linux-kernel - Re: [PATCH 1/2] drm: allow limiting the scatter list size.

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Message-ID: <03c03129-db27-c6da-df8f-909141d2a3f7@amd.com>
Date:   Tue, 18 Aug 2020 10:31:19 +0200
From:   Christian König <christian.koenig@....com>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
        "open list:DRM DRIVER FOR NVIDIA GEFORCE/QUADRO GPUS" 
        <nouveau@...ts.freedesktop.org>, Sandy Huang <hjc@...k-chips.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>,
        "open list:RADEON and AMDGPU DRM DRIVERS" 
        <amd-gfx@...ts.freedesktop.org>,
        Jonathan Hunter <jonathanh@...dia.com>,
        "open list:ARM/Rockchip SoC support" 
        <linux-rockchip@...ts.infradead.org>,
        Ben Skeggs <bskeggs@...hat.com>,
        Russell King <linux+etnaviv@...linux.org.uk>,
        "moderated list:DRM DRIVERS FOR XEN" <xen-devel@...ts.xenproject.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        "moderated list:DRM DRIVERS FOR VIVANTE GPU IP" 
        <etnaviv@...ts.freedesktop.org>,
        "open list:DRM DRIVERS FOR NVIDIA TEGRA" 
        <linux-tegra@...r.kernel.org>, Sean Paul <sean@...rly.run>,
        "moderated list:ARM/Rockchip SoC support" 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Alex Deucher <alexander.deucher@....com>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <freedreno@...ts.freedesktop.org>
Subject: Re: [PATCH 1/2] drm: allow limiting the scatter list size.

Am 18.08.20 um 10:27 schrieb Gerd Hoffmann:
> On Tue, Aug 18, 2020 at 09:57:59AM +0200, Christian König wrote:
>> Am 18.08.20 um 09:48 schrieb Gerd Hoffmann:
>>> Add max_segment argument to drm_prime_pages_to_sg().  When set pass it
>>> through to the __sg_alloc_table_from_pages() call, otherwise use
>>> SCATTERLIST_MAX_SEGMENT.
>>>
>>> Also add max_segment field to gem objects and pass it to
>>> drm_prime_pages_to_sg() calls in drivers and helpers.
>>>
>>> Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
>> I'm missing an explanation why this should be useful (it certainly is).
> virtio-gpu needs this to work properly with SEV (see patch 2/2 of this
> series).

Yeah, that's the problem patch 2/2 never showed up here :)

>> And the maximum segment size seems misplaced in the GEM object. This is
>> usually a property of the device or even completely constant.
> Placing it in drm_device instead would indeed work for virtio-gpu, so I
> guess you are suggesting that instead?

That is probably the best approach, yes.

For Intel and AMD it could even be global/constant, but it certainly 
doesn't needs to be kept around for each buffer.

Christian.

>
> take care,
>    Gerd
>