linux-kernel - Re: [PATCH 2/4] drm/format-helper: Add drm_fb_gray8_to_mono

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Date:   Tue, 1 Feb 2022 12:48:42 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org
Cc:     linux-fbdev@...r.kernel.org, Maxime Ripard <maxime@...no.tech>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        dri-devel@...ts.freedesktop.org,
        Noralf Trønnes <noralf@...nnes.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        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 2/4] drm/format-helper: Add
 drm_fb_gray8_to_mono_reversed()

Hello Thomas,

Thanks a lot for your feedback.

On 2/1/22 10:59, Thomas Zimmermann wrote:
> Hi
> 
> Am 31.01.22 um 21:12 schrieb Javier Martinez Canillas:
>> Add support to convert 8-bit grayscale to reversed monochrome for drivers
>> that control monochromatic displays, that only have 1 bit per pixel depth.
>>
>> This helper function was based on repaper_gray8_to_mono_reversed() from
>> the drivers/gpu/drm/tiny/repaper.c driver.
> 
> You could convert repaper to the new helper.
>

Yes, I plan to do it but was out of scope for this patch series.

>>
>> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
>> ---
>>
>>   drivers/gpu/drm/drm_format_helper.c | 35 +++++++++++++++++++++++++++++
>>   include/drm/drm_format_helper.h     |  2 ++
>>   2 files changed, 37 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_format_helper.c b/drivers/gpu/drm/drm_format_helper.c
>> index 0f28dd2bdd72..bf477c136082 100644
>> --- a/drivers/gpu/drm/drm_format_helper.c
>> +++ b/drivers/gpu/drm/drm_format_helper.c
>> @@ -584,3 +584,38 @@ int drm_fb_blit_toio(void __iomem *dst, unsigned int dst_pitch, uint32_t dst_for
>>   	return -EINVAL;
>>   }
>>   EXPORT_SYMBOL(drm_fb_blit_toio);
>> +
>> +/**
>> + * drm_fb_gray8_to_mono_reversed - Convert grayscale to reversed monochrome
>> + * @dst: reversed monochrome destination buffer
>> + * @src: 8-bit grayscale source buffer
>> + * @clip: Clip rectangle area to copy
>> + *
>> + * 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, void *src, const struct drm_rect *clip)
> 
> IMHO it would be better to have a function that converts xrgb8888 to 
> mono and let it handle the intermediate step of gray8.
>

That's a good idea. I'll add that too.

>> +{
>> +	size_t width = drm_rect_width(clip);
>> +	size_t height = drm_rect_width(clip);
>> +
>> +	u8 *mono = dst, *gray8 = src;
>> +	unsigned int y, xb, i;
>> +
>> +	for (y = 0; y < height; y++)
>> +		for (xb = 0; xb < width / 8; xb++) {
> 
> The inner loop should probably go into a separate helper function. See 
> the drm_fb_*_line() helpers in this file
>
> At some point, we's want to have a single blit helper that takes source 
> and destination formats/buffers. It would then pick the correct per-line 
> helper for the conversion. So yeah, we'd want something composable.
>

Agreed. I'll split that into a separate helper function.

Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat