Re: [PATCH 1/4 v3] drm: Add support of ARC PGU display controller

[Daniel Vetter]

On Thu, Mar 17, 2016 at 08:27:10PM +0000, Alexey Brodkin wrote:
> Hi Daniel,
> 
> On Tue, 2016-03-15 at 16:59 +0100, Daniel Vetter wrote:
> > On Tue, Mar 15, 2016 at 03:24:46PM +0000, Alexey Brodkin wrote:
> > > On Tue, 2016-03-15 at 09:10 +0100, Daniel Vetter wrote:
> > > > On Mon, Mar 14, 2016 at 11:15:59AM +0000, Alexey Brodkin wrote:
> > > > > On Mon, 2016-03-14 at 08:00 +0100, Daniel Vetter wrote:
> > > > > > On Fri, Mar 11, 2016 at 06:42:36PM +0300, Alexey Brodkin wrote:
> > > > > > > 
> > > > > > > +static struct drm_driver arcpgu_drm_driver = {
> > > > > > > +	.driver_features = DRIVER_MODESET | DRIVER_GEM | DRIVER_PRIME |
> > > > > > > +			   DRIVER_ATOMIC,
> > > > > > > +	.preclose = arcpgu_preclose,
> > > > > > > +	.lastclose = arcpgu_lastclose,
> > > > > > > +	.name = "drm-arcpgu",
> > > > > > > +	.desc = "ARC PGU Controller",
> > > > > > > +	.date = "20160219",
> > > > > > > +	.major = 1,
> > > > > > > +	.minor = 0,
> > > > > > > +	.patchlevel = 0,
> > > > > > > +	.fops = &arcpgu_drm_ops,
> > > > > > > +	.load = arcpgu_load,
> > > > > > > +	.unload = arcpgu_unload,
> > > > > > Load and unload hooks are deprecated (it's a classic midlayer mistake).
> > > > > > Please use drm_dev_alloc/register pairs directly instead, and put your
> > > > > > device setup code in-between. Similar for unloading. There's a bunch of
> > > > > > example drivers converted already.
> > > > > Ok I took "atmel-hlcdc" as example.
> > > > > And that's interesting.
> > > > > 
> > > > > If I put my arcpgu_load() in between drm_dev_alloc() and
> > > > > drm_dev_register() then I'm getting this on the driver probe:
> > > > > ---------------------------------->8-------------------------------
> > > > > [drm] Initialized drm 1.1.0 20060810
> > > > > arcpgu e0017000.pgu: arc_pgu ID: 0xabbabaab
> > > > > ------------[ cut here ]------------
> > > > > WARNING: CPU: 0 PID: 1 at lib/kobject.c:244 kobject_add_internal+0x17c/0x498()
> > > > > kobject_add_internal failed for card0-HDMI-A-1 (error: -2 parent: card0)
> > > > > Modules linked in:
> > > > > CPU: 0 PID: 1 Comm: swapper Not tainted 4.5.0-rc3-01062-ga447822-dirty #17
> > > > > 
> > > > > Stack Trace:
> > > > >   arc_unwind_core.constprop.1+0xa4/0x110
> > > > >   warn_slowpath_fmt+0x6e/0xfc
> > > > >   kobject_add_internal+0x17c/0x498
> > > > >   kobject_add+0x98/0xe4
> > > > >   device_add+0xc6/0x734
> > > > >   device_create_with_groups+0x12a/0x144
> > > > >   drm_sysfs_connector_add+0x54/0xe8
> > > > >   arcpgu_drm_hdmi_init+0xd4/0x17c
> > > > >   arcpgu_probe+0x138/0x24c
> > > > >   platform_drv_probe+0x2e/0x6c
> > > > >   really_probe+0x212/0x35c
> > > > >   __driver_attach+0x90/0x94
> > > > >   bus_for_each_dev+0x46/0x80
> > > > >   bus_add_driver+0x14e/0x1b4
> > > > >   driver_register+0x64/0x108
> > > > >   do_one_initcall+0x86/0x194
> > > > >   kernel_init_freeable+0xf0/0x188
> > > > > ---[ end trace c67166ad43ddcce2 ]---
> > > > > [drm:drm_sysfs_connector_add] adding "HDMI-A-1" to sysfs
> > > > > [drm:drm_sysfs_connector_add] *ERROR* failed to register connector device: -2
> > > > > arcpgu e0017000.pgu: failed to regiter DRM connector and helper funcs
> > > > > arcpgu: probe of e0017000.pgu failed with error -2
> > > > > ---------------------------------->8-------------------------------
> > > > > 
> > > > > But if I move arcpgu_load() after drm_dev_register() then everything
> > > > > starts properly and I may see HDMI screen works perfectly fine.
> > > > > 
> > > > > Any thoughts?
> > > > Oops, yeah missed that detail. If you look at atmel it has a loop to
> > > > register all the drm connectors _after_ calling drm_dev_register().
> > > > Totally forgot about that. Can you pls
> > > > - Extract a new drm_connector_register_all() function
> > > >   (atmel_hlcdc_dc_connector_plug_all seems to be the best template),
> > > >   including kerneldoc.
> > > > - Adjust kerneldoc of drm_dev_register() to mention
> > > >   drm_connector_register_all() and that ordering constraint.
> > > > - Roll that helper out to all the drivers that currently hand-roll it (one
> > > >   patch per driver).
> > > > 
> > > > I know a bit of work but imo not too much, and by doing some small
> > > > refactoring every time someone stumbles over a drm pitfall we keep the
> > > > subsystem clean&easy to understand. You're up for this? Would be a prep
> > > > series, I'll happily review it to get it merged fast. Just a few weeks ago
> > > > I merged 20+ patches to make ->mode_fixup hooks optional and remove dummy
> > > > ones all over the subsystem, in other words: You'll have my full attention
> > > > ;-)
> > > Sure, I'm ready to pay that price :)
> > > Stay tuned and patches will follow.
> > Awesome, looking forward to your patches.
> 
> Sorry it took longer for me to finally put my hands on that work but anyways.
> 
> I'm looking now at how drivers use existing drm_connector_unplug_all() and
> their implementation of what would be drm_connector_plug_all() and see
> in some implementations people wraps both helpers with
> mutex_{lock|unlock}(&dev->mode_config.mutex). But not everybody does this.
> 
> So essentially my questions are:
>  [1] If it's necessary to get hold of that mutex before execution of either helper?

In plug_all I think so, unplug_all has a FIXME comment about how locking
against sysfs is horrible and it's all going to blow up. But we did
recently change the connector sysfs files, so maybe that's fixed now. Not
sure.

>  [2] If this is really necessary then IMHO it makes sense to move mutex_lock/unlock
>      in helpers itself, right?

Yeah, locking in the helper makes imo sense. Aside: I'd vote for
register_all/unregister_all for consistency with drm_connector_register.
register/unregister has a very clear meaning of "publish the object to
userspace/other kernel subsystems". plug/unplug is confusing in DRM
because of hotplug.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch