linux-kernel - Re: [PATCH v2 3/3] drm/panfrost: Add initial panfrost driver

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Date:   Mon, 15 Apr 2019 11:18:37 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Steven Price <steven.price@....com>
Cc:     Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        David Airlie <airlied@...ux.ie>,
        iommu@...ts.linux-foundation.org,
        "Marty E . Plummer" <hanetzer@...rtmail.com>,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 3/3] drm/panfrost: Add initial panfrost driver

On Fri, Apr 05, 2019 at 05:42:33PM +0100, Steven Price wrote:
> On 05/04/2019 17:16, Alyssa Rosenzweig wrote:
> > acronym once ever and have it as a "??"), I'm not sure how to respond to
> > that... We don't know how to allocate memory for the GPU-internal data
> > structures (the tiler heap, for instance, but also a few others I've
> > just named "misc_0" and "scratchpad" -- guessing one of those is for
> > "TLS"). With kbase, I took the worst-case strategy of allocating
> > gigantic chunks on startup with tiny commit counts and GROW_ON_GPF set.
> > With the new driver, well, our memory consumption is scary since
> > implementing GROW_ON_GPF in an upstream-friendly way is a bit more work
> > and isn't expected to hit the 5.2 window.
> 
> Yes GROW_ON_GPF is pretty much required for the tiler heap - it's not
> (reasonably) possible to determine how big it should be. The Arm user
> space driver does the same approach (tiny commit count, but allow it to
> grow).

Jumping in here with a drive through comment ...

Growing gem bo and dma-buf is going to be endless amounts of fun, since we
hard-coded that their size is invariant.

I think the only reasonable way to implement this is if you allocate a
really huge bo, map it, but only put the pages in on faulting. Or when
really evil userspace tries to export it. Actually changing the underlying
buffer size is not going to work I think.

Note: I didn't read kbase, so might be totally wrong in how GROW_ON_GPF
works.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch