From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id AECBD941 for ; Mon, 1 Aug 2016 14:41:41 +0000 (UTC) Received: from www381.your-server.de (www381.your-server.de [78.46.137.84]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id C8A891EF for ; Mon, 1 Aug 2016 14:41:40 +0000 (UTC) To: Mauro Carvalho Chehab References: <98eb563b-5d62-74df-692a-f2aa4f7b07b8@xs4all.nl> <20160729111303.GA10376@sirena.org.uk> <2525670.QGOuaEkzC4@avalon> <93f7ce34-c2e9-583f-2e6f-1f23ae76a761@xs4all.nl> <20160801105531.2687069a@recife.lan> From: Lars-Peter Clausen Message-ID: Date: Mon, 1 Aug 2016 16:41:34 +0200 MIME-Version: 1.0 In-Reply-To: <20160801105531.2687069a@recife.lan> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: ksummit-discuss@lists.linuxfoundation.org, "vegard.nossum@gmail.com" , "rafael.j.wysocki" , Valentin Rothberg , Marek Szyprowski Subject: Re: [Ksummit-discuss] [TECH TOPIC] Addressing complex dependencies and semantics (v2) List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 08/01/2016 03:55 PM, Mauro Carvalho Chehab wrote: > Em Mon, 1 Aug 2016 15:33:22 +0200 > Lars-Peter Clausen escreveu: > >> On 08/01/2016 03:21 PM, Hans Verkuil wrote: >>> >>> >>> On 08/01/2016 03:09 PM, Laurent Pinchart wrote: >>>> On Friday 29 Jul 2016 12:13:03 Mark Brown wrote: >>>>> On Fri, Jul 29, 2016 at 09:45:55AM +0200, Hans Verkuil wrote: >>>>>> My main problem is not so much with deferred probe (esp. for cyclic >>>>>> dependencies it is a simple method of solving this, and simple is good). >>>>>> My main problem is that you can't tell the system that driver A needs to >>>>>> be probed after drivers B, C and D are probed first. >>>>>> >>>>>> That would allow us to get rid of v4l2-async.c which is a horrible hack. >>>>>> >>>>>> That code allows a bridge driver to wait until all dependent drivers are >>>>>> probed. This really should be core functionality. >>>>>> >>>>>> Do other subsystems do something similar like >>>>>> drivers/media/v4l2-core/v4l2-async.c? Does anyone know? >>>>> >>>>> ASoC does, it has an explicit card driver to join things together and >>>>> that just defers probe until everything it needs is present. This was >>>>> originally open coded in ASoC but once deferred probe was implemented we >>>>> converted to that. >>>> >>>> Asynchronous bindings of components, as done in ASoC, DRM and V4L2, is a >>>> problem largely solved (or rather hacked around), but I'm curious to know how >>>> ASoC handles device unbinding (due to device removal or manual unbinding >>>> through sysfs). With asynchronous binding we can more or less easily wait for >>>> all components to be present before creating circular dependencies, but >>>> breaking them to implement unbinding is an unsolved problem at least in V4L2. >>>> >>> >>> We need to prevent subdevice drivers from being unbound. It's easy enough to >>> do that (set suppress_bind_attrs to true), we just never did that. It's been >>> on my TODO list for ages to make a patch adding that flag... >>> >>> You can only unbind bridge drivers. Unbinding subdevs is pointless in general >>> and should be prohibited. Perhaps in the future with dynamically reconfigurable >>> video pipelines (FPGA) you want that, but then you need to do a lot of >>> additional work. For everything we have today we should just set >>> suppress_bind_attrs to true. >> >> suppress_bind_attrs is the lazy solution and as you pointed out does not >> work too well for all cases. > > Agreed. > > What we really need is a kind of "usage count" behavior to suppress > unbinds, e. g. a device driver can be unbound only if any other driver > using resources on it gets unbind first. > > That will solve most of unbind issues at the media subsystem. While that is a good idea for certain use cases, it still does not handle the case of hot-unplug where we can't delay unbind since the physical device is already gone.