Re: [PATCH v2 02/25] drm/dumb-buffers: Provide helper to set pitch and size

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi


Am 10.01.25 um 02:49 schrieb Andy Yan:
> Hi Thomas,
>
> At 2025-01-09 22:56:56, "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's 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. Again, current per-driver implementations
>> likely have subtle differences or bugs in how they support color modes.
>>
>> Future directions: one bug is present in the current input validation
>> in drm_mode_create_dumb(). The dumb-buffer overflow tests round up any
>> given bits-per-pixel value to a multiple of 8. So even one-bit formats,
>> such as DRM_FORMAT_C1, require 8 bits per pixel. While not common,
>> low-end displays use such formats; with a possible overcommitment of
>> memory. At some point, the validation logic in drm_mode_size_dumb() is
>> supposed to replace the erronous code.
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>> ---
>> drivers/gpu/drm/drm_dumb_buffers.c | 93 ++++++++++++++++++++++++++++++
>> include/drm/drm_dumb_buffers.h     | 14 +++++
>> 2 files changed, 107 insertions(+)
>> 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..fd39720bd617 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 +59,97 @@
>>   * 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)
>> +{
>> +	u32 fourcc;
>> +	const struct drm_format_info *info;
>> +	u64 pitch;
>> +
>> +	/*
>> +	 * 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);
> This will return -EINVAL with bpp drm_mode_legacy_fb_format doesn't support,
> such as(NV15, NV20, NV30, bpp is 10)[0]

Thanks for taking a look. That NV-related code at [0] is a 'somewhat 
non-idiomatic use' of the UAPI. The dumb-buffer interface really just 
supports a single plane. The fix would be a new ioctl that takes a DRM 
4cc constant and returns a buffer handle/pitch/size for each plane. But 
that's separate series throughout the various components.

There's also code XRGB16161616F. This is a viable format for the UAPI, 
but seems not very useful in practice.

>
> And there are also some AFBC based format with bpp can't be handled here, see:
> static __u32 drm_gem_afbc_get_bpp(struct drm_device *dev,
>                                    const struct drm_mode_fb_cmd2 *mode_cmd)
> {
>          const struct drm_format_info *info;
>                  
>          info = drm_get_format_info(dev, mode_cmd);
>                  
>          switch (info->format) {
>          case DRM_FORMAT_YUV420_8BIT:
>                  return 12;
>          case DRM_FORMAT_YUV420_10BIT:
>                  return 15;
>          case DRM_FORMAT_VUY101010:
>                  return 30;
>          default:
>                  return drm_format_info_bpp(info, 0);
>          }
> }

Same problem here. These YUV formats are multi-planar and there should 
be no dumb buffers for them.

As we still have to support these all use cases, I've modified the new 
helper to fallback to computing the pitch from the given bpp value. 
That's what drivers currently do. Could you please apply the attached 
patch on top of the series and report back the result of the test? You 
should see a kernel warning about the unknown color mode, but allocation 
should succeed.

Best regards
Thomas

>
>
> [0]https://gitlab.freedesktop.org/mesa/drm/-/blob/main/tests/modetest/buffers.c?ref_type=heads#L159
>
> This introduce a modetest failure on rockchip platform:
> # modetest -M rockchip -s 70@68:1920x1080 -P 32@68:1920x1080@NV30
> setting mode 1920x1080-60.00Hz on connectors 70, crtc 68
> testing 1920x1080@NV30 overlay plane 32
> failed to create dumb buffer: Invalid argument
>
> I think other platform with bpp can't handler by  drm_mode_legacy_fb_format will
> also see this kind of failure:
>
>
>
>> +	if (fourcc == DRM_FORMAT_INVALID)
>> +		return -EINVAL;
>> +	info = drm_format_info(fourcc);
>> +	if (!info)
>> +		return -EINVAL;
>> +	pitch = drm_format_info_min_pitch(info, 0, args->width);
>> +	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
>> -- 
>> 2.47.1
>>
>>
>> _______________________________________________
>> Linux-rockchip mailing list
>> Linux-rockchip@lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/linux-rockchip

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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From 2e7005654d76b71f78fe07fcf98a3570022f5034 Mon Sep 17 00:00:00 2001
From: Thomas Zimmermann <tzimmermann@suse.de>
Date: Fri, 10 Jan 2025 09:35:12 +0100
Subject: [PATCH] add fallback for unknown bpp

---
 drivers/gpu/drm/drm_dumb_buffers.c | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/drm_dumb_buffers.c b/drivers/gpu/drm/drm_dumb_buffers.c
index fd39720bd617..5f2d026c764c 100644
--- a/drivers/gpu/drm/drm_dumb_buffers.c
+++ b/drivers/gpu/drm/drm_dumb_buffers.c
@@ -119,9 +119,8 @@ int drm_mode_size_dumb(struct drm_device *dev,
 		       unsigned long pitch_align,
 		       unsigned long size_align)
 {
+	u64 pitch = 0;
 	u32 fourcc;
-	const struct drm_format_info *info;
-	u64 pitch;
 
 	/*
 	 * The scanline pitch depends on the buffer width and the color
@@ -135,12 +134,25 @@ int drm_mode_size_dumb(struct drm_device *dev,
 	 * color-mode constant.
 	 */
 	fourcc = drm_driver_color_mode_format(dev, args->bpp);
-	if (fourcc == DRM_FORMAT_INVALID)
-		return -EINVAL;
-	info = drm_format_info(fourcc);
-	if (!info)
-		return -EINVAL;
-	pitch = drm_format_info_min_pitch(info, 0, args->width);
+	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.
+		 */
+		drm_warn(dev, "Unknown color mode %d; guessing buffer size.\n", args->bpp);
+		if (args->bpp < 8)
+			pitch = DIV_ROUND_UP(args->width * args->bpp, SZ_8);
+		else
+			pitch = args->width * DIV_ROUND_UP(args->bpp, SZ_8);
+	}
+
 	if (!pitch || pitch > U32_MAX)
 		return -EINVAL;
 
-- 
2.47.1