Commit 4be9bd10 authored by Neil Armstrong's avatar Neil Armstrong
Browse files

drm/fb_helper: Allow leaking fbdev smem_start 



Since "drm/fb: Stop leaking physical address", the default behaviour of
the DRM fbdev emulation is to set the smem_base to 0 and pass the new
FBINFO_HIDE_SMEM_START flag.

The main reason is to avoid leaking physical addresse to user-space, and
it follows a general move over the kernel code to avoid user-space to
manipulate physical addresses and then use some other mechanisms like
dma-buf to transfer physical buffer handles over multiple subsystems.

But, a lot of devices depends on closed sources binaries to enable
OpenGL hardware acceleration that uses this smem_start value to
pass physical addresses to out-of-tree modules in order to render
into these physical adresses. These should use dma-buf buffers allocated
from the DRM display device instead and stop relying on fbdev overallocation
to gather DMA memory (some HW vendors delivers GBM and Wayland capable
binaries, but older unsupported devices won't have these new binaries
and are doomed until an Open Source solution like Lima finalizes).

Since these devices heavily depends on this kind of software and because
the smem_start population was available for years, it's a breakage to
stop leaking smem_start without any alternative solutions.

This patch adds a Kconfig depending on the EXPERT config and an unsafe
kernel module parameter tainting the kernel when enabled.

A clear comment and Kconfig help text was added to clarify why and when
this patch should be reverted, but in the meantime it's a necessary
feature to keep.

Cc: Dave Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Ben Skeggs <skeggsb@gmail.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: default avatarNeil Armstrong <narmstrong@baylibre.com>
Reviewed-by: default avatarMaxime Ripard <maxime.ripard@bootlin.com>
Tested-by: default avatarMaxime Ripard <maxime.ripard@bootlin.com>
Acked-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: default avatarDave Airlie <airlied@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1538136355-15383-1-git-send-email-narmstrong@baylibre.com
parent 87c2ee74

drivers/gpu/drm/Kconfig

+20 −0
Original line number Diff line number Diff line
@@ -110,6 +110,26 @@ config DRM_FBDEV_OVERALLOC 
	  is 100. Typical values for double buffering will be 200,

	  triple buffering 300.



config DRM_FBDEV_LEAK_PHYS_SMEM

	bool "Shamelessly allow leaking of fbdev physical address (DANGEROUS)"

	depends on DRM_FBDEV_EMULATION && EXPERT

	default n

	help

	  In order to keep user-space compatibility, we want in certain

	  use-cases to keep leaking the fbdev physical address to the

	  user-space program handling the fbdev buffer.

	  This affects, not only, Amlogic, Allwinner or Rockchip devices

	  with ARM Mali GPUs using an userspace Blob.

	  This option is not supported by upstream developers and should be

	  removed as soon as possible and be considered as a broken and

	  legacy behaviour from a modern fbdev device driver.



	  Please send any bug reports when using this to your proprietary

	  software vendor that requires this.



	  If in doubt, say "N" or spread the word to your closed source

	  library vendor.



config DRM_LOAD_EDID_FIRMWARE

	bool "Allow to specify an EDID data set instead of probing for it"

	depends on DRM

drivers/gpu/drm/drm_fb_helper.c

+31 −2
Original line number Diff line number Diff line
@@ -56,6 +56,25 @@ MODULE_PARM_DESC(drm_fbdev_overalloc, 
		 "Overallocation of the fbdev buffer (%) [default="

		 __MODULE_STRING(CONFIG_DRM_FBDEV_OVERALLOC) "]");



/*

 * In order to keep user-space compatibility, we want in certain use-cases

 * to keep leaking the fbdev physical address to the user-space program

 * handling the fbdev buffer.

 * This is a bad habit essentially kept into closed source opengl driver

 * that should really be moved into open-source upstream projects instead

 * of using legacy physical addresses in user space to communicate with

 * other out-of-tree kernel modules.

 *

 * This module_param *should* be removed as soon as possible and be

 * considered as a broken and legacy behaviour from a modern fbdev device.

 */

#if IS_ENABLED(CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM)

static bool drm_leak_fbdev_smem = false;

module_param_unsafe(drm_leak_fbdev_smem, bool, 0600);

MODULE_PARM_DESC(fbdev_emulation,

		 "Allow unsafe leaking fbdev physical smem address [default=false]");

#endif



static LIST_HEAD(kernel_fb_helper_list);

static DEFINE_MUTEX(kernel_fb_helper_lock);
@@ -2670,6 +2689,10 @@ __drm_fb_helper_initial_config_and_unlock(struct drm_fb_helper *fb_helper, 


	info = fb_helper->fbdev;

	info->var.pixclock = 0;

	/* Shamelessly allow physical address leaking to userspace */

#if IS_ENABLED(CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM)

	if (!drm_leak_fbdev_smem)

#endif

		/* don't leak any physical addresses to userspace */

		info->flags |= FBINFO_HIDE_SMEM_START;
@@ -3081,6 +3104,12 @@ int drm_fb_helper_generic_probe(struct drm_fb_helper *fb_helper, 
	fbi->screen_size = fb->height * fb->pitches[0];

	fbi->fix.smem_len = fbi->screen_size;

	fbi->screen_buffer = buffer->vaddr;

	/* Shamelessly leak the physical address to user-space */

#if IS_ENABLED(CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM)

	if (drm_leak_fbdev_smem && fbi->fix.smem_start == 0)

		fbi->fix.smem_start =

			page_to_phys(virt_to_page(fbi->screen_buffer));

#endif

	strcpy(fbi->fix.id, "DRM emulated");



	drm_fb_helper_fill_fix(fbi, fb->pitches[0], fb->format->depth);

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