Re: [PATCH 1/2] Revert "drm/mgag200: Add a workaround for low-latency"

[Thomas Zimmermann <tzimmermann@xxxxxxx>]

Am 16.05.24 um 18:17 schrieb Jocelyn Falempe:
> This reverts commit bfa4437fd3938ae2e186e7664b2db65bb8775670.
>
> This workaround doesn't work reliably on all servers.
> I'll replace it with an option to disable Write-Combine,
> which has more impact on performance, but fix the latency
> issue on all hardware.
>
> Signed-off-by: Jocelyn Falempe <jfalempe@xxxxxxxxxx>

Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>

> ---
>   drivers/gpu/drm/mgag200/Kconfig        | 12 ------------
>   drivers/gpu/drm/mgag200/mgag200_drv.c  | 17 -----------------
>   drivers/gpu/drm/mgag200/mgag200_mode.c |  8 --------
>   3 files changed, 37 deletions(-)
>
> diff --git a/drivers/gpu/drm/mgag200/Kconfig b/drivers/gpu/drm/mgag200/Kconfig
> index 5e4d48df4854c..b28c5e4828f47 100644
> --- a/drivers/gpu/drm/mgag200/Kconfig
> +++ b/drivers/gpu/drm/mgag200/Kconfig
> @@ -11,15 +11,3 @@ 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_IOBURST_WORKAROUND
> -	bool "Disable buffer caching"
> -	depends on DRM_MGAG200 && PREEMPT_RT && X86
> -	help
> -	  Enable a workaround to avoid I/O bursts within the mgag200 driver at
> -	  the expense of overall display performance.
> -	  It restores the <v5.10 behavior, by mapping the framebuffer in system
> -	  RAM as Write-Combining, and flushing the cache after each write.
> -	  This is only useful on x86_64 if you want to run processes with
> -	  deterministic latency.
> -	  If unsure, say N.
> diff --git a/drivers/gpu/drm/mgag200/mgag200_drv.c b/drivers/gpu/drm/mgag200/mgag200_drv.c
> index 65f2ed18b31c5..3883f25ca4d8b 100644
> --- a/drivers/gpu/drm/mgag200/mgag200_drv.c
> +++ b/drivers/gpu/drm/mgag200/mgag200_drv.c
> @@ -84,20 +84,6 @@ resource_size_t mgag200_probe_vram(void __iomem *mem, resource_size_t size)
>   	return offset - 65536;
>   }
>   
> -#if defined(CONFIG_DRM_MGAG200_IOBURST_WORKAROUND)
> -static struct drm_gem_object *mgag200_create_object(struct drm_device *dev, size_t size)
> -{
> -	struct drm_gem_shmem_object *shmem;
> -
> -	shmem = kzalloc(sizeof(*shmem), GFP_KERNEL);
> -	if (!shmem)
> -		return NULL;
> -
> -	shmem->map_wc = true;
> -	return &shmem->base;
> -}
> -#endif
> -
>   /*
>    * DRM driver
>    */
> @@ -113,9 +99,6 @@ static const struct drm_driver mgag200_driver = {
>   	.major = DRIVER_MAJOR,
>   	.minor = DRIVER_MINOR,
>   	.patchlevel = DRIVER_PATCHLEVEL,
> -#if defined(CONFIG_DRM_MGAG200_IOBURST_WORKAROUND)
> -	.gem_create_object = mgag200_create_object,
> -#endif
>   	DRM_GEM_SHMEM_DRIVER_OPS,
>   };
>   
> diff --git a/drivers/gpu/drm/mgag200/mgag200_mode.c b/drivers/gpu/drm/mgag200/mgag200_mode.c
> index fc54851d3384d..d3d820f7a77d7 100644
> --- a/drivers/gpu/drm/mgag200/mgag200_mode.c
> +++ b/drivers/gpu/drm/mgag200/mgag200_mode.c
> @@ -13,7 +13,6 @@
>   
>   #include <drm/drm_atomic.h>
>   #include <drm/drm_atomic_helper.h>
> -#include <drm/drm_cache.h>
>   #include <drm/drm_damage_helper.h>
>   #include <drm/drm_edid.h>
>   #include <drm/drm_format_helper.h>
> @@ -438,13 +437,6 @@ static void mgag200_handle_damage(struct mga_device *mdev, const struct iosys_ma
>   
>   	iosys_map_incr(&dst, drm_fb_clip_offset(fb->pitches[0], fb->format, clip));
>   	drm_fb_memcpy(&dst, fb->pitches, vmap, fb, clip);
> -
> -	/* Flushing the cache greatly improves latency on x86_64 */
> -#if defined(CONFIG_DRM_MGAG200_IOBURST_WORKAROUND)
> -	if (!vmap->is_iomem)
> -		drm_clflush_virt_range(vmap->vaddr + clip->y1 * fb->pitches[0],
> -				       drm_rect_height(clip) * fb->pitches[0]);
> -#endif
>   }
>   
>   /*

--
--
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)