linux-kernel - Re: [PATCH v2] wait while adding MMC host to ensure root mounts

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Date:	Tue, 02 Apr 2013 22:56:38 +0400
From:	Sergey Yanovich <ynvich@...il.com>
To:	Ulf Hansson <ulf.hansson@...aro.org>
Cc:	Chris Ball <cjb@...top.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	Namjae Jeon <linkinjeon@...il.com>,
	linux-kernel@...r.kernel.org, linux-mmc@...r.kernel.org
Subject: Re: [PATCH v2] wait while adding MMC host to ensure root mounts

On Tue, 2013-04-02 at 19:45 +0200, Ulf Hansson wrote:
> On 2 April 2013 12:35, Sergey Yanovich <ynvich@...il.com> wrote:
> > If the system is booted using initrd and root is not on an mmc card,
> > then mmc modules can be omitted from initrd. The probing will happen
> > only after root is mounted.
> 
> This will not solve the problem when having one device intended for
> rootfs and some other for something else. Of course, as long as the
> devices uses the same mmc module. Once inserted, all devices will be
> probed.

I agree that is this special case there will be boot time regression.
However, this case is not guaranteed to boot without the patch or some
workaround.

> > If root is on an mmc, kernel needs to wait for the mmc probe.
> >
> 
> True, although your patch is preventing the parallelism and instead
> doing things in synchronized manner.

mount_root() assumes it has waited for "known devices to complete their
probing" [init/do_mounts.c:545]. The patch has brought mmc into
compliance with the assumption.

If several devices can be probed in parallel, the bus should do it, but
not the driver.

> I think we must discuss alternative solutions instead.
> 
> Like an "mmc detect flush" mechanism or a "new card device notification" event.

There are 2 events to trigger root mount:
1. all known devices complete their probing
2. 1 is true and root_wait is specified and root device is found

So I see the only fast alternative to my patch: if root is on an mmc
card, set root_wait to 'true'.

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