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 4103E87A for ; Mon, 1 Aug 2016 16:19:10 +0000 (UTC) Received: from mailout3.w1.samsung.com (mailout3.w1.samsung.com [210.118.77.13]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id DC6F7B0 for ; Mon, 1 Aug 2016 16:19:08 +0000 (UTC) Received: from eucpsbgm2.samsung.com (unknown [203.254.199.245]) by mailout3.w1.samsung.com (Oracle Communications Messaging Server 7.0.5.31.0 64bit (built May 5 2014)) with ESMTP id <0OB80065CNZUV230@mailout3.w1.samsung.com> for ksummit-discuss@lists.linuxfoundation.org; Mon, 01 Aug 2016 17:19:06 +0100 (BST) To: Lars-Peter Clausen , 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> <579F6049.9030408@samsung.com> <63f6e3b4-3a48-182f-e8d5-87e720b60d5d@metafoo.de> <579F6C00.502@samsung.com> From: Andrzej Hajda Message-id: <579F766F.1000605@samsung.com> Date: Mon, 01 Aug 2016 18:18:55 +0200 MIME-version: 1.0 In-reply-to: Content-type: text/plain; charset=windows-1252 Content-transfer-encoding: 7bit Cc: "vegard.nossum@gmail.com" , "rafael.j.wysocki" , Marek Szyprowski , ksummit-discuss@lists.linuxfoundation.org, Valentin Rothberg 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 05:43 PM, Lars-Peter Clausen wrote: > On 08/01/2016 05:34 PM, Andrzej Hajda wrote: >> On 08/01/2016 04:54 PM, Lars-Peter Clausen wrote: >>> On 08/01/2016 04:44 PM, Andrzej Hajda wrote: >>>> 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. >>>> When I was investigating issues with unbind sysfs attribute I have found >>>> claim by Greg KH that unbind should be rather unavoidable, like in case >>>> of hw removal - kernel is not able to prevent users from removing usb >>>> device, even if it is in use. >>>> >>>> Assuming the claim is still valid, the only solution I see are callbacks >>>> notifying resource consumers about removal of the resources. >>> There are multiple options. >>> >>> One option, which I think is currently the most used option in the kernel, >>> is to unregister the resource when the provider is removed, but keep the >>> resource object alive as long as there are users. Any further operation on >>> such object will fail with an error. This works to the point where things >>> don't crash, but it wont function in any meaningful way. There is no way to >>> automatically recover if the resource reappears. >> For me it is not a real solution, it is just dirty workaround to just avoid >> invalid pointers. It 'works' only because unbinding is rarely used. >> For example, how the device is supposed to work if its regulator or clock >> disappeared? >> >>> Other options are as you pointed out notifier callbacks that allows the >>> resource use to be aware that a resource has disappeared and it might adjust >>> and continue to function with limited functionality. >>> >>> Another option is to teach the device core about critical resource >>> dependencies so that a consumer is automatically unbound by the core if any >>> of its resource dependencies are unregistered. The device can also >>> automatically be re-bound once the critical resources re-appear. >> That would be OK only for critical resources. >> >>> The most likely solution is probably a mixture of all of them. >> If we implement callbacks, we do not need other two 'options'. > Having to manually register callbacks for every resource in every driver > will result in a massive amount of boilerplate code. I'd rather avoid that. You can use helper to monitor multiple resources in one callback - it should not increase the code significantly. As I wrote in other e-mail I send already RFC in which the code in the driver was even shorter than before. See [1]. [1]: http://www.spinics.net/lists/dri-devel/msg73500.html > In addition to that doing resource tracking at the framework level will help > with probe ordering. > > Could you elaborate more? Regards Andrzej