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Subject: Re: DMA-buf and uncached system memory
From:   Thomas Zimmermann <tzimmermann@suse.de>
To:     =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        linux-media <linux-media@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        linaro-mm-sig@lists.linaro.org, lkml <linux-kernel@vger.kernel.org>
Cc:     "Sharma, Shashank" <Shashank.Sharma@amd.com>
References: <91ff0bbb-ea3a-2663-3453-dea96ccd6dd8@amd.com>
 <e6897f92-4c61-cd42-2822-43c50a744d4c@suse.de>
Message-ID: <302e06ad-f979-dc77-5d84-fa0923aa4632@suse.de>
Date:   Mon, 15 Feb 2021 13:00:46 +0100
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From: Thomas Zimmermann <tzimmermann@suse.de>
To: =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
 linux-media <linux-media@vger.kernel.org>,
 dri-devel <dri-devel@lists.freedesktop.org>, linaro-mm-sig@lists.linaro.org,
 lkml <linux-kernel@vger.kernel.org>
Cc: "Sharma, Shashank" <Shashank.Sharma@amd.com>
Message-ID: <302e06ad-f979-dc77-5d84-fa0923aa4632@suse.de>
Subject: Re: DMA-buf and uncached system memory
References: <91ff0bbb-ea3a-2663-3453-dea96ccd6dd8@amd.com>
 <e6897f92-4c61-cd42-2822-43c50a744d4c@suse.de>
In-Reply-To: <e6897f92-4c61-cd42-2822-43c50a744d4c@suse.de>

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Hi

Am 15.02.21 um 10:49 schrieb Thomas Zimmermann:
> Hi
>=20
> Am 15.02.21 um 09:58 schrieb Christian K=C3=B6nig:
>> Hi guys,
>>
>> we are currently working an Freesync and direct scan out from system=20
>> memory on AMD APUs in A+A laptops.
>>
>> On problem we stumbled over is that our display hardware needs to scan=
=20
>> out from uncached system memory and we currently don't have a way to=20
>> communicate that through DMA-buf.

Re-reading this paragrah, it sounds more as if you want to let the=20
exporter know where to move the buffer. Is this another case of the=20
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=20
>> generic for this?
>=20
> For vmap operations, we return the address as struct dma_buf_map, which=
=20
> contains additional information about the memory buffer. In vram=20
> helpers, we have the interface drm_gem_vram_offset() that returns the=20
> offset of the GPU device memory.
>=20
> Would it be feasible to combine both concepts into a dma-buf interface =

> that returns the device-memory offset plus the additional caching flag?=

>=20
> There'd be a structure and a getter function returning the structure.
>=20
> struct dma_buf_offset {
>  =C2=A0=C2=A0=C2=A0=C2=A0bool cached;
>  =C2=A0=C2=A0=C2=A0=C2=A0u64 address;
> };
>=20
> // return offset in *off
> int dma_buf_offset(struct dma_buf *buf, struct dma_buf_off *off);
>=20
> Whatever settings are returned by dma_buf_offset() are valid while the =

> dma_buf is pinned.
>=20
> Best regards
> Thomas
>=20
>>
>> After all the system memory access pattern is a PCIe extension and as =

>> such something generic.
>>
>> Regards,
>> Christian.
>> _______________________________________________
>> dri-devel mailing list
>> dri-devel@lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>=20
>=20
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>=20

--=20
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 N=C3=BCrnberg, Germany
(HRB 36809, AG N=C3=BCrnberg)
Gesch=C3=A4ftsf=C3=BChrer: Felix Imend=C3=B6rffer


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