Re: [PATCH v3 02/25] drm/dumb-buffers: Provide helper to set pitch and size

Geert Uytterhoeven
2025-02-18T21:26:07Z

Hi Thomas,

On Tue, 18 Feb 2025 at 15:26, Thomas Zimmermann <tzimmermann@suse.de> wrote:
> Add drm_modes_size_dumb(), a helper to calculate the dumb-buffer
> scanline pitch and allocation size. Implementations of struct
> drm_driver.dumb_create can call the new helper for their size
> computations.
>
> There is currently quite a bit of code duplication among DRM's
> memory managers. Each calculates scanline pitch and buffer size
> from the given arguments, but the implementations are inconsistent
> in how they treat alignment and format support. Later patches will
> unify this code on top of drm_mode_size_dumb() as much as possible.
>
> drm_mode_size_dumb() uses existing 4CC format helpers to interpret
> the given color mode. This makes the dumb-buffer interface behave
> similar the kernel's video= parameter. Current per-driver implementations
> again likely have subtle differences or bugs in how they support color
> modes.
>
> The dumb-buffer UAPI is only specified for known color modes. These
> values describe linear, single-plane RGB color formats or legacy index
> formats. Other values should not be specified. But some user space
> still does. So for unknown color modes, there are a number of known
> exceptions for which drm_mode_size_dumb() calculates the pitch from
> the bpp value, as before. All other values work the same but print
> an error.
>
> v3:
> - document the UAPI semantics
> - compute scanline pitch from for unknown color modes (Andy, Tomi)
>
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>

Thanks for your patch!

> --- a/drivers/gpu/drm/drm_dumb_buffers.c
> +++ b/drivers/gpu/drm/drm_dumb_buffers.c
> +/**
> + * drm_mode_size_dumb - Calculates the scanline and buffer sizes for dumb buffers
> + * @dev: DRM device
> + * @args: Parameters for the dumb buffer
> + * @pitch_align: Scanline alignment in bytes
> + * @size_align: Buffer-size alignment in bytes
> + *
> + * The helper drm_mode_size_dumb() calculates the size of the buffer
> + * allocation and the scanline size for a dumb buffer. Callers have to
> + * set the buffers width, height and color mode in the argument @arg.
> + * The helper validates the correctness of the input and tests for
> + * possible overflows. If successful, it returns the dumb buffer's
> + * required scanline pitch and size in &args.
> + *
> + * The parameter @pitch_align allows the driver to specifies an
> + * alignment for the scanline pitch, if the hardware requires any. The
> + * calculated pitch will be a multiple of the alignment. The parameter
> + * @size_align allows to specify an alignment for buffer sizes. The
> + * returned size is always a multiple of PAGE_SIZE.
> + *
> + * Returns:
> + * Zero on success, or a negative error code otherwise.
> + */
> +int drm_mode_size_dumb(struct drm_device *dev,
> + struct drm_mode_create_dumb *args,
> + unsigned long pitch_align,
> + unsigned long size_align)
> +{
> + u64 pitch = 0;
> + u32 fourcc;
> +
> + /*
> + * The scanline pitch depends on the buffer width and the color
> + * format. The latter is specified as a color-mode constant for
> + * which we first have to find the corresponding color format.
> + *
> + * Different color formats can have the same color-mode constant.
> + * For example XRGB8888 and BGRX8888 both have a color mode of 32.
> + * It is possible to use different formats for dumb-buffer allocation
> + * and rendering as long as all involved formats share the same
> + * color-mode constant.
> + */
> + fourcc = drm_driver_color_mode_format(dev, args->bpp);
> + if (fourcc != DRM_FORMAT_INVALID) {
> + const struct drm_format_info *info = drm_format_info(fourcc);
> +
> + if (!info)
> + return -EINVAL;
> + pitch = drm_format_info_min_pitch(info, 0, args->width);
> + } else if (args->bpp) {
> + /*
> + * Some userspace throws in arbitrary values for bpp and
> + * relies on the kernel to figure it out. In this case we
> + * fall back to the old method of using bpp directly. The
> + * over-commitment of memory from the rounding is acceptable
> + * for compatibility with legacy userspace. We have a number
> + * of deprecated legacy values that are explicitly supported.
> + */
> + switch (args->bpp) {
> + default:
> + drm_warn(dev, "Unknown color mode %d; guessing buffer size.\n",

%u

> + args->bpp);
> + fallthrough;
> + case 12:
> + case 15:
> + case 30: /* see drm_gem_afbc_get_bpp() */
> + case 10:

Perhaps keep them sorted numerically?

