linux-kernel - Re: [alsa-devel] [RFC PATCH] ASoC: Modify check condition of multiple bindings of components

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Date:	Wed, 14 Oct 2015 09:19:09 +0800
From:	Koro Chen <koro.chen@...iatek.com>
To:	Lars-Peter Clausen <lars@...afoo.de>
CC:	<alsa-devel@...a-project.org>, <srv_heupstream@...iatek.com>,
	<tiwai@...e.de>, <linux-kernel@...r.kernel.org>,
	<s.hauer@...gutronix.de>, <lgirdwood@...il.com>, <perex@...ex.cz>,
	<broonie@...nel.org>, <linux-mediatek@...ts.infradead.org>,
	<p.zabel@...gutronix.de>, <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [alsa-devel] [RFC PATCH] ASoC: Modify check condition of
 multiple bindings of components

On Tue, 2015-10-13 at 16:42 +0200, Lars-Peter Clausen wrote:
> On 10/13/2015 04:18 PM, Koro Chen wrote:
> > On Tue, 2015-10-13 at 15:44 +0200, Lars-Peter Clausen wrote:
> >> On 10/13/2015 03:37 PM, Koro Chen wrote:
> >>> The patch "ASoC: Prevent components from being bound to multiple cards"
> >>> adds check to prevent multiple cards from using the same component.
> >>> However, snd_soc_register_platform() or snd_soc_register_codec() will
> >>> also create components, and sharing the same platform by multiple cards
> >>> is then refused. This happens with a platform having multiple
> >>> independent DAIs that share the same DMA controller.
> >>>
> >>> Relax the condition by checking component->registered_as_component,
> >>> which is only true in case of snd_soc_register_component() and
> >>> will be false for components created by snd_soc_register_platform()
> >>> or snd_soc_register_codec().
> >>
> >> Binding a component to multiple cards results in internal data structure
> >> corruption, regardless of whether it is a raw component, CODEC or platform,
> >> which is why the check was added. So the proposed change wont work.
> >>
> > Thanks for your comment. Is it possible to share an example of how the
> > data structure will be corrupted? So I can study into this further.
> 
> Just look at soc_probe_component() and think about what happens if that runs
> twice for two different cards. Multiple calls to list_add() on the same
> list, controls are added multiple times, DAPM widgets are created multiple
> times, the card field will only point to the last card.
> 
When multiple binding happens, soc_probe_component() just returns zero
without doing anything after this patch (actually it also returned zero
before the patch "ASoC: Prevent components from being bound to multiple
cards"). So the component still binds to the first card. For this case I
think it should be fine?

> 
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