media: ivtv: add parameter to enable ivtvfb on x86 PAT systems

ivtvfb was previously disabled for x86 PAT-enabled systems
by commit 1bf1735b4780 ("x86/mm/pat, drivers/media/ivtv:
Use arch_phys_wc_add() and require PAT disabled") as a
workaround to abstract MTRR code away from device drivers.

The driver is not easily upgradable to the PAT-aware
ioremap_wc() API since the firmware hides the address
ranges that should be marked write-combined from the driver.
However, since a write-combined cache on the framebuffer
is only a performance enhancement not a requirement for
the framebuffer to function, completely disabling the driver
in this configuration is not necessary.

Add force_pat module parameter and a corresponding kernel
configuration parameter to optionally force initialization
on PAT-enabled x86 systems with a warning about the lack of
write-combined caching, and document the reasons the driver
cannot be easily updated to support wc caching on all systems.

Signed-off-by: Nick French <naf@ou.edu>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: fix typo, split long pr_ lines up]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

1 files changed, 20 insertions, 3 deletions

diff --git a/drivers/media/pci/ivtv/Kconfig b/drivers/media/pci/ivtv/Kconfig
index c72cbbd2d40c..06ca4e23f9fb 100644
--- a/drivers/media/pci/ivtv/Kconfig
+++ b/drivers/media/pci/ivtv/Kconfig
@@ -70,8 +70,25 @@ config VIDEO_FB_IVTV
 	  This is used in the Hauppauge PVR-350 card. There is a driver
 	  homepage at <http://www.ivtvdriver.org>.
 
-	  In order to use this module, you will need to boot with PAT disabled
-	  on x86 systems, using the nopat kernel parameter.
-
 	  To compile this driver as a module, choose M here: the
 	  module will be called ivtvfb.
+
+config VIDEO_FB_IVTV_FORCE_PAT
+	bool "force cx23415 framebuffer init with x86 PAT enabled"
+	depends on VIDEO_FB_IVTV && X86_PAT
+	default n
+	---help---
+	  With PAT enabled, the cx23415 framebuffer driver does not
+	  utilize write-combined caching on the framebuffer memory.
+	  For this reason, the driver will by default disable itself
+	  when initializied on a kernel with PAT enabled (i.e. not
+	  using the nopat kernel parameter).
+
+	  The driver is not easily upgradable to the PAT-aware
+	  ioremap_wc() API since the firmware hides the address
+	  ranges that should be marked write-combined from the driver.
+
+	  With this setting enabled, the framebuffer will initialize on
+	  PAT-enabled systems but the framebuffer memory will be uncached.
+
+	  If unsure, say N.