Re: [PATCH v2] drm/mgag200: Add a workaround for low-latency

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 





Am 08.02.24 um 10:51 schrieb Jocelyn Falempe:
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@xxxxxxxxxx>
Link: https://lore.kernel.org/dri-devel/20231019135655.313759-1-jfalempe@xxxxxxxxxx/ # 1

Acked-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 insertions(+)

diff --git a/drivers/gpu/drm/mgag200/Kconfig b/drivers/gpu/drm/mgag200/Kconfig
index b28c5e4828f4..5e4d48df4854 100644
--- a/drivers/gpu/drm/mgag200/Kconfig
+++ b/drivers/gpu/drm/mgag200/Kconfig
@@ -11,3 +11,15 @@ 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 54fce00e2136..573dbe256aa8 100644
--- a/drivers/gpu/drm/mgag200/mgag200_drv.c
+++ b/drivers/gpu/drm/mgag200/mgag200_drv.c
@@ -84,6 +84,20 @@ 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
   */
@@ -99,6 +113,9 @@ 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 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
  }
/*

base-commit: 1f36d634670d8001a45fe2f2dcae546819f9c7d8

--
--
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)




[Index of Archives]     [Linux DRI Users]     [Linux Intel Graphics]     [Linux USB Devel]     [Video for Linux]     [Linux Audio Users]     [Yosemite News]     [Linux Kernel]     [Linux SCSI]     [XFree86]     [Linux USB Devel]     [Video for Linux]     [Linux Audio Users]     [Linux Kernel]     [Linux SCSI]     [XFree86]
  Powered by Linux