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Subject: Re: [PATCH 1/2] drm: allow limiting the scatter list size.
To:     Gerd Hoffmann <kraxel@redhat.com>, dri-devel@lists.freedesktop.org
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        Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>,
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Am 18.08.20 um 09:48 schrieb Gerd Hoffmann:
> Add max_segment argument to drm_prime_pages_to_sg().  When set pass it
> through to the __sg_alloc_table_from_pages() call, otherwise use
> SCATTERLIST_MAX_SEGMENT.
>
> Also add max_segment field to gem objects and pass it to
> drm_prime_pages_to_sg() calls in drivers and helpers.
>
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

I'm missing an explanation why this should be useful (it certainly is).

And the maximum segment size seems misplaced in the GEM object. This is 
usually a property of the device or even completely constant.

Christian.

> ---
>   include/drm/drm_gem.h                       |  8 ++++++++
>   include/drm/drm_prime.h                     |  3 ++-
>   drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c |  3 ++-
>   drivers/gpu/drm/drm_gem_shmem_helper.c      |  3 ++-
>   drivers/gpu/drm/drm_prime.c                 | 10 +++++++---
>   drivers/gpu/drm/etnaviv/etnaviv_gem.c       |  3 ++-
>   drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c |  3 ++-
>   drivers/gpu/drm/msm/msm_gem.c               |  3 ++-
>   drivers/gpu/drm/msm/msm_gem_prime.c         |  3 ++-
>   drivers/gpu/drm/nouveau/nouveau_prime.c     |  3 ++-
>   drivers/gpu/drm/radeon/radeon_prime.c       |  3 ++-
>   drivers/gpu/drm/rockchip/rockchip_drm_gem.c |  6 ++++--
>   drivers/gpu/drm/tegra/gem.c                 |  3 ++-
>   drivers/gpu/drm/vgem/vgem_drv.c             |  3 ++-
>   drivers/gpu/drm/xen/xen_drm_front_gem.c     |  3 ++-
>   15 files changed, 43 insertions(+), 17 deletions(-)
>
> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> index 337a48321705..dea5e92e745b 100644
> --- a/include/drm/drm_gem.h
> +++ b/include/drm/drm_gem.h
> @@ -241,6 +241,14 @@ struct drm_gem_object {
>   	 */
>   	size_t size;
>   
> +	/**
> +	 * @max_segment:
> +	 *
> +	 * Max size for scatter list segments.  When unset the default
> +	 * (SCATTERLIST_MAX_SEGMENT) is used.
> +	 */
> +	size_t max_segment;
> +
>   	/**
>   	 * @name:
>   	 *
> diff --git a/include/drm/drm_prime.h b/include/drm/drm_prime.h
> index 9af7422b44cf..2c3689435cb4 100644
> --- a/include/drm/drm_prime.h
> +++ b/include/drm/drm_prime.h
> @@ -88,7 +88,8 @@ void drm_gem_dmabuf_vunmap(struct dma_buf *dma_buf, void *vaddr);
>   int drm_gem_prime_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma);
>   int drm_gem_dmabuf_mmap(struct dma_buf *dma_buf, struct vm_area_struct *vma);
>   
> -struct sg_table *drm_prime_pages_to_sg(struct page **pages, unsigned int nr_pages);
> +struct sg_table *drm_prime_pages_to_sg(struct page **pages, unsigned int nr_pages,
> +				       size_t max_segment);
>   struct dma_buf *drm_gem_prime_export(struct drm_gem_object *obj,
>   				     int flags);
>   
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> index 519ce4427fce..5e8a9760b33f 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
> @@ -303,7 +303,8 @@ static struct sg_table *amdgpu_dma_buf_map(struct dma_buf_attachment *attach,
>   	switch (bo->tbo.mem.mem_type) {
>   	case TTM_PL_TT:
>   		sgt = drm_prime_pages_to_sg(bo->tbo.ttm->pages,
> -					    bo->tbo.num_pages);
> +					    bo->tbo.num_pages,
> +					    obj->max_segment);
>   		if (IS_ERR(sgt))
>   			return sgt;
>   
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 4b7cfbac4daa..cfb979d808fd 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -656,7 +656,8 @@ struct sg_table *drm_gem_shmem_get_sg_table(struct drm_gem_object *obj)
>   
>   	WARN_ON(shmem->base.import_attach);
>   
> -	return drm_prime_pages_to_sg(shmem->pages, obj->size >> PAGE_SHIFT);
> +	return drm_prime_pages_to_sg(shmem->pages, obj->size >> PAGE_SHIFT,
> +				     obj->max_segment);
>   }
>   EXPORT_SYMBOL_GPL(drm_gem_shmem_get_sg_table);
>   
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 1693aa7c14b5..27c783fd6633 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -802,7 +802,8 @@ static const struct dma_buf_ops drm_gem_prime_dmabuf_ops =  {
>    *
>    * This is useful for implementing &drm_gem_object_funcs.get_sg_table.
