linux-kernel - Re: [Linux-graphics-maintainer] [PATCH v3 15/25] drm: vmwgfx: fix common struct sg

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Date:   Tue, 12 May 2020 02:19:25 +0200
From:   Roland Scheidegger <sroland@...are.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:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Airlie <airlied@...ux.ie>,
        VMware Graphics <linux-graphics-maintainer@...are.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Linux-graphics-maintainer] [PATCH v3 15/25] drm: vmwgfx: fix
 common struct sg_table related issues

I'm not exactly an expert on this, but looks alright to me.
Acked-by: Roland Scheidegger <sroland@...are.com>

Am 05.05.20 um 10:46 schrieb Marek Szyprowski:
> 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>
> ---
> 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/vmwgfx/vmwgfx_ttm_buffer.c | 17 ++++-------------
>  1 file changed, 4 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
> index bf0bc46..e50ae8b 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
> @@ -362,8 +362,7 @@ static void vmw_ttm_unmap_from_dma(struct vmw_ttm_tt *vmw_tt)
>  {
>  	struct device *dev = vmw_tt->dev_priv->dev->dev;
>  
> -	dma_unmap_sg(dev, vmw_tt->sgt.sgl, vmw_tt->sgt.nents,
> -		DMA_BIDIRECTIONAL);
> +	dma_unmap_sgtable(dev, vmw_tt->sgt, DMA_BIDIRECTIONAL);
>  	vmw_tt->sgt.nents = vmw_tt->sgt.orig_nents;
>  }
>  
> @@ -383,16 +382,8 @@ static void vmw_ttm_unmap_from_dma(struct vmw_ttm_tt *vmw_tt)
>  static int vmw_ttm_map_for_dma(struct vmw_ttm_tt *vmw_tt)
>  {
>  	struct device *dev = vmw_tt->dev_priv->dev->dev;
> -	int ret;
> -
> -	ret = dma_map_sg(dev, vmw_tt->sgt.sgl, vmw_tt->sgt.orig_nents,
> -			 DMA_BIDIRECTIONAL);
> -	if (unlikely(ret == 0))
> -		return -ENOMEM;
>  
> -	vmw_tt->sgt.nents = ret;
> -
> -	return 0;
> +	return dma_map_sgtable(dev, vmw_tt->sgt, DMA_BIDIRECTIONAL);
>  }
>  
>  /**
> @@ -449,10 +440,10 @@ static int vmw_ttm_map_dma(struct vmw_ttm_tt *vmw_tt)
>  		if (unlikely(ret != 0))
>  			goto out_sg_alloc_fail;
>  
> -		if (vsgt->num_pages > vmw_tt->sgt.nents) {
> +		if (vsgt->num_pages > vmw_tt->sgt.orig_nents) {
>  			uint64_t over_alloc =
>  				sgl_size * (vsgt->num_pages -
> -					    vmw_tt->sgt.nents);
> +					    vmw_tt->sgt.orig_nents);
>  
>  			ttm_mem_global_free(glob, over_alloc);
>  			vmw_tt->sg_alloc_size -= over_alloc;
>