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

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Message-ID: <alpine.DEB.2.10.1901211357140.11004@lmark-linux.qualcomm.com>
Date:   Mon, 21 Jan 2019 14:12:30 -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 Mon, Jan 21, 2019 at 11:44:10AM -0800, Liam Mark wrote:
> > 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.
> 
> This can't work.  The cpu can still easily speculate into this area.

Can you provide more detail on your concern here.
The use case I am thinking about here is a cached buffer which is accessed 
by a non IO-coherent device (quite a common use case for ION).

Guessing on your concern:
The speculative access can be an issue if you are going to access the 
buffer from the CPU after the device has written to it, however if you 
know you aren't going to do any CPU access before the buffer is again 
returned to the device then I don't think the speculative access is a 
concern.

> Moreover in general these operations should be cheap if the addresses
> aren't cached.
> 

I am thinking of use cases with cached buffers here, so CMO isn't cheap.

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