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 14:18:12 -0800 (PST)
From:   Liam Mark <lmark@...eaurora.org>
To:     "Andrew F. Davis" <afd@...com>
cc:     Christoph Hellwig <hch@...radead.org>,
        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,
        john.stultz@...aro.org
Subject: Re: [PATCH 3/4] dma-buf: add support for mapping with dma mapping
 attributes

On Mon, 21 Jan 2019, Andrew F. Davis wrote:

> On 1/21/19 2:20 PM, Liam Mark wrote:
> > 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.
> > 
> 
> I think we have different definitions of "client", I'm talking about the
> DMA-BUF client (the importer), that is who can set this flag. It seems
> you mean the userspace application, which has no control over this flag.
> 

I am also talking about dma-buf clients, I am referring to both the 
userspace and kernel component of the client. For example our Camera ION 
client has both a usersapce and kernel component and they have ION 
buffers, which they control the access to, which may or may not be 
accessed by the CPU in certain uses cases.

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