[PATCH] USB: misc: appledisplay: fix backlight update_status return code

[Mattias Jacobsson <2pi@mok.nu>]

Upon success the update_status handler returns a positive number
corresponding to the number of bytes transferred by usb_control_msg.
However the return code of the update_status handler should indicate if
an error occurred(negative) or how many bytes of the user's input to sysfs
that was consumed. Return code zero indicates all bytes were consumed.

The bug can for example result in the update_status handler being called
twice, the second time with only the "unconsumed" part of the user's input
to sysfs. Effectively setting an incorrect brightness.

Change the update_status handler to return zero for all successful
transactions and forward usb_control_msg's error code upon failure.

Signed-off-by: Mattias Jacobsson <2pi@mok.nu>
---
I've not found any documentation regarding the return code from the
update_status handler. The information above is based on looking at
other driver and how the return code is used for by the caller of the
update_status handler. Please let me know if it is incorrect.
---
 drivers/usb/misc/appledisplay.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/misc/appledisplay.c b/drivers/usb/misc/appledisplay.c
index d746c26a8055..bd539f3058bc 100644
--- a/drivers/usb/misc/appledisplay.c
+++ b/drivers/usb/misc/appledisplay.c
@@ -146,8 +146,11 @@ static int appledisplay_bl_update_status(struct backlight_device *bd)
 		pdata->msgdata, 2,
 		ACD_USB_TIMEOUT);
 	mutex_unlock(&pdata->sysfslock);
-	
-	return retval;
+
+	if (retval < 0)
+		return retval;
+	else
+		return 0;
 }
 
 static int appledisplay_bl_get_brightness(struct backlight_device *bd)
-- 
2.19.1