linux-kernel - Re: [PATCH v2 1/2] drm/file: Rehabilitate the firstopen hook for non-legacy drivers

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Message-ID: <87r2appmz2.fsf@anholt.net>
Date:   Fri, 29 Mar 2019 11:14:09 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     Daniel Stone <daniel@...ishbar.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Eben Upton <eben@...pberrypi.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v2 1/2] drm/file: Rehabilitate the firstopen hook for non-legacy drivers

Paul Kocialkowski <paul.kocialkowski@...tlin.com> writes:

> Hi,
>
> Le vendredi 29 mars 2019 à 16:25 +0100, Daniel Vetter a écrit :
>> On Fri, Mar 29, 2019 at 04:02:23PM +0100, Paul Kocialkowski wrote:
>> > Hi,
>> > 
>> > On Fri, 2019-03-29 at 09:09 +0000, Daniel Stone wrote:
>> > > Hi,
>> > > 
>> > > On Thu, 28 Mar 2019 at 18:53, Daniel Vetter <daniel@...ll.ch> wrote:
>> > > > On Thu, Mar 21, 2019 at 04:27:06PM +0100, Paul Kocialkowski wrote:
>> > > > > I don't see other options either, and using firstclose/lastopen feels
>> > > > > overall more readable in the driver code.
>> > > > > 
>> > > > > I'm not sure there is such a big overhead associated with allocating
>> > > > > the binner BO (it seems that the current implementation tries to alloc
>> > > > > until the specific memory constraints for the buffer are met, so
>> > > > > perhaps that can take time). But if there is, I suppose it's best to
>> > > > > have that when starting up rather than delaying the first render
>> > > > > operation.
>> > > > 
>> > > > I'm not entirely buying the "we don't need this for fbcon only" argument -
>> > > > there's plenty of dumb kms clients too (boot splash and whatever else
>> > > > there might be). If you don't want to keep this around I think allocating
>> > > > on first non-dumb bo allocation and dropping it when the last such fd
>> > > > closes sounds like a much better idea. Needs a bit more state, you need to
>> > > > track per drm_file whether you've already allocated a non-dumb bo, and a
>> > > > drm_device refcount, but that's not much. Firstopen feels like the wrong
>> > > > thing.
>> > > > 
>> > > > Another option would be first_renderopen or something like that, except
>> > > > you can also render on the legacy node and I'm not sure how much there's a
>> > > > demand for this in other drivers. In the end you have open/close
>> > > > callbacks, you can do all the driver specific things you want to do in
>> > > > there.
>> > > 
>> > > I'd like to avoid doing it in open where possible, since that hurts
>> > > device enumeration from userspace.
>> > 
>> > I've noticed the same issue with firstopen, where our buffer is
>> > allocated/liberated a couple of times during enumeration, before the
>> > final open that stays alive during use.
>> > 
>> > I'm not sure what is preferable between that and allocating when the
>> > GPU is first used. Slowing down the first GPU operation with the
>> > allocation does not sound too great either and it feels like the buffer
>> > should have been allocated earlier.
>> > 
>> > To me, it feels I think it's better to have delays due to allocation at
>> > enumeration / startup rather than later on, but I might be missing some
>> > elements to have a clear idea.
>> > 
>> > What do you think?
>> 
>> We'll have the delay somewhere on driver load. Better to have it only once
>> (when the driver starts using gem for real), than a bunch of time, at
>> least once while enumerating and then once more while actually
>> initializing. I think if you allocat this on first non-dumb gem_create,
>> and on first command submission (just so evil userspace can't screw up the
>> hw too badly), that should be perfectly fine.
>
> I'm not totally convinced that it's okay to have a delay outside of
> init/enumeration, even if it's a smaller delay.

You'll have non-dumb buffers created during GL context creation, so
early in xserver or other KMS-and-GL-using application init anyway.
Seems like a perfectly fine plan to me.

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