linux-kernel - Re: [PATCH v7 7/9] drm/virtio: Improve DMA API usage for shmem BOs

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Date:   Tue, 5 Jul 2022 17:03:16 +0100
From:   Emil Velikov <emil.velikov@...labora.com>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        kernel@...labora.com, David Airlie <airlied@...ux.ie>,
        Thomas Hellström <thomas_os@...pmail.org>,
        Emil Velikov <emil.l.velikov@...il.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Dmitry Osipenko <digetx@...il.com>,
        virtualization@...ts.linux-foundation.org,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH v7 7/9] drm/virtio: Improve DMA API usage for shmem BOs

On 2022/07/05, Gerd Hoffmann wrote:
>   Hi,
> 
> > > Also note that pci is not the only virtio transport we have.
> > 
> > The VirtIO indeed has other transports, but only PCI is really supported
> > in case of the VirtIO-GPU in kernel and in Qemu/crosvm, AFAICT. Hence
> > only the PCI transport was tested.
> 
> qemu -M microvm \
>   -global virtio-mmio.force-legacy=false \
>   -device virtio-gpu-device
> 
> Gives you a functional virtio-gpu device on virtio-mmio.
> 
> aarch64 virt machines support both pci and mmio too.
> s390x has virtio-gpu-ccw ...
> 

As the last person who was there - the problem is indeed when using
virtio on top of mmio. If that's no longer supported by the kernel then
the hacky code-path can be dropped.

Even in that case, I would suggest keeping it a separate commit.

HTH
Emil