On Sun, May 30, 2021 at 09:42:38PM +0200, Linus Walleij wrote:
> Remove interrupt disablement during backlight setting. It is
> way to dangerous and makes platforms instable by having it
> miss vblank IRQs leading to the graphics derailing.
>
> The code is using ndelay() which is not available on
> platforms such as ARM and will result in 32 * udelay(1)
> which is substantial.
>
> Fixes: 5317f37e48b9 ("backlight: Add Kinetic KTD253 backlight driver")
> Cc: Stephan Gerhold <stephan@xxxxxxxxxxx>
> Reported-by: newbyte@xxxxxxxxxxx
> Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
> ---
> drivers/video/backlight/ktd253-backlight.c | 6 ------
> 1 file changed, 6 deletions(-)
>
> diff --git a/drivers/video/backlight/ktd253-backlight.c b/drivers/video/backlight/ktd253-backlight.c
> index a7df5bcca9da..b79a47c2801f 100644
> --- a/drivers/video/backlight/ktd253-backlight.c
> +++ b/drivers/video/backlight/ktd253-backlight.c
> @@ -40,7 +40,6 @@ static int ktd253_backlight_update_status(struct backlight_device *bl)
> int brightness = backlight_get_brightness(bl);
> u16 target_ratio;
> u16 current_ratio = ktd253->ratio;
> - unsigned long flags;
>
> dev_dbg(ktd253->dev, "new brightness/ratio: %d/32\n", brightness);
>
> @@ -69,13 +68,9 @@ static int ktd253_backlight_update_status(struct backlight_device *bl)
> }
>
> /*
> - * WARNING:
> - * The loop to set the correct current level is performed
> - * with interrupts disabled as it is timing critical.
> * The maximum number of cycles of the loop is 32
> * so the time taken will be (T_LOW_NS + T_HIGH_NS + loop_time) * 32,
> */
> - local_irq_save(flags);
> while (current_ratio != target_ratio) {
> /*
> * These GPIO operations absolutely can NOT sleep so no
> @@ -92,7 +87,6 @@ static int ktd253_backlight_update_status(struct backlight_device *bl)
> else
> current_ratio--;
> }
> - local_irq_restore(flags);
IIUC if this code is prevented from progressing for 3ms or longer then
we risk deactivating the backlight instead of setting it. Does there need
to be a retrospective timeout check and recovery loop?
Daniel.
> ktd253->ratio = current_ratio;
>
> dev_dbg(ktd253->dev, "new ratio set to %d/32\n", target_ratio);
> --
> 2.31.1
>
