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Message-Id: <1CNRTQ.12RZKDP9M4XR3@crapouillou.net>
Date: Thu, 27 May 2021 13:43:13 +0100
From: Paul Cercueil <paul@...pouillou.net>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Maxime Ripard <mripard@...nel.org>,
Thomas Zimmermann <tzimmermann@...e.de>,
David Airlie <airlied@...ux.ie>,
Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
dri-devel@...ts.freedesktop.org, linux-mips@...r.kernel.org,
Christoph Hellwig <hch@...radead.org>, list@...ndingux.net,
Lokesh Vutla <lokeshvutla@...com>,
"Yadav, Pratyush" <p.yadav@...com>
Subject: Re: [PATCH v5 1/3] drm: Add support for GEM buffers backed by
non-coherent memory
Hi Tomi,
Le jeu., mai 27 2021 at 13:40:07 +0300, Tomi Valkeinen
<tomi.valkeinen@...asonboard.com> 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@...pouillou.net>
>> Acked-by: Thomas Zimmermann <tzimmermann@...e.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()?
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
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