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

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Date:   Fri, 18 Jan 2019 14:45:02 -0800
From:   Laura Abbott <labbott@...hat.com>
To:     Liam Mark <lmark@...eaurora.org>
Cc:     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 1/18/19 1:32 PM, Liam Mark wrote:
> On Fri, 18 Jan 2019, Laura Abbott wrote:
> 
>> On 1/18/19 10:37 AM, Liam Mark wrote:
>>> Add support for configuring dma mapping attributes when mapping
>>> and unmapping memory through dma_buf_map_attachment and
>>> dma_buf_unmap_attachment.
>>>
>>> Signed-off-by: Liam Mark <lmark@...eaurora.org>
>>> ---
>>>    include/linux/dma-buf.h | 3 +++
>>>    1 file changed, 3 insertions(+)
>>>
>>> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
>>> index 58725f890b5b..59bf33e09e2d 100644
>>> --- a/include/linux/dma-buf.h
>>> +++ b/include/linux/dma-buf.h
>>> @@ -308,6 +308,8 @@ struct dma_buf {
>>>     * @dev: device attached to the buffer.
>>>     * @node: list of dma_buf_attachment.
>>>     * @priv: exporter specific attachment data.
>>> + * @dma_map_attrs: DMA mapping attributes to be used in
>>> + *		   dma_buf_map_attachment() and dma_buf_unmap_attachment().
>>>     *
>>>     * This structure holds the attachment information between the dma_buf
>>> buffer
>>>     * and its user device(s). The list contains one attachment struct per
>>> device
>>> @@ -323,6 +325,7 @@ struct dma_buf_attachment {
>>>    	struct device *dev;
>>>    	struct list_head node;
>>>    	void *priv;
>>> +	unsigned long dma_map_attrs;
>>>    };
>>>      /**
>>>
>>
>> Did you miss part of this patch? This only adds it to the structure but
>> doesn't
>> add it to any API. The same commment applies to the follow up patch,
>> I don't quite see how it's being used.
>>
> 
> Were you asking for a cleaner DMA-buf API to set this field or were you
> asking for a change to an upstream client to make use of this field?
> 
> I have clients set the dma_map_attrs field directly on their
> dma_buf_attachment struct before calling dma_buf_map_attachment (if they
> need this functionality).
> Of course this is all being used in Android for out of tree drivers, but
> I assume it is just as useful to everyone else who has cached ION buffers
> which aren't always accessed by the CPU.
> 
> My understanding is that AOSP Android on Hikey 960 also is currently
> suffering from too many CMOs due to dma_map_attachemnt always applying
> CMOs, so this support should help them avoid it.
> 

Ahhhh I see how you intend this to be used now! I was missing
that clients would do attachment->dma_map_attrs = blah
and that was how it would be stored as opposed to passing
it in at the top level for dma_buf_map. I'll give this some
more thought but I think it could work if Sumit is okay
with the approach.

Thanks,
Laura