Re: [PATCH 2/2] drm/doc: Update docs about device instance setup

From: Thierry Reding <thierry.reding@gmail.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>,
	Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH 2/2] drm/doc: Update docs about device instance setup
Date: Mon, 10 Aug 2015 14:34:18 +0200	[thread overview]
Message-ID: <20150810123417.GA1262@ulmo.nvidia.com> (raw)
In-Reply-To: <20150810120749.GX17734@phenom.ffwll.local>

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On Mon, Aug 10, 2015 at 02:07:49PM +0200, Daniel Vetter wrote:
> On Mon, Aug 10, 2015 at 01:59:07PM +0200, Thierry Reding wrote:
> > On Mon, Aug 10, 2015 at 11:55:38AM +0200, Daniel Vetter wrote:
> > > ->load is depracated, bus functionst are deprecated and everyone
> > > should use drm_dev_alloc&register.
> > 
> > Why would you want to deprecated ->load()? Even if you use
> > drm_dev_alloc() and drm_dev_register(), there's still use for ->load()
> > because it gives you the subsystem-level initialization entry point.
> 
> ->load is called after the drm /dev node is registered and hence you can't
> really do any driver setup in there without risking races. We paper over
> that using drm_global_mutex, but that doesn't work for any other
> driver/userspace interface like sysfs/debugfs because of deadlocks.
> 
> And we can't just reorder ->load to happen before the /dev nodes are
> registered because a lot of drivers would fall over if we do that.
> 
> This is typical midlayer fail where the core calls into the driver instead
> of the other way round.

Okay, but then if we're going to deprecate ->load(), I think we should
also come up with an upgrade plan. As it is, this just says that users
shouldn't do ->load(), but it doesn't tell them what to do instead.

Would the proper procedure be to fix drivers so that ->load() can be
called between drm_dev_alloc() and drm_dev_register()? I suppose we
could add some sort of (temporary) flag for drivers to signal this and
then have drm_dev_register() call ->load() at the right time. If drivers
don't support it, we can keep what we have.

That, of course, doesn't get rid of the midlayer, so perhaps a better
way forward would be to tell driver writers that they should be doing
subsystem-level setup between drm_dev_alloc() and drm_dev_register().

Thierry

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