linux-kernel - Re: [RFC 1/2] core: Add generic object registry implementation

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Date:	Thu, 6 Nov 2014 10:48:22 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Andrzej Hajda <a.hajda@...sung.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [RFC 1/2] core: Add generic object registry implementation

On Wed, Nov 05, 2014 at 05:00:47PM +0100, Andrzej Hajda wrote:
> On 11/05/2014 03:04 PM, Thierry Reding wrote:
> > On Wed, Nov 05, 2014 at 01:36:24PM +0100, Andrzej Hajda wrote:
> >> On 11/04/2014 05:29 PM, Thierry Reding wrote:
> >>> From: Thierry Reding <treding@...dia.com>
> >>>
> >>> Add a generic implementation of an object registry. This targets drivers
> >>> and subsystems that provide auxiliary objects that other drivers need to
> >>> look up. The goal is to put the difficult parts (keep object references,
> >>> module usage count, ...) into core code so that individual subsystems do
> >>> not have to deal with them.
> >>>
> >>> The intention is for subsystems to instantiate a struct registry and use
> >>> a struct registry_record embedded into a subsystem-specific structure to
> >>> provide a subsystem-specific API around that.
> >>
> >>
> >> As I understand you want to use this registry for panels and bridges.
> >> Could you explain the idea and describe example scenario when these
> >> refcountings are useful. I guess it should be when panel attached to
> >> drmdrv want to disappear.
> > 
> > Correct. When a panel driver is unloaded it frees memory associated with
> > the panel. The goal of this registry is for the panel object to stay
> > around until all references are gone.
> > 
> >> Real lifetime of panel is limited by probe/remove callbacks of panel
> >> driver, do you want to prolong it behind these limits?
> > 
> > Yes.
> > 
> >> Do you want to have zombie panels, without hardware they abstract? For
> >> what purpose?
> > 
> > So that display drivers don't try to access objects that have been
> > freed.
> 
> Why do not just release panel references from drm_dev, I have
> successfully implemented dsi panels this way, thanks to dsi bus specific
> attach/detach callbacks and drm hotplug mechansim.

Like you say yourself, that's something that work only for DSI. Any
other type of panel can't do this.

> My point is we do not need to make the whole tricky double refcounting,

There's no double refcounting. We have no refcounting at all at the
moment.

> with total redesign of panels, revoke, zombies, etc.... It is enough to

It's not a total redesign. It just makes it more mature and implements
features that I think are useful (and needed) but that were left out for
the sake of simplicity. Now it turns out that this is actually quite
fragile and easy to get wrong.

> have just hot plug/unplug callbacks. This is why I have proposed few
> months ago interface_tracker framework. It can add hot(un)plug
> capability in a generic way to any framework.

That's something that this object registry could easily implement as
well. But instead of passing around void * and type IDs as in the
interface tracker it could deal with real objects for proper type-
safety.

Thierry

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