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Subject[PATCH v2] 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>
---
Changes in v2:
- Add wording to Kconfig parameter to memorialize the reasoning
this driver cannot easily be upgraded to use ioremap_wc()

drivers/media/pci/ivtv/Kconfig | 23 ++++++++++++++++++++---
drivers/media/pci/ivtv/ivtvfb.c | 19 +++++++++++++++++--
2 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/drivers/media/pci/ivtv/Kconfig b/drivers/media/pci/ivtv/Kconfig
index c72cbbd2d40c..e5ebf7ca145c 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 frambuffer 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.
diff --git a/drivers/media/pci/ivtv/ivtvfb.c b/drivers/media/pci/ivtv/ivtvfb.c
index 621b2f613d81..5df74721aa19 100644
--- a/drivers/media/pci/ivtv/ivtvfb.c
+++ b/drivers/media/pci/ivtv/ivtvfb.c
@@ -55,6 +55,7 @@
/* card parameters */
static int ivtvfb_card_id = -1;
static int ivtvfb_debug = 0;
+static bool ivtvfb_force_pat = IS_ENABLED(CONFIG_VIDEO_FB_IVTV_FORCE_PAT);
static bool osd_laced;
static int osd_depth;
static int osd_upper;
@@ -64,6 +65,7 @@ static int osd_xres;

module_param(ivtvfb_card_id, int, 0444);
module_param_named(debug,ivtvfb_debug, int, 0644);
+module_param_named(force_pat, ivtvfb_force_pat, bool, 0644);
module_param(osd_laced, bool, 0444);
module_param(osd_depth, int, 0444);
module_param(osd_upper, int, 0444);
@@ -79,6 +81,9 @@ MODULE_PARM_DESC(debug,
"Debug level (bitmask). Default: errors only\n"
"\t\t\t(debug = 3 gives full debugging)");

+MODULE_PARM_DESC(force_pat,
+ "Force initialization on x86 PAT-enabled systems (bool).\n");
+
/* Why upper, left, xres, yres, depth, laced ? To match terminology used
by fbset.
Why start at 1 for left & upper coordinate ? Because X doesn't allow 0 */
@@ -1169,8 +1174,18 @@ static int ivtvfb_init_card(struct ivtv *itv)

#ifdef CONFIG_X86_64
if (pat_enabled()) {
- pr_warn("ivtvfb needs PAT disabled, boot with nopat kernel parameter\n");
- return -ENODEV;
+ if (ivtvfb_force_pat) {
+ pr_info("PAT is enabled. Write-combined framebuffer "
+ "caching will be disabled. To enable caching, "
+ "boot with nopat kernel parameter\n");
+ } else {
+ pr_warn("ivtvfb needs PAT disabled for write-combined "
+ "framebuffer caching. Boot with nopat kernel "
+ "parameter to use caching, or use the "
+ "force_pat module parameter to run with "
+ "caching disabled\n");
+ return -ENODEV;
+ }
}
#endif

--
2.13.6
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