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

[Paul Kocialkowski <paul.kocialkowski@bootlin.com>]

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@ffwll.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.

Eric, do you have an opinion on what is the preference with VC4?

> Only way to avoid that is to allocate at driver load and pin it, but
> that's what we're trying to avoid here.

Exactly!

Cheers,

Paul

-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com