Revert "drm/mgag200: Add a workaround for low-latency"

This reverts commit bfa4437f.

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@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517151050.624797-2-jfalempe@redhat.com

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.

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,
 };
 

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>
@@ -439,13 +438,6 @@ 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
 }
 
 /*