>    */
> -struct sg_table *drm_prime_pages_to_sg(struct page **pages, unsigned int nr_pages)
> +struct sg_table *drm_prime_pages_to_sg(struct page **pages, unsigned int nr_pages,
> +				       size_t max_segment)
>   {
>   	struct sg_table *sg = NULL;
>   	int ret;
> @@ -813,8 +814,11 @@ struct sg_table *drm_prime_pages_to_sg(struct page **pages, unsigned int nr_page
>   		goto out;
>   	}
>   
> -	ret = sg_alloc_table_from_pages(sg, pages, nr_pages, 0,
> -				nr_pages << PAGE_SHIFT, GFP_KERNEL);
> +	if (max_segment == 0 || max_segment > SCATTERLIST_MAX_SEGMENT)
> +		max_segment = SCATTERLIST_MAX_SEGMENT;
> +	ret = __sg_alloc_table_from_pages(sg, pages, nr_pages, 0,
> +					  nr_pages << PAGE_SHIFT,
> +					  max_segment, GFP_KERNEL);
>   	if (ret)
>   		goto out;
>   
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.c b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> index f06e19e7be04..e5b6e7996f80 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
> @@ -103,7 +103,8 @@ struct page **etnaviv_gem_get_pages(struct etnaviv_gem_object *etnaviv_obj)
>   		int npages = etnaviv_obj->base.size >> PAGE_SHIFT;
>   		struct sg_table *sgt;
>   
> -		sgt = drm_prime_pages_to_sg(etnaviv_obj->pages, npages);
> +		sgt = drm_prime_pages_to_sg(etnaviv_obj->pages, npages,
> +					    etnaviv_obj->base.max_segment);
>   		if (IS_ERR(sgt)) {
>   			dev_err(dev->dev, "failed to allocate sgt: %ld\n",
>   				PTR_ERR(sgt));
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c b/drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c
> index 6d9e5c3c4dd5..f327676450bd 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c
> @@ -19,7 +19,8 @@ struct sg_table *etnaviv_gem_prime_get_sg_table(struct drm_gem_object *obj)
>   	if (WARN_ON(!etnaviv_obj->pages))  /* should have already pinned! */
>   		return ERR_PTR(-EINVAL);
>   
> -	return drm_prime_pages_to_sg(etnaviv_obj->pages, npages);
> +	return drm_prime_pages_to_sg(etnaviv_obj->pages, npages,
> +				     obj->max_segment);
>   }
>   
>   void *etnaviv_gem_prime_vmap(struct drm_gem_object *obj)
> diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
> index b2f49152b4d4..f805419bb84a 100644
> --- a/drivers/gpu/drm/msm/msm_gem.c
> +++ b/drivers/gpu/drm/msm/msm_gem.c
> @@ -126,7 +126,8 @@ static struct page **get_pages(struct drm_gem_object *obj)
>   
>   		msm_obj->pages = p;
>   
> -		msm_obj->sgt = drm_prime_pages_to_sg(p, npages);
> +		msm_obj->sgt = drm_prime_pages_to_sg(p, npages,
> +						     obj->max_segment);
>   		if (IS_ERR(msm_obj->sgt)) {
>   			void *ptr = ERR_CAST(msm_obj->sgt);
>   
> diff --git a/drivers/gpu/drm/msm/msm_gem_prime.c b/drivers/gpu/drm/msm/msm_gem_prime.c
> index d7c8948427fe..a5a412564c7f 100644
> --- a/drivers/gpu/drm/msm/msm_gem_prime.c
> +++ b/drivers/gpu/drm/msm/msm_gem_prime.c
> @@ -19,7 +19,8 @@ struct sg_table *msm_gem_prime_get_sg_table(struct drm_gem_object *obj)
>   	if (WARN_ON(!msm_obj->pages))  /* should have already pinned! */
>   		return NULL;
>   
> -	return drm_prime_pages_to_sg(msm_obj->pages, npages);
> +	return drm_prime_pages_to_sg(msm_obj->pages, npages,
> +				     obj->max_segment);
>   }
>   
>   void *msm_gem_prime_vmap(struct drm_gem_object *obj)
> diff --git a/drivers/gpu/drm/nouveau/nouveau_prime.c b/drivers/gpu/drm/nouveau/nouveau_prime.c
> index bae6a3eccee0..56a2e916d51a 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_prime.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_prime.c
> @@ -32,7 +32,8 @@ struct sg_table *nouveau_gem_prime_get_sg_table(struct drm_gem_object *obj)
>   	struct nouveau_bo *nvbo = nouveau_gem_object(obj);
>   	int npages = nvbo->bo.num_pages;
>   
> -	return drm_prime_pages_to_sg(nvbo->bo.ttm->pages, npages);
