diff options
| author | Jocelyn Falempe <jfalempe@redhat.com> | 2024-02-08 10:51:10 +0100 |
|---|---|---|
| committer | Jocelyn Falempe <jfalempe@redhat.com> | 2024-02-26 16:37:51 +0100 |
| commit | bfa4437fd3938ae2e186e7664b2db65bb8775670 (patch) | |
| tree | 380c56f5019a3f1f2d57ea70d5ad6f41ad99ff1b /drivers/gpu/drm/mgag200/mgag200_mode.c | |
| parent | 0475184905387dc481927f87e4abd63c3d8fa51d (diff) | |
drm/mgag200: Add a workaround for low-latency
We found a regression in v5.10 on real-time server, using the
rt-kernel and the mgag200 driver. It's some really specialized
workload, with <10us latency expectation on isolated core.
After the v5.10, the real time tasks missed their <10us latency
when something prints on the screen (fbcon or printk)
The regression has been bisected to 2 commits:
commit 0b34d58b6c32 ("drm/mgag200: Enable caching for SHMEM pages")
commit 4862ffaec523 ("drm/mgag200: Move vmap out of commit tail")
The first one changed the system memory framebuffer from Write-Combine
to the default caching.
Before the second commit, the mgag200 driver used to unmap the
framebuffer after each frame, which implicitly does a cache flush.
Both regressions are fixed by this commit, which restore WC mapping
for the framebuffer in system memory, and add a cache flush.
This is only needed on x86_64, for low-latency workload,
so the new kconfig DRM_MGAG200_IOBURST_WORKAROUND depends on
PREEMPT_RT and X86.
For more context, the whole thread can be found here [1]
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/dri-devel/20231019135655.313759-1-jfalempe@redhat.com/ # 1
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240208095125.377908-1-jfalempe@redhat.com
Diffstat (limited to 'drivers/gpu/drm/mgag200/mgag200_mode.c')
| -rw-r--r-- | drivers/gpu/drm/mgag200/mgag200_mode.c | 8 |
1 files changed, 8 insertions, 0 deletions
diff --git a/drivers/gpu/drm/mgag200/mgag200_mode.c b/drivers/gpu/drm/mgag200/mgag200_mode.c index 0eb769dd76ce..e17cb4c5f774 100644 --- a/drivers/gpu/drm/mgag200/mgag200_mode.c +++ b/drivers/gpu/drm/mgag200/mgag200_mode.c @@ -13,6 +13,7 @@ #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> @@ -436,6 +437,13 @@ 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 } /* |