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 16:52:54 +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 14:43 schrieb Javier Martinez Canillas:
> Add support to convert XR24 and 8-bit grayscale to reversed monochrome for
> drivers that control monochromatic panels, that only have 1 bit per pixel.
> 
> The drm_fb_gray8_to_mono_reversed() helper was based on the function that
> does the same in the drivers/gpu/drm/tiny/repaper.c driver.
> 
> Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> ---
> 
> (no changes since v1)
> 
>   drivers/gpu/drm/drm_format_helper.c | 80 +++++++++++++++++++++++++++++
>   include/drm/drm_format_helper.h     |  7 +++
>   2 files changed, 87 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_format_helper.c b/drivers/gpu/drm/drm_format_helper.c
> index 0f28dd2bdd72..cdce4b7c25d9 100644
> --- a/drivers/gpu/drm/drm_format_helper.c
> +++ b/drivers/gpu/drm/drm_format_helper.c
> @@ -584,3 +584,83 @@ int drm_fb_blit_toio(void __iomem *dst, unsigned int dst_pitch, uint32_t dst_for
>   	return -EINVAL;
>   }
>   EXPORT_SYMBOL(drm_fb_blit_toio);
> +
> +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)


> +		u8 byte = 0x00;
> +
> +		for (i = 0; i < 8; i++) {
> +			int x = xb * 8 + i;
> +
> +			byte >>= 1;
> +			if (src[x] >> 7)
> +				byte |= BIT(7);
> +		}
> +		*dst++ = byte;
> +	}
> +}
> +
> +/**
> + * drm_fb_gray8_to_mono_reversed - Convert grayscale to reversed monochrome
> + * @dst: reversed monochrome destination buffer
> + * @dst_pitch: Number of bytes between two consecutive scanlines within dst
> + * @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, 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).

> +
> +	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.

> +		gray8 += dst_pitch;

'gray8 += fb->pitches[0]' would be correct.

> +	}
> +}
> +
> +/**
> + * drm_fb_xrgb8888_to_mono_reversed - Convert XRGB8888 to reversed monochrome
> + * @dst: reversed monochrome destination buffer
> + * @dst_pitch: Number of bytes between two consecutive scanlines within dst
> + * @src: XRGB8888 source buffer
> + * @fb: DRM framebuffer
> + * @clip: Clip rectangle area to copy
> + *
> + * DRM doesn't have native monochrome support.
> + * Such drivers can announce the commonly supported XR24 format to userspace
> + * and use this function to convert to the native format.
> + *
> + * This function uses drm_fb_xrgb8888_to_gray8() to convert to grayscale and
> + * then the result is converted from grayscale to reversed monohrome.
> + */
> +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);
   }

Best regards
Thomas

> +}
> +EXPORT_SYMBOL(drm_fb_xrgb8888_to_mono_reversed);
> diff --git a/include/drm/drm_format_helper.h b/include/drm/drm_format_helper.h
> index b30ed5de0a33..85e551a5cbe6 100644
> --- a/include/drm/drm_format_helper.h
> +++ b/include/drm/drm_format_helper.h
> @@ -43,4 +43,11 @@ int drm_fb_blit_toio(void __iomem *dst, unsigned int dst_pitch, uint32_t dst_for
>   		     const void *vmap, const struct drm_framebuffer *fb,
>   		     const struct drm_rect *rect);
>   
> +void drm_fb_gray8_to_mono_reversed(void *dst, unsigned int dst_pitch, const void *src,
> +				   const struct drm_rect *clip);
> +
> +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);
> +
>   #endif /* __LINUX_DRM_FORMAT_HELPER_H */

-- 
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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