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From:   Daniel Stone <daniel@fooishbar.org>
Date:   Wed, 16 Feb 2022 13:46:18 +0000
Message-ID: <CAPj87rOgk8A8s6MTqxcTO5EkS=ABpHQV3sHsAp7Yn2t3-N_SoQ@mail.gmail.com>
Subject: Re: [PATCH v6 1/3] drm/lsdc: add drm driver for loongson display controller
To:     Sui Jingfeng <15330273260@189.cn>
Cc:     Maxime Ripard <maxime@cerno.tech>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        suijingfeng <suijingfeng@loongson.cn>,
        David Airlie <airlied@linux.ie>,
        dri-devel@lists.freedesktop.org,
        Randy Dunlap <rdunlap@infradead.org>,
        Roland Scheidegger <sroland@vmware.com>,
        linux-mips@vger.kernel.org, Krzysztof Kozlowski <krzk@kernel.org>,
        linux-kernel@vger.kernel.org,
        Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>,
        Rob Herring <robh+dt@kernel.org>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Sam Ravnborg <sam@ravnborg.org>
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On Wed, 9 Feb 2022 at 15:41, Sui Jingfeng <15330273260@189.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