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

[Thomas Zimmermann]

Hi,

just nits below.

Am 16.05.24 um 18:17 schrieb 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

The formatting look off. Maybe make this one single paragraph.

No comma after 'option'.

>
> 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
No comma after Write-Combine
> +	  performances. However this increases the system latency a lot, even
Just say "This can interfere with real-time tasks; even if they are 
running on other CPU cores then the graphics output."

> +	  for realtime tasks running on other CPU cores. Typically 40us-80us
> +	  latencies are measured with hwlat when Write Combine is enabled.

Leave out the next sentence: "Typically ..." The measureed numbers 
depend on the hardware and everyone is encouraged to test on their own 
system. You could mention  the numbers in the commit description, as you 
already mention the affected systems there.

> +	  Recommended if you run realtime tasks on a server with a Matrox G200.

I still think that we should not encourage anyone to use this option. 
Maybe say "Enable this option only if you run..."

> \ 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");

I would not print this drm_info() here. The user has selected the config 
option, so they should know what happens. It's also listed in /proc/mtrr 
IIRC.

Best regards
Thomas

> +	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;
>   }

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)