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

Simona Vetter
2025-03-07T13:20:10Z

On Fri, Mar 07, 2025 at 09:42:25AM +0100, Simona Vetter wrote:
> On Tue, Feb 18, 2025 at 03:23:25PM +0100, Thomas Zimmermann 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>
> > ---
> > drivers/gpu/drm/drm_dumb_buffers.c | 116 +++++++++++++++++++++++++++++
> > include/drm/drm_dumb_buffers.h | 14 ++++
> > include/uapi/drm/drm_mode.h | 46 +++++++++++-
> > 3 files changed, 175 insertions(+), 1 deletion(-)
> > create mode 100644 include/drm/drm_dumb_buffers.h
> >
> > diff --git a/drivers/gpu/drm/drm_dumb_buffers.c b/drivers/gpu/drm/drm_dumb_buffers.c
> > index 9916aaf5b3f2..600ab281712b 100644
> > --- a/drivers/gpu/drm/drm_dumb_buffers.c
> > +++ b/drivers/gpu/drm/drm_dumb_buffers.c
> > @@ -25,6 +25,8 @@
> >
> > #include <drm/drm_device.h>
> > #include <drm/drm_drv.h>
> > +#include <drm/drm_dumb_buffers.h>
> > +#include <drm/drm_fourcc.h>
> > #include <drm/drm_gem.h>
> > #include <drm/drm_mode.h>
> >
> > @@ -57,6 +59,120 @@
> > * a hardware-specific ioctl to allocate suitable buffer objects.
> > */
> >
> > +static int drm_mode_align_dumb(struct drm_mode_create_dumb *args,
> > + unsigned long pitch_align,
> > + unsigned long size_align)
> > +{
> > + u32 pitch = args->pitch;
> > + u32 size;
> > +
> > + if (!pitch)
> > + return -EINVAL;
> > +
> > + if (pitch_align)
> > + pitch = roundup(pitch, pitch_align);
> > +
> > + /* overflow checks for 32bit size calculations */
> > + if (args->height > U32_MAX / pitch)
> > + return -EINVAL;
> > +
> > + if (!size_align)
> > + size_align = PAGE_SIZE;
> > + else if (!IS_ALIGNED(size_align, PAGE_SIZE))
> > + return -EINVAL;
> > +
> > + size = ALIGN(args->height * pitch, size_align);
> > + if (!size)
> > + return -EINVAL;
> > +
> > + args->pitch = pitch;
> > + args->size = size;
> > +
> > + return 0;
> > +}
> > +
> > +/**
> > + * 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",
> > + args->bpp);
> > + fallthrough;
>
> We cannot let userspace trigger dmesg warnings (or anything else really
> that spams logs). Also I think for future proofing it would be good if we
> just reject anything we don't currently know about instead of silently
> letting this mess become worse. Hence my vote is to reject unknown bpp
> hack values.
>
> > + case 12:
> > + case 15:
> > + case 30: /* see drm_gem_afbc_get_bpp() */
>
> This is a bit too cryptic to me, I think if you want to do comments I'd
> just put a long-form one above each value that explains where we've found
> it and why it happens. I'm also assuming these all have depth = 0, which I
> guess is something we should check just to keep this as strict as
> possible? Or do they have matching depth?

Correction from irc: Thomas pointed out that there's no depth in the
create_dumb ioctl, I was mixing this up with addfb and failed to check. So
please disregard this part, I was fabricating stuff out of some very thin
air (and probably not enough coffee in the brain too).
-Sima

>
> Cheers, Sima
>
> > + case 10:
> > + 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 |
> > + * +------------+------------------------+------------------------+
> > + * | 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;
> > --
> > 2.48.1
> >
>
> --
> Simona Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

--
Simona Vetter
Software Engineer, Intel Corporation