linux-kernel - Re: [PATCH v3] staging: android: ion: Add implementation of dma_buf_vmap and dma_buf

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Message-ID: <7d836085-cd8d-300b-56be-e8db4ff37afc@intel.com>
Date:   Wed, 7 Feb 2018 07:25:58 +0200
From:   Alexey Skidanov <alexey.skidanov@...el.com>
To:     Laura Abbott <labbott@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     devel@...verdev.osuosl.org, tkjos@...roid.com, rve@...roid.com,
        linux-kernel@...r.kernel.org, maco@...roid.com,
        sumit.semwal@...aro.org
Subject: Re: [PATCH v3] staging: android: ion: Add implementation of
 dma_buf_vmap and dma_buf_vunmap



On 02/07/2018 01:56 AM, Laura Abbott wrote:
> On 01/31/2018 10:10 PM, Alexey Skidanov wrote:
>>
>> On 01/31/2018 03:00 PM, Greg KH wrote:
>>> On Wed, Jan 31, 2018 at 02:03:42PM +0200, Alexey Skidanov wrote:
>>>> Any driver may access shared buffers, created by ion, using
>>>> dma_buf_vmap and
>>>> dma_buf_vunmap dma-buf API that maps/unmaps previosuly allocated
>>>> buffers into
>>>> the kernel virtual address space. The implementation of these API is
>>>> missing in
>>>> the current ion implementation.
>>>>
>>>> Signed-off-by: Alexey Skidanov <alexey.skidanov@...el.com>
>>>> ---
>>>
>>> No review from any other Intel developers? :(
>> Will add.
>>>
>>> Anyway, what in-tree driver needs access to these functions?
>> I'm not sure that there are the in-tree drivers using these functions
>> and ion as> buffer exporter because they are not implemented in ion :)
>> But there are some in-tre> drivers using these APIs (gpu drivers) with
>> other buffer exporters.
> 
> It's still not clear why you need to implement these APIs.
How the importing kernel module may access the content of the buffer? :)
With the current ion implementation it's only possible by dma_buf_kmap,
mapping one page at a time. For pretty large buffers, it might have some
performance impact.
(Probably, the page by page mapping is the only way to access large
buffers on 32 bit systems, where the vmalloc range is very small. By the
way, the current ion dma_map_kmap doesn't really map only 1 page at a
time - it uses the result of vmap() that might fail on 32 bit systems.)

> Are you planning to use Ion with GPU drivers? I'm especially
> interested in this if you have a non-Android use case.
Yes, my use case is the non-Android one. But not with GPU drivers.
> 
> Thanks,
> Laura

Thanks,
Alexey