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

diff --git a/drivers/gpu/drm/mgag200/Kconfig b/drivers/gpu/drm/mgag200/Kconfig
index b28c5e4828f47bc3b46666d31a26d02f9cb71752..3096944a8f0ab76ab8ac2e8b85c7123e05bf6419 100644 (file)
--- 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. 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

diff --git a/drivers/gpu/drm/mgag200/mgag200_drv.c b/drivers/gpu/drm/mgag200/mgag200_drv.c
index 3883f25ca4d8bd0dea8c5adf7e4f608ae5b2de72..62080cf0f2da497f652f54089ec344acaa635c60 100644 (file)
--- a/drivers/gpu/drm/mgag200/mgag200_drv.c
+++ b/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;
 }