linux-kernel - Re: [PATCH v2 0/5] Introduce DMA_HEAP_ALLOC_AND_READ

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Message-ID: <33775a86-206a-4724-a662-3eb5c70b8f03@vivo.com>
Date: Tue, 30 Jul 2024 19:33:14 +0800
From: Huan Yang <link@...o.com>
To: Christian König <christian.koenig@....com>,
 Sumit Semwal <sumit.semwal@...aro.org>,
 Benjamin Gaignard <benjamin.gaignard@...labora.com>,
 Brian Starkey <Brian.Starkey@....com>, John Stultz <jstultz@...gle.com>,
 "T.J. Mercier" <tjmercier@...gle.com>, linux-media@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
 linux-kernel@...r.kernel.org, opensource.kernel@...o.com
Subject: Re: [PATCH v2 0/5] Introduce DMA_HEAP_ALLOC_AND_READ_FILE heap flag


在 2024/7/30 18:42, Christian König 写道:
> Am 30.07.24 um 11:05 schrieb Huan Yang:
>>
>> 在 2024/7/30 16:56, Daniel Vetter 写道:
>>> [????????? daniel.vetter@...ll.ch ????????? 
>>> https://aka.ms/LearnAboutSenderIdentification?????????????]
>>>
>>> On Tue, Jul 30, 2024 at 03:57:44PM +0800, Huan Yang wrote:
>>>> UDMA-BUF step:
>>>>    1. memfd_create
>>>>    2. open file(buffer/direct)
>>>>    3. udmabuf create
>>>>    4. mmap memfd
>>>>    5. read file into memfd vaddr
>>> Yeah this is really slow and the worst way to do it. You absolutely 
>>> want
>>> to start _all_ the io before you start creating the dma-buf, ideally 
>>> with
>>> everything running in parallel. But just starting the direct I/O with
>>> async and then creating the umdabuf should be a lot faster and avoid
>> That's greate,  Let me rephrase that, and please correct me if I'm 
>> wrong.
>>
>> UDMA-BUF step:
>>   1. memfd_create
>>   2. mmap memfd
>>   3. open file(buffer/direct)
>>   4. start thread to async read
>>   3. udmabuf create
>>
>> With this, can improve
>>
>>> needlessly serialization operations.
>>>
>>> The other issue is that the mmap has some overhead, but might not be 
>>> too
>>> bad.
>> Yes, the time spent on page fault in mmap should be negligible 
>> compared to the time spent on file read.
>
> You should try to avoid mmap as much as possible. Especially the TLB 
> invalidation overhead is really huge on platforms with a large number 
> of CPUs.
But, if not mmap, how to read file from fd to fd? sendfile may 
help(shmemfs may support it, I do not test it), but use pipe buffer is 
not good through test.
>
> Regards,
> Christian.
>
>>> -Sima
>>> -- 
>>> Daniel Vetter
>>> Software Engineer, Intel Corporation
>>> http://blog.ffwll.ch/ 
>>>
>