[v5,2/6] drm/format-helper: Add drm_fb_xrgb8888_to_mono_reversed()

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.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
---

Changes in v5:
- Use drm_WARN_ON* macros instead of deprecated ones (Thomas Zimmermann)

Changes in v4:
- Rename end_offset to end_len (Thomas Zimmermann)
- Warn once if dst_pitch is not a multiple of 8 (Thomas Zimmermann)
- Drop drm_fb_gray8_to_mono_reversed() that's not used (Thomas Zimmermann)
- Allocate single buffer for both copy cma memory and gray8 (Thomas Zimmermann)
- Add Thomas Zimmermann Reviewed-by tag to patch adding XR24 -> mono helper.

Changes in v3:
- Also add a drm_fb_xrgb8888_to_mono_reversed() helper (Thomas Zimmermann)
- Split lines copy to drm_fb_gray8_to_mono_reversed_line() (Thomas Zimmermann)
- Handle case where the source buffer is not aligned to 8 (Thomas Zimmermann)

 drivers/gpu/drm/drm_format_helper.c | 110 ++++++++++++++++++++++++++++
 include/drm/drm_format_helper.h     |   4 +
 2 files changed, 114 insertions(+)

On Fri, Feb 11, 2022 at 03:33:54PM +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.


I believe there is enough room for (micro-)optimizations (like moving out
invariant conditionals from the loop), but since it's almost a direct copy
of the existing code let's improve it later on.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
> ---
> 
> Changes in v5:
> - Use drm_WARN_ON* macros instead of deprecated ones (Thomas Zimmermann)
> 
> Changes in v4:
> - Rename end_offset to end_len (Thomas Zimmermann)
> - Warn once if dst_pitch is not a multiple of 8 (Thomas Zimmermann)
> - Drop drm_fb_gray8_to_mono_reversed() that's not used (Thomas Zimmermann)
> - Allocate single buffer for both copy cma memory and gray8 (Thomas Zimmermann)
> - Add Thomas Zimmermann Reviewed-by tag to patch adding XR24 -> mono helper.
> 
> Changes in v3:
> - Also add a drm_fb_xrgb8888_to_mono_reversed() helper (Thomas Zimmermann)
> - Split lines copy to drm_fb_gray8_to_mono_reversed_line() (Thomas Zimmermann)
> - Handle case where the source buffer is not aligned to 8 (Thomas Zimmermann)
> 
>  drivers/gpu/drm/drm_format_helper.c | 110 ++++++++++++++++++++++++++++
>  include/drm/drm_format_helper.h     |   4 +
>  2 files changed, 114 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_format_helper.c b/drivers/gpu/drm/drm_format_helper.c
> index b981712623d3..bc0f49773868 100644
> --- a/drivers/gpu/drm/drm_format_helper.c
> +++ b/drivers/gpu/drm/drm_format_helper.c
> @@ -12,9 +12,11 @@
>  #include <linux/slab.h>
>  #include <linux/io.h>
>  
> +#include <drm/drm_device.h>
>  #include <drm/drm_format_helper.h>
>  #include <drm/drm_framebuffer.h>
>  #include <drm/drm_fourcc.h>
> +#include <drm/drm_print.h>
>  #include <drm/drm_rect.h>
>  
>  static unsigned int clip_offset(const struct drm_rect *clip, unsigned int pitch, unsigned int cpp)
> @@ -591,3 +593,111 @@ 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, 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;

Just to point out again, this is 100% invariant and may be moved out.

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

This one almost, can be moved out after refactoring.

> +		for (i = start; i < end; i++) {
> +			unsigned int x = xb * 8 + i;
> +
> +			byte >>= 1;
> +			if (src[x] >> 7)
> +				byte |= BIT(7);
> +		}
> +		*dst++ = byte;
> +	}
> +}
> +
> +/**
> + * 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 *vaddr,
> +				      const struct drm_framebuffer *fb, const struct drm_rect *clip)
> +{
> +	unsigned int linepixels = drm_rect_width(clip);
> +	unsigned int lines = clip->y2 - clip->y1;
> +	unsigned int cpp = fb->format->cpp[0];
> +	unsigned int len_src32 = linepixels * cpp;
> +	struct drm_device *dev = fb->dev;
> +	unsigned int start_offset, end_len;
> +	unsigned int y;
> +	u8 *mono = dst, *gray8;
> +	u32 *src32;
> +
> +	if (drm_WARN_ON(dev, fb->format->format != DRM_FORMAT_XRGB8888))
> +		return;
> +
> +	/*
> +	 * 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);
> +
> +	drm_WARN_ONCE(dev, dst_pitch % 8 != 0, "dst_pitch is not a multiple of 8\n");
> +
> +	/*
> +	 * The cma memory is write-combined so reads are uncached.
> +	 * 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);
> +	if (!src32)
> +		return;
> +
> +	gray8 = (u8 *)src32 + len_src32;
> +
> +	/*
> +	 * 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;
> +
> +	vaddr += clip_offset(clip, fb->pitches[0], cpp);
> +	for (y = 0; y < lines; y++) {
> +		src32 = memcpy(src32, vaddr, len_src32);
> +		drm_fb_xrgb8888_to_gray8_line(gray8, src32, linepixels);
> +		drm_fb_gray8_to_mono_reversed_line(mono, gray8, dst_pitch,
> +						   start_offset, end_len);
> +		vaddr += fb->pitches[0];
> +		mono += dst_pitch;
> +	}
> +
> +	kfree(src32);
> +}
> +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..0b0937c0b2f6 100644
> --- a/include/drm/drm_format_helper.h
> +++ b/include/drm/drm_format_helper.h
> @@ -43,4 +43,8 @@ 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_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 */
> -- 
> 2.34.1
>

Hello Geert,

Thanks for your feedback.

On 2/12/22 16:54, Geert Uytterhoeven wrote:

[snip]

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

I think the original version from Noralf is easier to read and understand.

It makes explicit that the bit is set if the grayscale value is >= 128.

[snip]

>> +void drm_fb_xrgb8888_to_mono_reversed(void *dst, unsigned int dst_pitch, const void *vaddr,
>> +                                     const struct drm_framebuffer *fb, const struct drm_rect *clip)
>> +{

[snip]

>> +       u8 *mono = dst, *gray8;
>> +       u32 *src32;

[snip]

>> +        *
>> +        * 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);
>> +       if (!src32)
>> +               return;
>> +
>> +       gray8 = (u8 *)src32 + len_src32;
> 
> The cast can be removed if src32 is changed to "void *".
> For symmetry, gray8 and mono can be changed to "void *", too.
>

Yes, but these being "u32 *" and "u8 *" also express that the source buffer
contains 32-bit XRGB8888 pixels, the intermediate buffer a 8-bit grayscale
pixel format and the destination buffer a 8-bit packed reversed monochrome.

Using "void *" for these would make that less clear when reading the code IMO.

Best regards,