Re: [PATCH v2 11/17] drm/spacemit: add Saturn DPU DRM device driver

[Sarah Emery <sarah.emery@canonical.com>]

Hi,

We have incorporated this patchset into the Ubuntu kernel for display 
support on the K3 platforms.

During testing, we found an issue: the screen would display artifacts 
all over, kind of like black lines blinking.

I have implemented the following patch to fix the issue:

drm/spacemit: map scanout buffers write-combining

The K3 display controller is not DMA-coherent: the DPU fetches scanout
buffers straight from memory while fbcon and userspace write them through
a cached CPU mapping. The shmem helpers clean the buffer only once, when
the sg table is first built, so nothing flushes later CPU writes. Dirty
cache lines shadow what the DPU scans out, and erased fbcon text lingers
on screen until the lines are evicted by chance.

Add a gem_create_object() hook that sets map_wc on the shmem objects.
The K3 harts implement Svpbmt, so the CPU mapping becomes genuinely
non-cacheable and writes reach memory immediately; the one-time clean in
dma_map_sgtable() still covers the initial contents.

---
  drivers/gpu/drm/spacemit/spacemit_drm.c | 16 ++++++++++++++++
  1 file changed, 16 insertions(+)

diff --git a/drivers/gpu/drm/spacemit/spacemit_drm.c 
b/drivers/gpu/drm/spacemit/spacemit_drm.c
index 12b2fe8b6fc99..fe354219b996d 100644
--- a/drivers/gpu/drm/spacemit/spacemit_drm.c
+++ b/drivers/gpu/drm/spacemit/spacemit_drm.c
@@ -12,6 +12,7 @@
  #include <drm/drm_fbdev_shmem.h>
  #include <linux/component.h>
  #include <linux/of_graph.h>
+#include <linux/slab.h>

  #include "spacemit_drm.h"
  #include "spacemit_crtc.h"
@@ -75,6 +76,20 @@ int spacemit_drm_mode_config_init(struct drm_device *drm)
         return 0;
  }

+static struct drm_gem_object *spacemit_gem_create_object(struct 
drm_device *drm,
+                                                        size_t size)
+{
+       struct drm_gem_shmem_object *shmem;
+
+       shmem = kzalloc_obj(*shmem);
+       if (!shmem)
+               return ERR_PTR(-ENOMEM);
+
+       shmem->map_wc = true;
+
+       return &shmem->base;
+}
+
  DEFINE_DRM_GEM_FOPS(spacemit_drm_fops);

  struct drm_driver spacemit_drm_drv = {
@@ -82,6 +97,7 @@ struct drm_driver spacemit_drm_drv = {
         .fops = &spacemit_drm_fops,

         DRM_GEM_SHMEM_DRIVER_OPS,
+       .gem_create_object = spacemit_gem_create_object,
         DRM_FBDEV_SHMEM_DRIVER_OPS,

         .name           = DRIVER_NAME,
--

I'm sending this here in the hope that it will be useful for a future 
revision.
Is there a v3 in preparation?

Thanks,

Sarah