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SubjectRe: [PATCH v2 1/4] drm/format-helper: Add drm_fb_{xrgb8888,gray8}_to_mono_reversed()
From
Hi

Am 04.02.22 um 20:31 schrieb Javier Martinez Canillas:
> Hello Thomas,
>
> Thanks a lot for your feedback.
>
> On 2/4/22 16:52, Thomas Zimmermann wrote:
>
> [snip]
>
>>> +static void drm_fb_gray8_to_mono_reversed_line(u8 *dst, const u8 *src, size_t pixels)
>>> +{
>>> + unsigned int xb, i;
>>> +
>>> + for (xb = 0; xb < pixels / 8; xb++) {
>>
>> In practice, all mode widths are multiples of 8 because VGA mandated it.
>> So it's ok-ish to assume this here. You should probably at least print a
>> warning somewhere if (pixels % 8 != 0)
>>
>
> Agreed.

After sending the mail, I realized that some code copies only parts of
the source around; specifically for damage handling. None of this is
aligned to multiple of 8. So the copying could start and end in the
middle of bytes. You'd need a pixel-offset value of some sort.

If you don't want to handle this now, maybe at least detect this case
and put a warning somewhere.

>
> [snip]
>
>>> + * DRM doesn't have native monochrome or grayscale support.
>>> + * Such drivers can announce the commonly supported XR24 format to userspace
>>> + * and use drm_fb_xrgb8888_to_gray8() to convert to grayscale and then this
>>> + * helper function to convert to the native format.
>>> + */
>>> +void drm_fb_gray8_to_mono_reversed(void *dst, unsigned int dst_pitch, const void *src,
>>> + const struct drm_rect *clip)
>>
>> There's a bug here. You want to pass in a drm_framebuffer as fourth
>> argument.
>>
>>> +{
>>> +
>>> + size_t height = drm_rect_height(clip);
>>> + size_t width = drm_rect_width(clip);
>>> + unsigned int y;
>>> + const u8 *gray8 = src;
>>> + u8 *mono = dst;
>>> +
>>> + if (!dst_pitch)
>>> + dst_pitch = width;
>>
>> The dst_pitch is given in bytes. You have to device by 8. Here would be
>> a good place to warn if (width % 8 != 0).
>>
>
> Ok.
>
>>> +
>>> + for (y = 0; y < height; y++) {
>>> + drm_fb_gray8_to_mono_reversed_line(mono, gray8, dst_pitch);
>>> + mono += (dst_pitch / 8);
>>
>> The dst_pitch is already given in bytes.
>>
>
> Yes, I know but for reversed mono we want only 1/8 of the width since we
> are converting from 8 bits per pixel greyscale to 1 bit per pixel mono.
>
> Or am I misunderstanding what you meant ?

I mean that if there are 80 pixel on a scanline, the value of dst_pitch
is already 10. These pitch values are always given in bytes.

Best regards
Thomas

>
>>> + gray8 += dst_pitch;
>>
>> 'gray8 += fb->pitches[0]' would be correct.
>>
>
> Ok.
>
> [snip]
>
>>> + */
>>> +void drm_fb_xrgb8888_to_mono_reversed(void *dst, unsigned int dst_pitch, const void *src,
>>> + const struct drm_framebuffer *fb,
>>> + const struct drm_rect *clip)
>>> +{
>>> + if (WARN_ON(fb->format->format != DRM_FORMAT_XRGB8888))
>>> + return;
>>> +
>>> + if (!dst_pitch)
>>> + dst_pitch = drm_rect_width(clip);
>>> +
>>> + drm_fb_xrgb8888_to_gray8(dst, dst_pitch, src, fb, clip);
>>> + drm_fb_gray8_to_mono_reversed(dst, dst_pitch, dst, fb, clip);
>>
>> Converting from dst into dst can give incorrect results. At some point
>> we probably want to add restrict qualifiers to these pointers, to help
>> the compiler with optimizing.
>>
>> A better approach here is to pull the per-line conversion from
>> drm_fb_xrgb8888_to_gray8() into a separate helper and implement a
>> line-by-line conversion here. something like this:
>>
>> drm_fb_xrgb8888_to_mono_reversed()
>> {
>> char *tmp = kmalloc(size of a single line of gray8)
>>
>> for (heigth) {
>> drm_fb_xrgb8888_to_gray8_line(tmp, ..., src, ...);
>> drm_fb_gray8_to_mono_reversed(dst, ..., tmp, ...);
>>
>> src += fb->pitches[0]
>> dst += dst_pitch;
>> }
>>
>> kfree(tmp);
>> }
>>
>
> I see. Yes, that sounds a much better approach. I'll change it in v3.
>
> Best regards,

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev
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