[PATCH v2] drm/drm_fb_helper: fix fbdev with sparc64

From: Sam Ravnborg <sam@ravnborg.org>
To: dri-devel@lists.freedesktop.org, Dave Airlie <airlied@gmail.com>,
	Daniel Vetter <daniel@ffwll.ch>,
	Thomas Zimmermann <tzimmermann@suse.de>
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>,
	Gerd Hoffmann <kraxel@redhat.com>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	sparclinux@vger.kernel.org, Sam Ravnborg <sam@ravnborg.org>,
	"David S. Miller" <davem@davemloft.net>
Subject: [PATCH v2] drm/drm_fb_helper: fix fbdev with sparc64
Date: Sat, 25 Jul 2020 21:10:12 +0200	[thread overview]
Message-ID: <20200725191012.GA434957@ravnborg.org> (raw)

From 1323a7433691aee112a9b2df8041b5024895a77e Mon Sep 17 00:00:00 2001
From: Sam Ravnborg <sam@ravnborg.org>
Date: Thu, 9 Jul 2020 21:30:16 +0200
Subject: [PATCH v2 1/1] drm/drm_fb_helper: fix fbdev with sparc64

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.

For many architectures memcpy_toio() is a simple memcpy().
One side-effect that is unknown is if this has any impact on other
architectures.
So far the analysis tells that this change is OK for other arch's.
but testing would be good.

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>
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
---

This fix introducing a flag in mode_config is at best a band-aid
solution until we have a proper fix.
We need to propagate the info about iomem so it is not a driver flag
thing.

There is also the issue with sys* versus cfb* functions, where cfb*
functions are used for iomem.
I did not manage to make the bochs driver work with the cfb* functions,
for some unknown reason booting would be stuck waiting for the console
mutex when usign the cfb* functions.

I consider this fix OK to get the kernel working for sparc64 with the
bochs driver for now. And with the fbdev_uses_iomem flag no other
drivers will see any changes.

	Sam

 drivers/gpu/drm/bochs/bochs_kms.c | 1 +
 drivers/gpu/drm/drm_fb_helper.c   | 6 +++++-
 include/drm/drm_mode_config.h     | 9 +++++++++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bochs/bochs_kms.c b/drivers/gpu/drm/bochs/bochs_kms.c
index 05d8373888e8..079f46f5cdb6 100644
--- a/drivers/gpu/drm/bochs/bochs_kms.c
+++ b/drivers/gpu/drm/bochs/bochs_kms.c
@@ -146,6 +146,7 @@ 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 5609e164805f..89cfd68ef400 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -399,7 +399,11 @@ 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 6c3ef49b46b3..c24c066bdd9c 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -865,6 +865,15 @@ 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()
+	 */
+	bool fbdev_use_iomem;
+
 	/**
 	 * @quirk_addfb_prefer_xbgr_30bpp:
 	 *
-- 
2.25.1

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