Re: [PATCH] drm/vgem: fix cache synchronization on arm/arm64

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jul 17, 2019 at 02:15:37PM -0700, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.org>
> 
> drm_cflush_pages() is no-op on arm/arm64.  But instead we can use
> dma_sync API.
> 
> Fixes failures w/ vgem_test.
> 
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> ---
> An alternative approach to the series[1] I sent yesterday
> 
> On the plus side, it keeps the WC buffers and avoids any drm core
> changes.  On the minus side, I don't think it will work (at least
> on arm64) prior to v5.0[2], so the fix can't be backported very
> far.

Yeah seems a lot more reasonable.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 
> [1] https://patchwork.freedesktop.org/series/63771/
> [2] depends on 356da6d0cde3323236977fce54c1f9612a742036
> 
>  drivers/gpu/drm/vgem/vgem_drv.c | 130 ++++++++++++++++++++------------
>  1 file changed, 83 insertions(+), 47 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vgem/vgem_drv.c b/drivers/gpu/drm/vgem/vgem_drv.c
> index 76d95b5e289c..6c9b5e20b3d4 100644
> --- a/drivers/gpu/drm/vgem/vgem_drv.c
> +++ b/drivers/gpu/drm/vgem/vgem_drv.c
> @@ -47,10 +47,16 @@ static struct vgem_device {
>  	struct platform_device *platform;
>  } *vgem_device;
>  
> +static void sync_and_unpin(struct drm_vgem_gem_object *bo);
> +static struct page **pin_and_sync(struct drm_vgem_gem_object *bo);
> +
>  static void vgem_gem_free_object(struct drm_gem_object *obj)
>  {
>  	struct drm_vgem_gem_object *vgem_obj = to_vgem_bo(obj);
>  
> +	if (!obj->import_attach)
> +		sync_and_unpin(vgem_obj);
> +
>  	kvfree(vgem_obj->pages);
>  	mutex_destroy(&vgem_obj->pages_lock);
>  
> @@ -78,40 +84,15 @@ static vm_fault_t vgem_gem_fault(struct vm_fault *vmf)
>  		return VM_FAULT_SIGBUS;
>  
>  	mutex_lock(&obj->pages_lock);
> +	if (!obj->pages)
> +		pin_and_sync(obj);
>  	if (obj->pages) {
>  		get_page(obj->pages[page_offset]);
>  		vmf->page = obj->pages[page_offset];
>  		ret = 0;
>  	}
>  	mutex_unlock(&obj->pages_lock);
> -	if (ret) {
> -		struct page *page;
> -
> -		page = shmem_read_mapping_page(
> -					file_inode(obj->base.filp)->i_mapping,
> -					page_offset);
> -		if (!IS_ERR(page)) {
> -			vmf->page = page;
> -			ret = 0;
> -		} else switch (PTR_ERR(page)) {
> -			case -ENOSPC:
> -			case -ENOMEM:
> -				ret = VM_FAULT_OOM;
> -				break;
> -			case -EBUSY:
> -				ret = VM_FAULT_RETRY;
> -				break;
> -			case -EFAULT:
> -			case -EINVAL:
> -				ret = VM_FAULT_SIGBUS;
> -				break;
> -			default:
> -				WARN_ON(PTR_ERR(page));
> -				ret = VM_FAULT_SIGBUS;
> -				break;
> -		}
>  
> -	}
>  	return ret;
>  }
>  
> @@ -277,32 +258,93 @@ static const struct file_operations vgem_driver_fops = {
>  	.release	= drm_release,
>  };
>  
> -static struct page **vgem_pin_pages(struct drm_vgem_gem_object *bo)
> +/* Called under pages_lock, except in free path (where it can't race): */
> +static void sync_and_unpin(struct drm_vgem_gem_object *bo)
>  {
> -	mutex_lock(&bo->pages_lock);
> -	if (bo->pages_pin_count++ == 0) {
> -		struct page **pages;
