[PATCH] drm/fb-helper: Fix clipping when damage area spans a single scanline

[PATCH] drm/fb-helper: Fix clipping when damage area spans a single scanline

[Francesco Lavra]

When the damage area resulting from a dirty memory range spans a single
scanline, the width of the rectangle is calculated dynamically because it
may not coincide with the framebuffer width.
If the dirty range ends exactly at the end of the scanline, the `bit_end`
variable is incorrectly assigned a 0 value, which results in a bogus clip
rectangle where the x2 coordinate is 0. This prevents the dirty scanline
from being flushed to the hardware.
Change the calculation of the `bit_end` value to fix the x2 coordinate
value in the above edge case.

Fixes: ded74cafeea9 ("drm/fb-helper: Clip damage area horizontally")
Signed-off-by: Francesco Lavra <flavra@baylibre.com>
---
 drivers/gpu/drm/drm_fb_helper.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 4b47aa0dab35..d545342d24b3 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -608,7 +608,7 @@ static void drm_fb_helper_memory_range_to_clip(struct fb_info *info, off_t off,
 		 * the number of horizontal pixels that need an update.
 		 */
 		off_t bit_off = (off % line_length) * 8;
-		off_t bit_end = (end % line_length) * 8;
+		off_t bit_end = bit_off + len * 8;
 
 		x1 = bit_off / info->var.bits_per_pixel;
 		x2 = DIV_ROUND_UP(bit_end, info->var.bits_per_pixel);
-- 
2.39.5

Re: [PATCH] drm/fb-helper: Fix clipping when damage area spans a single scanline

[Thomas Zimmermann]

Hi

Am 10.02.26 um 18:35 schrieb Francesco Lavra:
> When the damage area resulting from a dirty memory range spans a single
> scanline, the width of the rectangle is calculated dynamically because it
> may not coincide with the framebuffer width.
> If the dirty range ends exactly at the end of the scanline, the `bit_end`
> variable is incorrectly assigned a 0 value, which results in a bogus clip
> rectangle where the x2 coordinate is 0. This prevents the dirty scanline
> from being flushed to the hardware.
> Change the calculation of the `bit_end` value to fix the x2 coordinate
> value in the above edge case.
>
> Fixes: ded74cafeea9 ("drm/fb-helper: Clip damage area horizontally")
> Signed-off-by: Francesco Lavra <flavra@baylibre.com>

The whole logic in that helper would need an update. But for now

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>

Best regards
Thomas

> ---
>   drivers/gpu/drm/drm_fb_helper.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 4b47aa0dab35..d545342d24b3 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -608,7 +608,7 @@ static void drm_fb_helper_memory_range_to_clip(struct fb_info *info, off_t off,
>   		 * the number of horizontal pixels that need an update.
>   		 */
>   		off_t bit_off = (off % line_length) * 8;
> -		off_t bit_end = (end % line_length) * 8;
> +		off_t bit_end = bit_off + len * 8;
>   
>   		x1 = bit_off / info->var.bits_per_pixel;
>   		x2 = DIV_ROUND_UP(bit_end, info->var.bits_per_pixel);

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)

Re: [PATCH] drm/fb-helper: Fix clipping when damage area spans a single scanline

[Francesco Lavra]

Friendly ping

On Tue, 2026-02-10 at 18:35 +0100, Francesco Lavra wrote:
> When the damage area resulting from a dirty memory range spans a single
> scanline, the width of the rectangle is calculated dynamically because it
> may not coincide with the framebuffer width.
> If the dirty range ends exactly at the end of the scanline, the `bit_end`
> variable is incorrectly assigned a 0 value, which results in a bogus clip
> rectangle where the x2 coordinate is 0. This prevents the dirty scanline
> from being flushed to the hardware.
> Change the calculation of the `bit_end` value to fix the x2 coordinate
> value in the above edge case.
> 
> Fixes: ded74cafeea9 ("drm/fb-helper: Clip damage area horizontally")
> Signed-off-by: Francesco Lavra <flavra@baylibre.com>
> ---
>  drivers/gpu/drm/drm_fb_helper.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c
> b/drivers/gpu/drm/drm_fb_helper.c
> index 4b47aa0dab35..d545342d24b3 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -608,7 +608,7 @@ static void drm_fb_helper_memory_range_to_clip(struct
> fb_info *info, off_t off,
>                  * the number of horizontal pixels that need an update.
>                  */
>                 off_t bit_off = (off % line_length) * 8;
> -               off_t bit_end = (end % line_length) * 8;
> +               off_t bit_end = bit_off + len * 8;
>  
>                 x1 = bit_off / info->var.bits_per_pixel;
>                 x2 = DIV_ROUND_UP(bit_end, info->var.bits_per_pixel);

Re: [PATCH] drm/fb-helper: Fix clipping when damage area spans a single scanline

[Thomas Zimmermann]

Hi

Am 30.04.26 um 17:35 schrieb Francesco Lavra:
> Friendly ping

Sorry, forgot this patch. It's merged now. Thanks a lot.

Best regards
Thomas

>
> On Tue, 2026-02-10 at 18:35 +0100, Francesco Lavra wrote:
>> When the damage area resulting from a dirty memory range spans a single
>> scanline, the width of the rectangle is calculated dynamically because it
>> may not coincide with the framebuffer width.
>> If the dirty range ends exactly at the end of the scanline, the `bit_end`
>> variable is incorrectly assigned a 0 value, which results in a bogus clip
>> rectangle where the x2 coordinate is 0. This prevents the dirty scanline
>> from being flushed to the hardware.
>> Change the calculation of the `bit_end` value to fix the x2 coordinate
>> value in the above edge case.
>>
>> Fixes: ded74cafeea9 ("drm/fb-helper: Clip damage area horizontally")
>> Signed-off-by: Francesco Lavra <flavra@baylibre.com>
>> ---
>>   drivers/gpu/drm/drm_fb_helper.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/drm_fb_helper.c
>> b/drivers/gpu/drm/drm_fb_helper.c
>> index 4b47aa0dab35..d545342d24b3 100644
>> --- a/drivers/gpu/drm/drm_fb_helper.c
>> +++ b/drivers/gpu/drm/drm_fb_helper.c
>> @@ -608,7 +608,7 @@ static void drm_fb_helper_memory_range_to_clip(struct
>> fb_info *info, off_t off,
>>                   * the number of horizontal pixels that need an update.
>>                   */
>>                  off_t bit_off = (off % line_length) * 8;
>> -               off_t bit_end = (end % line_length) * 8;
>> +               off_t bit_end = bit_off + len * 8;
>>   
>>                  x1 = bit_off / info->var.bits_per_pixel;
>>                  x2 = DIV_ROUND_UP(bit_end, info->var.bits_per_pixel);

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)