[Bug 14662] drm/i915: Select CONFIG_SHMEM

[Hugh Dickins]

Hi Chris,

I'm slightly worried by this patch which went into Linus's git yesterday.

Thank you for looking into the issue, and coming up with what Mateusz
has found to be a good workaround.

But is it the right fix?

drm/i915 ought to be working without CONFIG_SHMEM: it then falls back
to using ramfs instead of shmem.  So why isn't that working here?

I'd take the NULL function pointer in read_cache_page_async() to be a
NULL filler function; but that should be pointing to simple_readpage(),
as specified in ramfs_aops.

You don't go into more detail in the comments: do you have any more
info on how this came about?  I don't get it.

I'd prefer not to "select SHMEM" there, because it shouldn't be
necessary; and will put CONFIG_SHMEM=y into the .configs of users
who chose not to have it before, and who will forget to reverse it
if we come up with a better fix letter.

If there is a better fix: I just don't understand what happened here.

Thanks,
Hugh

From: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Date: Sun, 22 Nov 2009 15:40:31 +0000 (+0000)
Subject: drm/i915: Select CONFIG_SHMEM
X-Git-Url: http://git.kernel.org/gitweb.cgi?p=linux%2Fkernel%2Fgit%2Ftorvalds%2Flinux-2.6.git;a=commitdiff_plain;h=ca9ab10033d190c1ede85fdf456307bdfdabf079

drm/i915: Select CONFIG_SHMEM

The driver requires shmfs as the backing filesystem to handle the buffer
objects, so ensure it is selected if the user chooses to build our
driver.

Fixes: Bug 14662 - Dell E5500 kernel panic with KMS
http://bugzilla.kernel.org/show_bug.cgi?id=14662

The revealing nature of the panic is the NULL function pointer
dereference in read_cache_page_async().

Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Reported-and-tested-by: Mateusz Kaduk <mateusz.kaduk@xxxxxxxxx>
Signed-off-by: Eric Anholt <eric@xxxxxxxxxx>
Cc: stable@xxxxxxxxxx
---

diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index f831ea1..96eddd1 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -92,6 +92,7 @@ config DRM_I830
 config DRM_I915
 	tristate "i915 driver"
 	depends on AGP_INTEL
+	select SHMEM
 	select DRM_KMS_HELPER
 	select FB_CFB_FILLRECT
 	select FB_CFB_COPYAREA
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