> +	struct drm_device *dev = bo->base.dev;
> +
> +	if (bo->table) {
> +		dma_sync_sg_for_cpu(dev->dev, bo->table->sgl,
> +				bo->table->nents, DMA_BIDIRECTIONAL);
> +		sg_free_table(bo->table);
> +		kfree(bo->table);
> +		bo->table = NULL;
> +	}
> +
> +	if (bo->pages) {
> +		drm_gem_put_pages(&bo->base, bo->pages, true, true);
> +		bo->pages = NULL;
> +	}
> +}
> +
> +static struct page **pin_and_sync(struct drm_vgem_gem_object *bo)
> +{
> +	struct drm_device *dev = bo->base.dev;
> +	int npages = bo->base.size >> PAGE_SHIFT;
> +	struct page **pages;
> +	struct sg_table *sgt;
> +
> +	WARN_ON(!mutex_is_locked(&bo->pages_lock));
> +
> +	pages = drm_gem_get_pages(&bo->base);
> +	if (IS_ERR(pages)) {
> +		bo->pages_pin_count--;
> +		mutex_unlock(&bo->pages_lock);
> +		return pages;
> +	}
>  
> -		pages = drm_gem_get_pages(&bo->base);
> -		if (IS_ERR(pages)) {
> -			bo->pages_pin_count--;
> -			mutex_unlock(&bo->pages_lock);
> -			return pages;
> -		}
> +	sgt = drm_prime_pages_to_sg(pages, npages);
> +	if (IS_ERR(sgt)) {
> +		dev_err(dev->dev,
> +			"failed to allocate sgt: %ld\n",
> +			PTR_ERR(bo->table));
> +		drm_gem_put_pages(&bo->base, pages, false, false);
> +		mutex_unlock(&bo->pages_lock);
> +		return ERR_CAST(bo->table);
> +	}
> +
> +	/*
> +	 * Flush the object from the CPU cache so that importers
> +	 * can rely on coherent indirect access via the exported
> +	 * dma-address.
> +	 */
> +	dma_sync_sg_for_device(dev->dev, sgt->sgl,
> +			sgt->nents, DMA_BIDIRECTIONAL);
> +
> +	bo->pages = pages;
> +	bo->table = sgt;
> +
> +	return pages;
> +}
> +
> +static struct page **vgem_pin_pages(struct drm_vgem_gem_object *bo)
> +{
> +	struct page **pages;
>  
> -		bo->pages = pages;
> +	mutex_lock(&bo->pages_lock);
> +	if (bo->pages_pin_count++ == 0 && !bo->pages) {
> +		pages = pin_and_sync(bo);
> +	} else {
> +		WARN_ON(!bo->pages);
> +		pages = bo->pages;
>  	}
>  	mutex_unlock(&bo->pages_lock);
>  
> -	return bo->pages;
> +	return pages;
>  }
>  
>  static void vgem_unpin_pages(struct drm_vgem_gem_object *bo)
>  {
> +	/*
> +	 * We shouldn't hit this for imported bo's.. in the import
> +	 * case we don't own the scatter-table
> +	 */
> +	WARN_ON(bo->base.import_attach);
> +
>  	mutex_lock(&bo->pages_lock);
>  	if (--bo->pages_pin_count == 0) {
> -		drm_gem_put_pages(&bo->base, bo->pages, true, true);
> -		bo->pages = NULL;
> +		WARN_ON(!bo->table);
> +		sync_and_unpin(bo);
>  	}
>  	mutex_unlock(&bo->pages_lock);
>  }
> @@ -310,18 +352,12 @@ static void vgem_unpin_pages(struct drm_vgem_gem_object *bo)
>  static int vgem_prime_pin(struct drm_gem_object *obj)
>  {
>  	struct drm_vgem_gem_object *bo = to_vgem_bo(obj);
> -	long n_pages = obj->size >> PAGE_SHIFT;
>  	struct page **pages;
>  
>  	pages = vgem_pin_pages(bo);
>  	if (IS_ERR(pages))
>  		return PTR_ERR(pages);
>  
> -	/* Flush the object from the CPU cache so that importers can rely
> -	 * on coherent indirect access via the exported dma-address.
> -	 */
> -	drm_clflush_pages(pages, n_pages);
> -
>  	return 0;
>  }
>  
> -- 
> 2.21.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch