[PATCH 2/2] drm/mgag200: Add an option to disable Write-Combine

[Jocelyn Falempe]

Unfortunately, the G200 ioburst workaround doesn't work on some servers
like Dell poweredge XR11, XR5610, or HPE XL260
In this case completely disabling WC is the only option to achieve
low-latency.
So this adds a new Kconfig option, to disable WC mapping of the G200

Signed-off-by: Jocelyn Falempe <jfalempe at redhat.com>
---
 drivers/gpu/drm/mgag200/Kconfig       | 10 ++++++++++
 drivers/gpu/drm/mgag200/mgag200_drv.c |  7 +++++++
 2 files changed, 17 insertions(+)

diff --git a/drivers/gpu/drm/mgag200/Kconfig b/drivers/gpu/drm/mgag200/Kconfig
index b28c5e4828f47..73ab5730b74d9 100644
--- a/drivers/gpu/drm/mgag200/Kconfig
+++ b/drivers/gpu/drm/mgag200/Kconfig
@@ -11,3 +11,13 @@ config DRM_MGAG200
 	 MGA G200 desktop chips and the server variants. It requires 0.3.0
 	 of the modesetting userspace driver, and a version of mga driver
 	 that will fail on KMS enabled devices.
+
+config DRM_MGAG200_DISABLE_WRITECOMBINE
+	bool "Disable Write Combine mapping of VRAM"
+	depends on DRM_MGAG200 && PREEMPT_RT
+	help
+	  The VRAM of the G200 is mapped with Write-Combine, to improve
+	  performances. However this increases the system latency a lot, even
+	  for realtime tasks running on other CPU cores. Typically 40us-80us
+	  latencies are measured with hwlat when Write Combine is enabled.
+	  Recommended if you run realtime tasks on a server with a Matrox G200.
\ No newline at end of file
diff --git a/drivers/gpu/drm/mgag200/mgag200_drv.c b/drivers/gpu/drm/mgag200/mgag200_drv.c
index 3883f25ca4d8b..7461e3f984eff 100644
--- a/drivers/gpu/drm/mgag200/mgag200_drv.c
+++ b/drivers/gpu/drm/mgag200/mgag200_drv.c
@@ -146,12 +146,19 @@ int mgag200_device_preinit(struct mga_device *mdev)
 	}
 	mdev->vram_res = res;
 
+#if defined(CONFIG_DRM_MGAG200_DISABLE_WRITECOMBINE)
+	drm_info(dev, "Disable Write Combine\n");
+	mdev->vram = devm_ioremap(dev->dev, res->start, resource_size(res));
+	if (!mdev->vram)
+		return -ENOMEM;
+#else
 	mdev->vram = devm_ioremap_wc(dev->dev, res->start, resource_size(res));
 	if (!mdev->vram)
 		return -ENOMEM;
 
 	/* Don't fail on errors, but performance might be reduced. */
 	devm_arch_phys_wc_add(dev->dev, res->start, resource_size(res));
+#endif
 
 	return 0;
 }
-- 
2.45.0