Re: [PATCH v4 1/1] drm: allow limiting the scatter list size.

From: Gerd Hoffmann <kraxel@redhat.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel <dri-devel@lists.freedesktop.org>,
	"Christian König" <christian.koenig@amd.com>,
	"Alex Deucher" <alexander.deucher@amd.com>,
	"David Airlie" <airlied@linux.ie>,
	"Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>,
	"Maxime Ripard" <mripard@kernel.org>,
	"Thomas Zimmermann" <tzimmermann@suse.de>,
	"Lucas Stach" <l.stach@pengutronix.de>,
	"Russell King" <linux+etnaviv@armlinux.org.uk>,
	"Christian Gmeiner" <christian.gmeiner@gmail.com>,
	"Rob Clark" <robdclark@gmail.com>, "Sean Paul" <sean@poorly.run>,
	"Ben Skeggs" <bskeggs@redhat.com>,
	"Sandy Huang" <hjc@rock-chips.com>,
	"Heiko Stübner" <heiko@sntech.de>,
	"Thierry Reding" <thierry.reding@gmail.com>,
	"Jonathan Hunter" <jonathanh@nvidia.com>,
	"Oleksandr Andrushchenko" <oleksandr_andrushchenko@epam.com>,
	"open list:RADEON and AMDGPU DRM DRIVERS"
	<amd-gfx@lists.freedesktop.org>,
	"open list" <linux-kernel@vger.kernel.org>,
	"moderated list:DRM DRIVERS FOR VIVANTE GPU IP"
	<etnaviv@lists.freedesktop.org>,
	"open list:DRM DRIVER FOR MSM ADRENO GPU"
	<linux-arm-msm@vger.kernel.org>,
	"open list:DRM DRIVER FOR MSM ADRENO GPU"
	<freedreno@lists.freedesktop.org>,
	"open list:DRM DRIVER FOR NVIDIA GEFORCE/QUADRO GPUS"
	<nouveau@lists.freedesktop.org>,
	"moderated list:ARM/Rockchip SoC support"
	<linux-arm-kernel@lists.infradead.org>,
	"open list:ARM/Rockchip SoC support"
	<linux-rockchip@lists.infradead.org>,
	"open list:DRM DRIVERS FOR NVIDIA TEGRA"
	<linux-tegra@vger.kernel.org>,
	"moderated list:DRM DRIVERS FOR XEN"
	<xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v4 1/1] drm: allow limiting the scatter list size.
Date: Tue, 8 Sep 2020 07:48:58 +0200	[thread overview]
Message-ID: <20200908054858.um34wojjv6uhi7d3@sirius.home.kraxel.org> (raw)
In-Reply-To: <CAKMK7uGjT73rh=9iuCKAXvC_CaOuygm8PgOQgofkTgH7wRysFw@mail.gmail.com>

On Mon, Sep 07, 2020 at 03:53:02PM +0200, Daniel Vetter wrote:
> On Mon, Sep 7, 2020 at 1:24 PM Gerd Hoffmann <kraxel@redhat.com> wrote:
> >
> > Add drm_device argument to drm_prime_pages_to_sg(), so we can
> > call dma_max_mapping_size() to figure the segment size limit
> > and call into __sg_alloc_table_from_pages() with the correct
> > limit.
> >
> > This fixes virtio-gpu with sev.  Possibly it'll fix other bugs
> > too given that drm seems to totaly ignore segment size limits
> > so far ...
> >
> > v2: place max_segment in drm driver not gem object.
> > v3: move max_segment next to the other gem fields.
> > v4: just use dma_max_mapping_size().
> >
> > Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> 
> Uh, are you sure this works in all cases for virtio?

Sure, I've tested it ;)

> The comments I've found suggest very much not ... Or is that all very
> old stuff only that no one cares about anymore?

I think these days it is possible to override dma_ops per device, which
in turn allows virtio to deal with the quirks without the rest of the
kernel knowing about these details.

I also think virtio-gpu can drop the virtio_has_dma_quirk() checks, just
use the dma api path unconditionally and depend on virtio core having
setup dma_ops in a way that it JustWorks[tm].  I'll look into that next.

take care,
  Gerd