))
 		return -EINVAL;
 	param.type = *(u32 *)buf;
-	count -= sizeof(u32);
 	buf += sizeof(u32);
 
 	/* The remaining buffer is the data payload */
-	if (count > gsmi_dev.data_buf->length)
+	if ((count - sizeof(u32)) > gsmi_dev.data_buf->length)
 		return -EINVAL;
 	param.data_len = count - sizeof(u32);
 
@@ -504,7 +503,7 @@ static ssize_t eventlog_write(struct file *filp, struct kobject *kobj,
 
 	spin_unlock_irqrestore(&gsmi_dev.lock, flags);
 
-	return rc;
+	return (rc == 0) ? count : rc;
 
 }