linux-kernel - Re: [PATCH] drm/tiny: Refactor framebuffer's size calculation

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Message-ID: <a37d83c7-dc2d-4e1d-a8d4-5628e0ac52a2@suse.de>
Date: Tue, 21 Oct 2025 08:57:07 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@...il.com>,
 lanzano.alex@...il.com, maarten.lankhorst@...ux.intel.com,
 mripard@...nel.org, airlied@...il.com, simona@...ll.ch
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 skhan@...uxfoundation.org, david.hunter.linux@...il.com, khalid@...nel.org,
 linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH] drm/tiny: Refactor framebuffer's size calculation



Am 21.10.25 um 08:51 schrieb Thomas Zimmermann:
> Hi
>
> Am 20.10.25 um 13:57 schrieb Mehdi Ben Hadj Khelifa:
>> Use drm_format_info_min_pitch() to calculate the framebuffer line pitch
>> instead of directly multiplying width and height. This aligns with DRM
>> helpers for determining per-line byte size and avoids manual assumptions
>> about bytes per pixel.
>>
>> Suggested-by: Thomas Zimmermann <tzimmermann@...e.de>
>> Signed-off-by: Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@...il.com>
>> ---
>>   drivers/gpu/drm/tiny/repaper.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/tiny/repaper.c 
>> b/drivers/gpu/drm/tiny/repaper.c
>> index 4824f863fdba..aeff49bc6ba7 100644
>> --- a/drivers/gpu/drm/tiny/repaper.c
>> +++ b/drivers/gpu/drm/tiny/repaper.c
>> @@ -517,6 +517,8 @@ static int repaper_fb_dirty(struct 
>> drm_framebuffer *fb, const struct iosys_map *
>>       unsigned int dst_pitch = 0;
>>       struct iosys_map dst;
>>       struct drm_rect clip;
>> +    const struct drm_format_info *info = fb->format;
>
> This is the wrong format. You're allocating the output buffer here, 
> but you're using the input format. IIUC the output format is 
> DRM_FORMAT_R1. The input is _XRGB8888.
>
> Best regards
> Thomas
>
>> +    size_t pitch;
>>       int idx, ret = 0;
>>       u8 *buf = NULL;
>>   @@ -534,7 +536,9 @@ static int repaper_fb_dirty(struct 
>> drm_framebuffer *fb, const struct iosys_map *
>>       DRM_DEBUG("Flushing [FB:%d] st=%ums\n", fb->base.id,
>>             epd->factored_stage_time);
>>   -    buf = kmalloc(fb->width * fb->height / 8, GFP_KERNEL);
>> +    pitch = drm_format_info_min_pitch(info, 0, fb->width);
>> +
>> +    buf = kmalloc_array(fb->height, pitch, GFP_KERNEL);

And TBH I'm not sure if we should really convert this call. It's not an 
array in the sense of that API.

Good candidates look like 'kmalloc(count * sizeof(<some type or variable>))'

>>       if (!buf) {
>>           ret = -ENOMEM;
>>           goto out_exit;
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)