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 13:49:37 -0600
From:   "Andrew F. Davis" <afd@...com>
To:     Liam Mark <lmark@...eaurora.org>,
        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>,
        <john.stultz@...aro.org>
Subject: Re: [PATCH 3/4] dma-buf: add support for mapping with dma mapping
 attributes

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?

> 
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> a Linux Foundation Collaborative Project
>