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

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Message-ID: <5cffc63b-c42a-df4c-7270-488735c9e014@suse.de>
Date:   Fri, 11 Feb 2022 12:59:30 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        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, 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()

Hi

Am 11.02.22 um 12:10 schrieb Andy Shevchenko:
> 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.

Splitting the loop is much nicer, but leaves corner cases where start 
and end is in the same byte. Doing this sounds like a premature 
optimization to me.

Best regards
Thomas

> 
>> +		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() ?
> 

-- 
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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