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

ivtvfb was previously disabled for x86 PAT-enabled systems
by commit 1bf1735b47 ("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>

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.

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 */
@@ -1167,8 +1172,15 @@ 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.\n");
+			pr_info("To enable caching, boot with nopat kernel parameter\n");
+		} else {
+			pr_warn("ivtvfb needs PAT disabled for write-combined framebuffer caching.\n");
+			pr_warn("Boot with nopat kernel parameter to use caching, or use the\n");
+			pr_warn("force_pat module parameter to run with caching disabled\n");
+			return -ENODEV;
+		}
 	}
 #endif