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@xxxxxxxxxx> --- 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
