linux-kernel - Re: [PATCH v2 1/4] drm/format-helper: Add drm_fb_{xrgb8888,gray8}_to_mono

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Date:   Fri, 4 Feb 2022 21:35:05 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-fbdev@...r.kernel.org, Sam Ravnborg <sam@...nborg.org>,
        dri-devel@...ts.freedesktop.org,
        Noralf Trønnes <noralf@...nnes.org>,
        Maxime Ripard <maxime@...no.tech>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>
Subject: Re: [PATCH v2 1/4] drm/format-helper: Add
 drm_fb_{xrgb8888,gray8}_to_mono_reversed()

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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