Re: [PATCH] drm/gem-shmem: Optimize DMA mapping for exported buffers

[Thomas Zimmermann]

Hi

Am 25.03.25 um 14:53 schrieb Christian König:
Am 25.03.25 um 14:37 schrieb Jacek Lawrynowicz:
Use DMA_ATTR_SKIP_CPU_SYNC flag for exported buffers during DMA mapping.
The same flag is already used by drm_gem_map_dma_buf() for imported
buffers.
Signed-off-by: Jacek Lawrynowicz <jacek.lawrynow...@linux.intel.com>
---
  drivers/gpu/drm/drm_gem_shmem_helper.c |  8 ++++++--
  include/drm/drm_gem.h                  | 12 ++++++++++++
  2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c 
b/drivers/gpu/drm/drm_gem_shmem_helper.c
index d99dee67353a1..8938d8e3de52f 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -699,7 +699,7 @@ EXPORT_SYMBOL_GPL(drm_gem_shmem_get_sg_table);
  static struct sg_table *drm_gem_shmem_get_pages_sgt_locked(struct 
drm_gem_shmem_object *shmem)
  {
        struct drm_gem_object *obj = &shmem->base;
-       int ret;
+       int ret, flags = 0;
        struct sg_table *sgt;
  
  	if (shmem->sgt)
@@ -716,8 +716,12 @@ static struct sg_table 
*drm_gem_shmem_get_pages_sgt_locked(struct drm_gem_shmem_
                ret = PTR_ERR(sgt);
                goto err_put_pages;
        }
+
+       if (drm_gem_is_exported())
+               flags |= DMA_ATTR_SKIP_CPU_SYNC;
We should probably just unconditionally set this flag or not at all.

A question to both of you: does this have an effect on performance? I'm 
asking because i have a bug report where someone exports a buffer from 
gem-shmem with miserable performance. Would this flag make a difference?

Best regards
Thomas


Otherwise we could run into quite some surprises.

Regards,
Christian.

+
        /* Map the pages for use by the h/w. */
-       ret = dma_map_sgtable(obj->dev->dev, sgt, DMA_BIDIRECTIONAL, 0);
+       ret = dma_map_sgtable(obj->dev->dev, sgt, DMA_BIDIRECTIONAL, flags);
        if (ret)
                goto err_free_sgt;
  
diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index 2bf893eabb4b2..7c355a44d0a49 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -589,6 +589,18 @@ static inline bool drm_gem_is_imported(const struct 
drm_gem_object *obj)
        return obj->dma_buf && (obj->dma_buf->priv != obj);
  }
  
+/**
+ * drm_gem_is_exported() - Tests if GEM object's buffer has been exported
+ * @obj: the GEM object
+ *
+ * Returns:
+ * True if the GEM object's buffer has been exported, false otherwise
+ */
+static inline bool drm_gem_is_exported(const struct drm_gem_object *obj)
+{
+       return obj->dma_buf && (obj->dma_buf->priv == obj);
+}
+
  #ifdef CONFIG_LOCKDEP
  /**
   * drm_gem_gpuva_set_lock() - Set the lock protecting accesses to the gpuva 
list.

--
--
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)