> +	return drm_prime_pages_to_sg(nvbo->bo.ttm->pages, npages,
> +				     obj->max_segment);
>   }
>   
>   void *nouveau_gem_prime_vmap(struct drm_gem_object *obj)
> diff --git a/drivers/gpu/drm/radeon/radeon_prime.c b/drivers/gpu/drm/radeon/radeon_prime.c
> index b906e8fbd5f3..503e35625045 100644
> --- a/drivers/gpu/drm/radeon/radeon_prime.c
> +++ b/drivers/gpu/drm/radeon/radeon_prime.c
> @@ -36,7 +36,8 @@ struct sg_table *radeon_gem_prime_get_sg_table(struct drm_gem_object *obj)
>   	struct radeon_bo *bo = gem_to_radeon_bo(obj);
>   	int npages = bo->tbo.num_pages;
>   
> -	return drm_prime_pages_to_sg(bo->tbo.ttm->pages, npages);
> +	return drm_prime_pages_to_sg(bo->tbo.ttm->pages, npages,
> +				     obj->max_segment);
>   }
>   
>   void *radeon_gem_prime_vmap(struct drm_gem_object *obj)
> diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_gem.c b/drivers/gpu/drm/rockchip/rockchip_drm_gem.c
> index b9275ba7c5a5..444657e03c16 100644
> --- a/drivers/gpu/drm/rockchip/rockchip_drm_gem.c
> +++ b/drivers/gpu/drm/rockchip/rockchip_drm_gem.c
> @@ -85,7 +85,8 @@ static int rockchip_gem_get_pages(struct rockchip_gem_object *rk_obj)
>   
>   	rk_obj->num_pages = rk_obj->base.size >> PAGE_SHIFT;
>   
> -	rk_obj->sgt = drm_prime_pages_to_sg(rk_obj->pages, rk_obj->num_pages);
> +	rk_obj->sgt = drm_prime_pages_to_sg(rk_obj->pages, rk_obj->num_pages,
> +					    rk_obj->base.max_segment);
>   	if (IS_ERR(rk_obj->sgt)) {
>   		ret = PTR_ERR(rk_obj->sgt);
>   		goto err_put_pages;
> @@ -442,7 +443,8 @@ struct sg_table *rockchip_gem_prime_get_sg_table(struct drm_gem_object *obj)
>   	int ret;
>   
>   	if (rk_obj->pages)
> -		return drm_prime_pages_to_sg(rk_obj->pages, rk_obj->num_pages);
> +		return drm_prime_pages_to_sg(rk_obj->pages, rk_obj->num_pages,
> +					     obj->max_segment);
>   
>   	sgt = kzalloc(sizeof(*sgt), GFP_KERNEL);
>   	if (!sgt)
> diff --git a/drivers/gpu/drm/tegra/gem.c b/drivers/gpu/drm/tegra/gem.c
> index 723df142a981..8d98b02a8d21 100644
> --- a/drivers/gpu/drm/tegra/gem.c
> +++ b/drivers/gpu/drm/tegra/gem.c
> @@ -284,7 +284,8 @@ static int tegra_bo_get_pages(struct drm_device *drm, struct tegra_bo *bo)
>   
>   	bo->num_pages = bo->gem.size >> PAGE_SHIFT;
>   
> -	bo->sgt = drm_prime_pages_to_sg(bo->pages, bo->num_pages);
> +	bo->sgt = drm_prime_pages_to_sg(bo->pages, bo->num_pages,
> +					bo->gem.max_segment);
>   	if (IS_ERR(bo->sgt)) {
>   		err = PTR_ERR(bo->sgt);
>   		goto put_pages;
> diff --git a/drivers/gpu/drm/vgem/vgem_drv.c b/drivers/gpu/drm/vgem/vgem_drv.c
> index 313339bbff90..d25c93b5a2c1 100644
> --- a/drivers/gpu/drm/vgem/vgem_drv.c
> +++ b/drivers/gpu/drm/vgem/vgem_drv.c
> @@ -321,7 +321,8 @@ static struct sg_table *vgem_prime_get_sg_table(struct drm_gem_object *obj)
>   {
>   	struct drm_vgem_gem_object *bo = to_vgem_bo(obj);
>   
> -	return drm_prime_pages_to_sg(bo->pages, bo->base.size >> PAGE_SHIFT);
> +	return drm_prime_pages_to_sg(bo->pages, bo->base.size >> PAGE_SHIFT,
> +				     obj->max_segment);
>   }
>   
>   static struct drm_gem_object* vgem_prime_import(struct drm_device *dev,
> diff --git a/drivers/gpu/drm/xen/xen_drm_front_gem.c b/drivers/gpu/drm/xen/xen_drm_front_gem.c
> index f0b85e094111..362fe5311b1b 100644
> --- a/drivers/gpu/drm/xen/xen_drm_front_gem.c
> +++ b/drivers/gpu/drm/xen/xen_drm_front_gem.c
> @@ -179,7 +179,8 @@ struct sg_table *xen_drm_front_gem_get_sg_table(struct drm_gem_object *gem_obj)
>   	if (!xen_obj->pages)
>   		return ERR_PTR(-ENOMEM);
>   
> -	return drm_prime_pages_to_sg(xen_obj->pages, xen_obj->num_pages);
> +	return drm_prime_pages_to_sg(xen_obj->pages, xen_obj->num_pages,
> +				     gem_obj->max_segment);
>   }
>   
>   struct drm_gem_object *