Re: [PATCH v5 1/3] drm: Add support for GEM buffers backed by non-coherent memory
From: Paul Cercueil
Date: Thu May 27 2021 - 11:31:48 EST
Hi Tomi,
Le jeu., mai 27 2021 at 13:40:07 +0300, Tomi Valkeinen
<tomi.valkeinen@xxxxxxxxxxxxxxxx> a écrit :
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@xxxxxxxxxxxxxxx>
Acked-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
---
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()?
My bad, dma_mmap_wc() is not just a regular mmap with writecombine page
protection.
The solution, I guess, would be to call dma_mmap_wc() if
(!cma_obj->map_noncoherent). I can send a patch later this week, unless
you already have one?
Cheers,
-Paul