Re: [PATCH v5 1/3] drm: Add support for GEM buffers backed by non-coherent memory

On 23/05/2021 20:04, Paul Cercueil wrote:

Having GEM buffers backed by non-coherent memory is interesting in the particular case where it is faster to render to a non-coherent buffer then sync the data cache, than to render to a write-combine buffer, and (by extension) much faster than using a shadow buffer. This is true for instance on some Ingenic SoCs, where even simple blits (e.g. memcpy) are about three times faster using this method.

Add a 'map_noncoherent' flag to the drm_gem_cma_object structure, which can be set by the drivers when they create the dumb buffer.

Since this really only applies to software rendering, disable this flag as soon as the CMA objects are exported via PRIME.

v3: New patch. Now uses a simple 'map_noncoherent' flag to control how the objects are mapped, and use the new dma_mmap_pages function.

v4: Make sure map_noncoherent is always disabled when creating GEM objects meant to be used with dma-buf.

Signed-off-by: Paul Cercueil paul@crapouillou.net Acked-by: Thomas Zimmermann tzimmermann@suse.de

---

drivers/gpu/drm/drm_gem_cma_helper.c | 38 +++++++++++++++++++++------- include/drm/drm_gem_cma_helper.h | 3 +++ 2 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_cma_helper.c b/drivers/gpu/drm/drm_gem_cma_helper.c index 7942cf05cd93..235c7a63da2b 100644 --- a/drivers/gpu/drm/drm_gem_cma_helper.c +++ b/drivers/gpu/drm/drm_gem_cma_helper.c @@ -46,6 +46,7 @@ static const struct drm_gem_object_funcs drm_gem_cma_default_funcs = {

• __drm_gem_cma_create - Create a GEM CMA object without allocating memory
• @drm: DRM device
• @size: size of the object to allocate

• • @private: true if used for internal purposes
  • This function creates and initializes a GEM CMA object of the given size,
  • but doesn't allocate any memory to back the object.

@@ -55,11 +56,11 @@ static const struct drm_gem_object_funcs drm_gem_cma_default_funcs = {

• error code on failure.

*/ static struct drm_gem_cma_object * -__drm_gem_cma_create(struct drm_device *drm, size_t size) +__drm_gem_cma_create(struct drm_device *drm, size_t size, bool private) { struct drm_gem_cma_object *cma_obj; struct drm_gem_object *gem_obj;

• int ret;

• int ret = 0;

  if (drm->driver->gem_create_object) gem_obj = drm->driver->gem_create_object(drm, size);

@@ -73,7 +74,14 @@ __drm_gem_cma_create(struct drm_device *drm, size_t size)

cma_obj = container_of(gem_obj, struct drm_gem_cma_object, base);

• ret = drm_gem_object_init(drm, gem_obj, size);

• if (private) {

• drm_gem_private_object_init(drm, gem_obj, size);
  

• /* Always use writecombine for dma-buf mappings */
  

• cma_obj->map_noncoherent = false;
  

• } else {

• ret = drm_gem_object_init(drm, gem_obj, size);
  

• } if (ret) goto error;

@@ -111,12 +119,19 @@ struct drm_gem_cma_object *drm_gem_cma_create(struct drm_device *drm,

size = round_up(size, PAGE_SIZE);

• cma_obj = __drm_gem_cma_create(drm, size);

• cma_obj = __drm_gem_cma_create(drm, size, false); if (IS_ERR(cma_obj)) return cma_obj;

• cma_obj->vaddr = dma_alloc_wc(drm->dev, size, &cma_obj->paddr,

• 		      GFP_KERNEL | __GFP_NOWARN);
  

• if (cma_obj->map_noncoherent) {

• cma_obj->vaddr = dma_alloc_noncoherent(drm->dev, size,
  

• 				       &cma_obj->paddr,
  

• 				       DMA_TO_DEVICE,
  

• 				       GFP_KERNEL | __GFP_NOWARN);
  

• } else {

• cma_obj->vaddr = dma_alloc_wc(drm->dev, size, &cma_obj->paddr,
  

• 			      GFP_KERNEL | __GFP_NOWARN);
  

• } if (!cma_obj->vaddr) { drm_dbg(drm, "failed to allocate buffer with size %zu\n", size);

@@ -432,7 +447,7 @@ drm_gem_cma_prime_import_sg_table(struct drm_device *dev, return ERR_PTR(-EINVAL);

/* Create a CMA GEM buffer. */

• cma_obj = __drm_gem_cma_create(dev, attach->dmabuf->size);

• cma_obj = __drm_gem_cma_create(dev, attach->dmabuf->size, true); if (IS_ERR(cma_obj)) return ERR_CAST(cma_obj);

@@ -499,8 +514,13 @@ int drm_gem_cma_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)

cma_obj = to_drm_gem_cma_obj(obj);

• ret = dma_mmap_wc(cma_obj->base.dev->dev, vma, cma_obj->vaddr,

• 	  cma_obj->paddr, vma->vm_end - vma->vm_start);
  

• vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
• if (!cma_obj->map_noncoherent)

• vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
  

• ret = dma_mmap_pages(cma_obj->base.dev->dev,

• 	     vma, vma->vm_end - vma->vm_start,
  

• 	     virt_to_page(cma_obj->vaddr));
  

This breaks mmap on TI's J7 EVM (tidss driver). All DRM apps just die when doing mmap. Changing these lines back to dma_mmap_wc() makes it work.

Is dma_alloc_wc() even compatible with dma_mmap_pages()?

Tomi