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Date:   Mon, 21 Jan 2019 12:20:42 -0800 (PST)
From:   Liam Mark <lmark@codeaurora.org>
To:     "Andrew F. Davis" <afd@ti.com>
cc:     Christoph Hellwig <hch@infradead.org>,
        Laura Abbott <labbott@redhat.com>, sumit.semwal@linaro.org,
        arve@android.com, tkjos@android.com, maco@android.com,
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        devel@driverdev.osuosl.org, dri-devel@lists.freedesktop.org,
        linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org,
        john.stultz@linaro.org
Subject: Re: [PATCH 3/4] dma-buf: add support for mapping with dma mapping
 attributes
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On Mon, 21 Jan 2019, Andrew F. Davis wrote:

> On 1/21/19 1:44 PM, Liam Mark wrote:
> > On Mon, 21 Jan 2019, Christoph Hellwig wrote:
> > 
> >> On Sat, Jan 19, 2019 at 08:50:41AM -0800, Laura Abbott wrote:
> >>>> And who is going to decide which ones to pass?  And who documents
> >>>> which ones are safe?
> >>>>
> >>>> I'd much rather have explicit, well documented dma-buf flags that
> >>>> might get translated to the DMA API flags, which are not error checked,
> >>>> not very well documented and way to easy to get wrong.
> >>>>
> >>>
> >>> I'm not sure having flags in dma-buf really solves anything
> >>> given drivers can use the attributes directly with dma_map
> >>> anyway, which is what we're looking to do. The intention
> >>> is for the driver creating the dma_buf attachment to have
> >>> the knowledge of which flags to use.
> >>
> >> Well, there are very few flags that you can simply use for all calls of
> >> dma_map*.  And given how badly these flags are defined I just don't want
> >> people to add more places where they indirectly use these flags, as
> >> it will be more than enough work to clean up the current mess.
> >>
> >> What flag(s) do you want to pass this way, btw?  Maybe that is where
> >> the problem is.
> >>
> > 
> > The main use case is for allowing clients to pass in 
> > DMA_ATTR_SKIP_CPU_SYNC in order to skip the default cache maintenance 
> > which happens in dma_buf_map_attachment and dma_buf_unmap_attachment. In 
> > ION the buffers aren't usually accessed from the CPU so this allows 
> > clients to often avoid doing unnecessary cache maintenance.
> > 
> 
> How can a client know that no CPU access has occurred that needs to be
> flushed out?
> 

I have left this to clients, but if they own the buffer they can have the 
knowledge as to whether CPU access is needed in that use case (example for 
post-processing).

For example with the previous version of ION we left all decisions of 
whether cache maintenance was required up to the client, they would use 
the ION cache maintenance IOCTL to force cache maintenance only when it 
was required.
In these cases almost all of the access was being done by the device and 
in the rare cases CPU access was required clients would initiate the 
required cache maintenance before and after the CPU access.

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