drm/mgag200: Add an option to disable Write-Combine

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@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517151050.624797-3-jfalempe@redhat.com

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. This can interfere with real-time tasks; even if they
+	  are running on other CPU cores than the graphics output.
+	  Enable this option only if you run realtime tasks on a server with a
+	  Matrox G200.
\ No newline at end of file

drivers/gpu/drm/mgag200/mgag200_drv.c

@@ -146,12 +146,18 @@ int mgag200_device_preinit(struct mga_device *mdev)
 	}
 	mdev->vram_res = res;
 
+#if defined(CONFIG_DRM_MGAG200_DISABLE_WRITECOMBINE)
+	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;
 }