linux-kernel - Re: DMA-buf and uncached system memory

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Date:   Mon, 15 Feb 2021 13:00:46 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Christian König <christian.koenig@....com>,
        linux-media <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linaro-mm-sig@...ts.linaro.org, lkml <linux-kernel@...r.kernel.org>
Cc:     "Sharma, Shashank" <Shashank.Sharma@....com>
Subject: Re: DMA-buf and uncached system memory

Hi

Am 15.02.21 um 10:49 schrieb Thomas Zimmermann:
> Hi
> 
> Am 15.02.21 um 09:58 schrieb Christian König:
>> Hi guys,
>>
>> we are currently working an Freesync and direct scan out from system 
>> memory on AMD APUs in A+A laptops.
>>
>> On problem we stumbled over is that our display hardware needs to scan 
>> out from uncached system memory and we currently don't have a way to 
>> communicate that through DMA-buf.

Re-reading this paragrah, it sounds more as if you want to let the 
exporter know where to move the buffer. Is this another case of the 
missing-pin-flag problem?

Best regards
Thomas

>>
>> For our specific use case at hand we are going to implement something 
>> driver specific, but the question is should we have something more 
>> generic for this?
> 
> For vmap operations, we return the address as struct dma_buf_map, which 
> contains additional information about the memory buffer. In vram 
> helpers, we have the interface drm_gem_vram_offset() that returns the 
> offset of the GPU device memory.
> 
> Would it be feasible to combine both concepts into a dma-buf interface 
> that returns the device-memory offset plus the additional caching flag?
> 
> There'd be a structure and a getter function returning the structure.
> 
> struct dma_buf_offset {
>      bool cached;
>      u64 address;
> };
> 
> // return offset in *off
> int dma_buf_offset(struct dma_buf *buf, struct dma_buf_off *off);
> 
> Whatever settings are returned by dma_buf_offset() are valid while the 
> dma_buf is pinned.
> 
> Best regards
> Thomas
> 
>>
>> After all the system memory access pattern is a PCIe extension and as 
>> such something generic.
>>
>> Regards,
>> Christian.
>> _______________________________________________
>> dri-devel mailing list
>> dri-devel@...ts.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 
> 
> _______________________________________________
> dri-devel mailing list
> dri-devel@...ts.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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