linux-kernel - Re: [PATCH 2/3] media/videobuf1|2: Mark follow

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Date:   Tue, 16 Mar 2021 16:52:44 +0100
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>, Tomasz Figa <tfiga@...omium.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Kees Cook <keescook@...omium.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        "J??r??me Glisse" <jglisse@...hat.com>, Jan Kara <jack@...e.cz>,
        Pawel Osciak <pawel@...iak.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Michel Lespinasse <walken@...gle.com>
Subject: Re: [PATCH 2/3] media/videobuf1|2: Mark follow_pfn usage as unsafe

On Tue, Mar 16, 2021 at 4:46 PM Christoph Hellwig <hch@...radead.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