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SubjectRe: [PATCH 13/14] staging: android: ion: Do not sync CPU cache on map/unmap
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
>
> _______________________________________________
> devel mailing list
> devel@linuxdriverproject.org
> http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
>

Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

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