linux-kernel - Re: [PATCH v4 2/6] drm/format-helper: Add drm_fb_xrgb8888_to_mono

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Message-ID: <YgZEPo0ERFRloDyA@smile.fi.intel.com>
Date:   Fri, 11 Feb 2022 13:10:54 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-fbdev@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Maxime Ripard <maxime@...no.tech>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Sam Ravnborg <sam@...nborg.org>,
        Noralf Trønnes <noralf@...nnes.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 v4 2/6] drm/format-helper: Add
 drm_fb_xrgb8888_to_mono_reversed()

On Fri, Feb 11, 2022 at 10:19:23AM +0100, Javier Martinez Canillas wrote:
> Add support to convert from XR24 to reversed monochrome for drivers that
> control monochromatic display panels, that only have 1 bit per pixel.
> 
> The function does a line-by-line conversion doing an intermediate step
> first from XR24 to 8-bit grayscale and then to reversed monochrome.
> 
> The drm_fb_gray8_to_mono_reversed_line() helper was based on code from
> drivers/gpu/drm/tiny/repaper.c driver.

...

> +static void drm_fb_gray8_to_mono_reversed_line(u8 *dst, const u8 *src, unsigned int pixels,
> +					       unsigned int start_offset, unsigned int end_len)
> +{
> +	unsigned int xb, i;
> +
> +	for (xb = 0; xb < pixels; xb++) {
> +		unsigned int start = 0, end = 8;
> +		u8 byte = 0x00;

> +		if (xb == 0 && start_offset)
> +			start = start_offset;

This is invariant to the loop, can be moved out.

> +		if (xb == pixels - 1 && end_len)
> +			end = end_len;

Ditto. However it may require to factor out the following loop to a helper.

> +		for (i = start; i < end; i++) {
> +			unsigned int x = xb * 8 + i;
> +
> +			byte >>= 1;
> +			if (src[x] >> 7)
> +				byte |= BIT(7);
> +		}
> +		*dst++ = byte;
> +	}
> +}

...

> +	/*
> +	 * The reversed mono destination buffer contains 1 bit per pixel
> +	 * and destination scanlines have to be in multiple of 8 pixels.
> +	 */
> +	if (!dst_pitch)
> +		dst_pitch = DIV_ROUND_UP(linepixels, 8);

round_up() ?

> +	WARN_ONCE(dst_pitch % 8 != 0, "dst_pitch is not a multiple of 8\n");


I would move this to the if conditional, i.e.

	if (dst_pitch)
		WARN_ONCE(dst_pitch % 8 != 0, "dst_pitch is not a multiple of 8\n");
	else
		dst_pitch = round_up(linepixels, 8);

> +	/*
> +	 * The cma memory is write-combined so reads are uncached.

CMA

> +	 * Speed up by fetching one line at a time.
> +	 *
> +	 * Also, format conversion from XR24 to reversed monochrome
> +	 * are done line-by-line but are converted to 8-bit grayscale
> +	 * as an intermediate step.
> +	 *
> +	 * Allocate a buffer to be used for both copying from the cma
> +	 * memory and to store the intermediate grayscale line pixels.
> +	 */
> +	src32 = kmalloc(len_src32 + linepixels, GFP_KERNEL);

size_add() ?

> +	if (!src32)
> +		return;

...

> +	/*
> +	 * For damage handling, it is possible that only parts of the source
> +	 * buffer is copied and this could lead to start and end pixels that
> +	 * are not aligned to multiple of 8.
> +	 *
> +	 * Calculate if the start and end pixels are not aligned and set the
> +	 * offsets for the reversed mono line conversion function to adjust.
> +	 */
> +	start_offset = clip->x1 % 8;
> +	end_len = clip->x2 % 8;

ALIGN() ?

-- 
With Best Regards,
Andy Shevchenko