Re: [PATCH 3/4] dma-buf: add support for mapping with dma mapping attributes

[Laura Abbott]

On 1/18/19 1:32 PM, Liam Mark wrote:
> On Fri, 18 Jan 2019, Laura Abbott wrote:
>
> > On 1/18/19 10:37 AM, Liam Mark wrote:
> > > Add support for configuring dma mapping attributes when mapping
> > > and unmapping memory through dma_buf_map_attachment and
> > > dma_buf_unmap_attachment.
> > >
> > > Signed-off-by: Liam Mark <lmark@xxxxxxxxxxxxxx>
> > > ---
> > >    include/linux/dma-buf.h | 3 +++
> > >    1 file changed, 3 insertions(+)
> > >
> > > diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> > > index 58725f890b5b..59bf33e09e2d 100644
> > > --- a/include/linux/dma-buf.h
> > > +++ b/include/linux/dma-buf.h
> > > @@ -308,6 +308,8 @@ struct dma_buf {
> > >     * @dev: device attached to the buffer.
> > >     * @node: list of dma_buf_attachment.
> > >     * @priv: exporter specific attachment data.
> > > + * @dma_map_attrs: DMA mapping attributes to be used in
> > > + *		   dma_buf_map_attachment() and dma_buf_unmap_attachment().
> > >     *
> > >     * This structure holds the attachment information between the dma_buf
> > > buffer
> > >     * and its user device(s). The list contains one attachment struct per
> > > device
> > > @@ -323,6 +325,7 @@ struct dma_buf_attachment {
> > >    	struct device *dev;
> > >    	struct list_head node;
> > >    	void *priv;
> > > +	unsigned long dma_map_attrs;
> > >    };
> > >      /**
> >
> > Did you miss part of this patch? This only adds it to the structure but
> > doesn't
> > add it to any API. The same commment applies to the follow up patch,
> > I don't quite see how it's being used.
>
> Were you asking for a cleaner DMA-buf API to set this field or were you
> asking for a change to an upstream client to make use of this field?
>
> I have clients set the dma_map_attrs field directly on their
> dma_buf_attachment struct before calling dma_buf_map_attachment (if they
> need this functionality).
> Of course this is all being used in Android for out of tree drivers, but
> I assume it is just as useful to everyone else who has cached ION buffers
> which aren't always accessed by the CPU.
>
> My understanding is that AOSP Android on Hikey 960 also is currently
> suffering from too many CMOs due to dma_map_attachemnt always applying
> CMOs, so this support should help them avoid it.

Ahhhh I see how you intend this to be used now! I was missing
that clients would do attachment->dma_map_attrs = blah
and that was how it would be stored as opposed to passing
it in at the top level for dma_buf_map. I'll give this some
more thought but I think it could work if Sumit is okay
with the approach.

Thanks,
Laura