Re: [PATCH 13/14] staging: android: ion: Do not sync CPU cache on map/unmap

[Liam Mark <lmark@codeaurora.org>]

On Fri, 11 Jan 2019, Andrew F. Davis wrote:

> Buffers may not be mapped from the CPU so skip cache maintenance here.
> Accesses from the CPU to a cached heap should be bracketed with
> {begin,end}_cpu_access calls so maintenance should not be needed anyway.
> 
> Signed-off-by: Andrew F. Davis <afd@ti.com>
> ---
>  drivers/staging/android/ion/ion.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c
> index 14e48f6eb734..09cb5a8e2b09 100644
> --- a/drivers/staging/android/ion/ion.c
> +++ b/drivers/staging/android/ion/ion.c
> @@ -261,8 +261,8 @@ static struct sg_table *ion_map_dma_buf(struct dma_buf_attachment *attachment,
>  
>  	table = a->table;
>  
> -	if (!dma_map_sg(attachment->dev, table->sgl, table->nents,
> -			direction))
> +	if (!dma_map_sg_attrs(attachment->dev, table->sgl, table->nents,
> +			      direction, DMA_ATTR_SKIP_CPU_SYNC))

Unfortunately I don't think you can do this for a couple reasons.
You can't rely on {begin,end}_cpu_access calls to do cache maintenance.
If the calls to {begin,end}_cpu_access were made before the call to 
dma_buf_attach then there won't have been a device attached so the calls 
to {begin,end}_cpu_access won't have done any cache maintenance.

Also ION no longer provides DMA ready memory, so if you are not doing CPU 
access then there is no requirement (that I am aware of) for you to call 
{begin,end}_cpu_access before passing the buffer to the device and if this 
buffer is cached and your device is not IO-coherent then the cache maintenance
in ion_map_dma_buf and ion_unmap_dma_buf is required.

>  		return ERR_PTR(-ENOMEM);
>  
>  	return table;
> @@ -272,7 +272,8 @@ static void ion_unmap_dma_buf(struct dma_buf_attachment *attachment,
>  			      struct sg_table *table,
>  			      enum dma_data_direction direction)
>  {
> -	dma_unmap_sg(attachment->dev, table->sgl, table->nents, direction);
> +	dma_unmap_sg_attrs(attachment->dev, table->sgl, table->nents,
> +			   direction, DMA_ATTR_SKIP_CPU_SYNC);
>  }
>  
>  static int ion_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
> -- 
> 2.19.1
> 
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