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SubjectRe: [PATCH 2/2] Fix a bug on appledisplay.c regarding signedness

bug description: If the brightness level on the apple display is 128 or
higher, the driver will complain on startup:

"Error while getting initial brightness: -128"

Actual Driver
Brightness level Reported value
0 0
... ...
126 126
127 127
128 -128 (aha! :-)
129 -127
... ...
255 -1

Also, the reported brightness via /sys interface will present the same

I realized that this was simply because the monitor transmits its data
as an unsigned char (uint8_t), and in the code it is treated as a signed

I consulted Caskey L. Dickson's acdctl sources to ascertain this. He
used uint8_t there. I also checked the usb_* functions definitions to be
sure that I was not second-guessing them. Yep, they accept (void *) in
the relevant parameters, so it's up to us to decide the type of the

I've tested it and works like a charm now.

I have emailed the author of the driver, Michael Hanselmann, and he said:
On Wed, Dec 02, 2009 at 05:14:26PM +0000, Michael Hanselmann wrote:
> If I remember correctly, Linux on PowerPC uses unsigned chars by
> default. I never tested the driver on a non-PowerPC platform and hence
> may never have seen this.

Please, review and push the patch, if you deem it adequate.

Thanks a bunch!

Signed-off-by: pancho horrillo <>

diff -purN a/drivers/usb/misc/appledisplay.c b/drivers/usb/misc/appledisplay.c
--- a/drivers/usb/misc/appledisplay.c 2009-11-10 01:32:31.000000000 +0100
+++ b/drivers/usb/misc/appledisplay.c 2009-12-02 20:05:00.000000000 +0100
@@ -72,8 +72,8 @@ struct appledisplay {
struct usb_device *udev; /* usb device */
struct urb *urb; /* usb request block */
struct backlight_device *bd; /* backlight device */
- char *urbdata; /* interrupt URB data buffer */
- char *msgdata; /* control message data buffer */
+ uint8_t *urbdata; /* interrupt URB data buffer */
+ uint8_t *msgdata; /* control message data buffer */

struct delayed_work work;
int button_pressed;
pancho horrillo

To be conscious that
you are ignorant is a great step
to knowledge.

Benjamin Disraeli

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