> + case 64: /* used by Mesa */
> + pitch = args->width * DIV_ROUND_UP(args->bpp, SZ_8);
> + break;
> + }
> + }
> +
> + if (!pitch || pitch > U32_MAX)
> + return -EINVAL;
> +
> + args->pitch = pitch;
> +
> + return drm_mode_align_dumb(args, pitch_align, size_align);
> +}
> +EXPORT_SYMBOL(drm_mode_size_dumb);
> +
> int drm_mode_create_dumb(struct drm_device *dev,
> struct drm_mode_create_dumb *args,
> struct drm_file *file_priv)
> diff --git a/include/drm/drm_dumb_buffers.h b/include/drm/drm_dumb_buffers.h
> new file mode 100644
> index 000000000000..6fe36004b19d
> --- /dev/null
> +++ b/include/drm/drm_dumb_buffers.h
> @@ -0,0 +1,14 @@
> +/* SPDX-License-Identifier: MIT */
> +
> +#ifndef __DRM_DUMB_BUFFERS_H__
> +#define __DRM_DUMB_BUFFERS_H__
> +
> +struct drm_device;
> +struct drm_mode_create_dumb;
> +
> +int drm_mode_size_dumb(struct drm_device *dev,
> + struct drm_mode_create_dumb *args,
> + unsigned long pitch_align,
> + unsigned long size_align);
> +
> +#endif
> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index c082810c08a8..eea09103b1a6 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -1058,7 +1058,7 @@ struct drm_mode_crtc_page_flip_target {
> * struct drm_mode_create_dumb - Create a KMS dumb buffer for scanout.
> * @height: buffer height in pixels
> * @width: buffer width in pixels
> - * @bpp: bits per pixel
> + * @bpp: color mode
> * @flags: must be zero
> * @handle: buffer object handle
> * @pitch: number of bytes between two consecutive lines
> @@ -1066,6 +1066,50 @@ struct drm_mode_crtc_page_flip_target {
> *
> * User-space fills @height, @width, @bpp and @flags. If the IOCTL succeeds,
> * the kernel fills @handle, @pitch and @size.
> + *
> + * The value of @bpp is a color-mode number describing a specific format
> + * or a variant thereof. The value often corresponds to the number of bits
> + * per pixel for most modes, although there are exceptions. Each color mode
> + * maps to a DRM format plus a number of modes with similar pixel layout.
> + * Framebuffer layout is always linear.
> + *
> + * Support for all modes and formats is optional. Even if dumb-buffer
> + * creation with a certain color mode succeeds, it is not guaranteed that
> + * the DRM driver supports any of the related formats. Most drivers support
> + * a color mode of 32 with a format of DRM_FORMAT_XRGB8888 on their primary
> + * plane.
> + *
> + * +------------+------------------------+------------------------+
> + * | Color mode | Framebuffer format | Compatibles |
> + * +============+========================+========================+
> + * | 32 | * DRM_FORMAT_XRGB8888 | * DRM_FORMAT_XBGR8888 |
> + * | | | * DRM_FORMAT_RGBX8888 |
> + * | | | * DRM_FORMAT_BGRX8888 |
> + * +------------+------------------------+------------------------+
> + * | 24 | * DRM_FORMAT_RGB888 | * DRM_FORMAT_BGR888 |
> + * +------------+------------------------+------------------------+
> + * | 16 | * DRM_FORMAT_RGB565 | * DRM_FORMAT_BGR565 |
> + * +------------+------------------------+------------------------+
> + * | 15 | * DRM_FORMAT_XRGB1555 | * DRM_FORMAT_XBGR1555 |
> + * | | | * DRM_FORMAT_RGBX1555 |
> + * | | | * DRM_FORMAT_BGRX1555 |
> + * +------------+------------------------+------------------------+
> + * | 8 | * DRM_FORMAT_C8 | * DRM_FORMAT_R8 |

+ DRM_FORMAT_D8? (and 4/2/1 below)

And DRM_FORMAT_Y8, if/when Tomi's series introducing that is accepted...

> + * +------------+------------------------+------------------------+
> + * | 4 | * DRM_FORMAT_C4 | * DRM_FORMAT_R4 |
> + * +------------+------------------------+------------------------+
> + * | 2 | * DRM_FORMAT_C2 | * DRM_FORMAT_R2 |
> + * +------------+------------------------+------------------------+
> + * | 1 | * DRM_FORMAT_C1 | * DRM_FORMAT_R1 |
> + * +------------+------------------------+------------------------+
> + *
> + * Color modes of 10, 12, 15, 30 and 64 are only supported for use by
> + * legacy user space. Please don't use them in new code. Other modes
> + * are not support.
> + *
> + * Do not attempt to allocate anything but linear framebuffer memory
> + * with single-plane RGB data. Allocation of other framebuffer
> + * layouts requires dedicated ioctls in the respective DRM driver.
> */
> struct drm_mode_create_dumb {
> __u32 height;

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds