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

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Date:   Mon, 21 Jan 2019 11:44:10 -0800 (PST)
From:   Liam Mark <lmark@...eaurora.org>
To:     Christoph Hellwig <hch@...radead.org>
cc:     Laura Abbott <labbott@...hat.com>, sumit.semwal@...aro.org,
        arve@...roid.com, tkjos@...roid.com, maco@...roid.com,
        joel@...lfernandes.org, christian@...uner.io,
        devel@...verdev.osuosl.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
        afd@...com, john.stultz@...aro.org
Subject: Re: [PATCH 3/4] dma-buf: add support for mapping with dma mapping
 attributes

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.


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