[PATCH 5.4 35/67] drm/drm_fb_helper: fix fbdev with sparc64

Greg Kroah-Hartman
2020-08-10T15:36:42Z

From: Sam Ravnborg <sam@ravnborg.org>

[ Upstream commit 2a1658bf922ffd9b7907e270a7d9cdc9643fc45d ]

Recent kernels have been reported to panic using the bochs_drm
framebuffer under qemu-system-sparc64 which was bisected to
commit 7a0483ac4ffc ("drm/bochs: switch to generic drm fbdev emulation").

The backtrace indicates that the shadow framebuffer copy in
drm_fb_helper_dirty_blit_real() is trying to access the real
framebuffer using a virtual address rather than use an IO access
typically implemented using a physical (ASI_PHYS) access on SPARC.

The fix is to replace the memcpy with memcpy_toio() from io.h.

memcpy_toio() uses writeb() where the original fbdev code
used sbus_memcpy_toio(). The latter uses sbus_writeb().

The difference between writeb() and sbus_memcpy_toio() is
that writeb() writes bytes in little-endian, where sbus_writeb() writes
bytes in big-endian. As endian does not matter for byte writes they are
the same. So we can safely use memcpy_toio() here.

Note that this only fixes bochs, in general fbdev helpers still have
issues with mixing up system memory and __iomem space. Fixing that will
require a lot more work.

v3:
- Improved changelog (Daniel)
- Added FIXME to fbdev_use_iomem (Daniel)

v2:
- Added missing __iomem cast (kernel test robot)
- Made changelog readable and fix typos (Mark)
- Add flag to select iomem - and set it in the bochs driver

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reported-by: kernel test robot <lkp@intel.com>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: sparclinux@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200709193016.291267-1-sam@ravnborg.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200725191012.GA434957@ravnborg.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/gpu/drm/bochs/bochs_kms.c | 1 +
drivers/gpu/drm/drm_fb_helper.c | 6 +++++-
include/drm/drm_mode_config.h | 12 ++++++++++++
3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bochs/bochs_kms.c b/drivers/gpu/drm/bochs/bochs_kms.c
index 02a9c1ed165bb..fa50ab2523d4b 100644
--- a/drivers/gpu/drm/bochs/bochs_kms.c
+++ b/drivers/gpu/drm/bochs/bochs_kms.c
@@ -194,6 +194,7 @@ int bochs_kms_init(struct bochs_device *bochs)
bochs->dev->mode_config.preferred_depth = 24;
bochs->dev->mode_config.prefer_shadow = 0;
bochs->dev->mode_config.prefer_shadow_fbdev = 1;
+ bochs->dev->mode_config.fbdev_use_iomem = true;
bochs->dev->mode_config.quirk_addfb_prefer_host_byte_order = true;

bochs->dev->mode_config.funcs = &bochs_mode_funcs;
diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 8d193a58363d4..6b8502bcf0fd3 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -390,7 +390,11 @@ static void drm_fb_helper_dirty_blit_real(struct drm_fb_helper *fb_helper,
unsigned int y;

for (y = clip->y1; y < clip->y2; y++) {
- memcpy(dst, src, len);
+ if (!fb_helper->dev->mode_config.fbdev_use_iomem)
+ memcpy(dst, src, len);
+ else
+ memcpy_toio((void __iomem *)dst, src, len);
+
src += fb->pitches[0];
dst += fb->pitches[0];
}
diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
index 3bcbe30339f04..198b9d0600081 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -865,6 +865,18 @@ struct drm_mode_config {
*/
bool prefer_shadow_fbdev;

+ /**
+ * @fbdev_use_iomem:
+ *
+ * Set to true if framebuffer reside in iomem.
+ * When set to true memcpy_toio() is used when copying the framebuffer in
+ * drm_fb_helper.drm_fb_helper_dirty_blit_real().
+ *
+ * FIXME: This should be replaced with a per-mapping is_iomem
+ * flag (like ttm does), and then used everywhere in fbdev code.
+ */
+ bool fbdev_use_iomem;
+
/**
* @quirk_addfb_prefer_xbgr_30bpp:
*
--
2.25.1