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:

quoted

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.

...

quoted

+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;

quoted

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

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

quoted

+		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

quoted

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

...

quoted

+	/*
+	 * 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() ?

quoted

+	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);

quoted

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

CMA

quoted

+	 * 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() ?

quoted

+	if (!src32)
+		return;

...

quoted

+	/*
+	 * 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