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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue, 16 Mar 2021 16:52:44 +0100
Message-ID: <CAKMK7uF8Lv0P4TuoctjUiVHtRzAnXf9a50JaYgm0rV+v+7=LFw@mail.gmail.com>
Subject: Re: [PATCH 2/3] media/videobuf1|2: Mark follow_pfn usage as unsafe
To:     Christoph Hellwig <hch@infradead.org>
Cc:     DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>, Tomasz Figa <tfiga@chromium.org>,
        Hans Verkuil <hverkuil-cisco@xs4all.nl>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Kees Cook <keescook@chromium.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        "J??r??me Glisse" <jglisse@redhat.com>, Jan Kara <jack@suse.cz>,
        Pawel Osciak <pawel@osciak.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        Michel Lespinasse <walken@google.com>
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On Tue, Mar 16, 2021 at 4:46 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Tue, Mar 16, 2021 at 04:33:02PM +0100, Daniel Vetter wrote:
> > The media model assumes that buffers are all preallocated, so that
> > when a media pipeline is running we never miss a deadline because the
> > buffers aren't allocated or available.
> >
> > This means we cannot fix the v4l follow_pfn usage through
> > mmu_notifier, without breaking how this all works. The only real fix
> > is to deprecate userptr support for VM_IO | VM_PFNMAP mappings and
> > tell everyone to cut over to dma-buf memory sharing for zerocopy.
> >
> > userptr for normal memory will keep working as-is, this only affects
> > the zerocopy userptr usage enabled in 50ac952d2263 ("[media]
> > videobuf2-dma-sg: Support io userptr operations on io memory").
>
> Maybe I'm missing something, but wasn't the conclusion last time that
> this hackish early device to device copy support can just go away?

My understanding is mostly, but with some objections. And I kinda
don't want to let this die in a bikeshed and then not getting rid of
follow_pfn as a result. There's enough people who acked this, and the
full removal got some nack from Mauro iirc.

Maybe if no bug report ever shows up for 1-2 years we can sunset it
for real&completely.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch