linux-kernel - Re: [PATCH v6 1/3] drm/lsdc: add drm driver for loongson display controller

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Message-ID: <adec818a-5c66-3b4a-6f27-bcb29b6c67d1@189.cn>
Date:   Wed, 16 Feb 2022 22:13:05 +0800
From:   Sui Jingfeng <15330273260@....cn>
To:     Daniel Stone <daniel@...ishbar.org>
Cc:     Maxime Ripard <maxime@...no.tech>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        suijingfeng <suijingfeng@...ngson.cn>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org,
        Randy Dunlap <rdunlap@...radead.org>,
        Roland Scheidegger <sroland@...are.com>,
        linux-mips@...r.kernel.org, Krzysztof Kozlowski <krzk@...nel.org>,
        linux-kernel@...r.kernel.org,
        Andrey Zhizhikin <andrey.zhizhikin@...ca-geosystems.com>,
        Rob Herring <robh+dt@...nel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH v6 1/3] drm/lsdc: add drm driver for loongson display
 controller


On 2022/2/16 21:46, Daniel Stone wrote:
> On Wed, 9 Feb 2022 at 15:41, Sui Jingfeng <15330273260@....cn> wrote:
>> On 2022/2/9 16:43, Maxime Ripard wrote:
>>> More fundamentally (and this extends to the CMA, caching and VRAM stuff
>>> you explained above), why can't the driver pick the right decision all
>>> the time and why would that be under the user control?
>> The right decision for ls7a1000 is to use VRAM based helper, But sometimes
>> we need CMA helper based solution. Because: The PRIME support is lost, use
>> lsdc with etnaviv is not possible any more.
>>
>>    Buffer sharing with etnaviv is no longer possible, loongson display controllers
>>    are simple which require scanout buffers to be physically contiguous.
> Other systems have this limitation, and Mesa's 'kmsro' concept makes
> this work transparently, as long as your driver can export dmabufs
> when running in 'VRAM' mode.
>
> Cheers,
> Daniel

When using vram helper based driver, the framebuffer  is locate at video 
ram. the backing memory fb is manage by TTM.

while bo of etnaviv is locate at system ram. Currently i can't figure 
out how does the buffer going to be shared.