linux-kernel - Re: [PATCH v3 10/25] drm: panfrost: fix common struct sg

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Message-ID: <eaaeae03-3fd4-fdd7-ec21-3218a5f74a28@arm.com>
Date:   Mon, 11 May 2020 16:51:21 +0100
From:   Steven Price <steven.price@....com>
To:     Marek Szyprowski <m.szyprowski@...sung.com>,
        dri-devel@...ts.freedesktop.org, iommu@...ts.linux-foundation.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org
Cc:     Rob Herring <robh@...nel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 10/25] drm: panfrost: fix common struct sg_table
 related issues

On 05/05/2020 09:45, Marek Szyprowski wrote:
> The Documentation/DMA-API-HOWTO.txt states that dma_map_sg returns the
> numer of the created entries in the DMA address space. However the
> subsequent calls to dma_sync_sg_for_{device,cpu} and dma_unmap_sg must be
> called with the original number of the entries passed to dma_map_sg. The
> sg_table->nents in turn holds the result of the dma_map_sg call as stated
> in include/linux/scatterlist.h. A common mistake was to ignore a result
> of the dma_map_sg function and don't use the sg_table->orig_nents at all.
> 
> To avoid such issues, lets use common dma-mapping wrappers operating
> directly on the struct sg_table objects and adjust references to the
> nents and orig_nents respectively.
> 
> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>

The change looks good to me:

Reviewed-by: Steven Price <steven.price@....com>

Although I would have appreciated the commit message being modified to 
match the specifics of Panfrost - the return of dma_mpa_sg() wasn't 
being ignored, but the use of orig_nents/nents was indeed wrong.

Steve

> ---
> For more information, see '[PATCH v3 00/25] DRM: fix struct sg_table nents
> vs. orig_nents misuse' thread: https://lkml.org/lkml/2020/5/5/187
> ---
>   drivers/gpu/drm/panfrost/panfrost_gem.c | 4 ++--
>   drivers/gpu/drm/panfrost/panfrost_mmu.c | 5 ++---
>   2 files changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.c b/drivers/gpu/drm/panfrost/panfrost_gem.c
> index 17b654e..95d7e80 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
> @@ -41,8 +41,8 @@ static void panfrost_gem_free_object(struct drm_gem_object *obj)
>   
>   		for (i = 0; i < n_sgt; i++) {
>   			if (bo->sgts[i].sgl) {
> -				dma_unmap_sg(pfdev->dev, bo->sgts[i].sgl,
> -					     bo->sgts[i].nents, DMA_BIDIRECTIONAL);
> +				dma_unmap_sgtable(pfdev->dev, &bo->sgts[i],
> +						  DMA_BIDIRECTIONAL);
>   				sg_free_table(&bo->sgts[i]);
>   			}
>   		}
> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> index ed28aeb..9926111 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -517,10 +517,9 @@ static int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as,
>   	if (ret)
>   		goto err_pages;
>   
> -	if (!dma_map_sg(pfdev->dev, sgt->sgl, sgt->nents, DMA_BIDIRECTIONAL)) {
> -		ret = -EINVAL;
> +	ret = dma_map_sgtable(pfdev->dev, sgt, DMA_BIDIRECTIONAL);
> +	if (ret)
>   		goto err_map;
> -	}
>   
>   	mmu_map_sg(pfdev, bomapping->mmu, addr,
>   		   IOMMU_WRITE | IOMMU_READ | IOMMU_NOEXEC, sgt);
>