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

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Message-ID: <dfb3d547-9842-443b-80a2-94bf715bb46e@gmail.com>
Date: Tue, 21 Oct 2025 09:42:46 +0100
From: Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@...il.com>
To: Thomas Zimmermann <tzimmermann@...e.de>, 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

On 10/21/25 7:57 AM, Thomas Zimmermann wrote:
> 
> 
> 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>))'
> 
I will leave kmalloc in v3. Do you want me to send deep testing 
information on this patch?
BR,
Mehdi>>>       if (!buf) {
>>>           ret = -ENOMEM;
>>>           goto out_exit;
>>
>