linux-kernel - Re: [PATCH 2/2] serial: pl011: Move uart_register

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Date:	Thu, 13 Feb 2014 18:42:49 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Tushar Behera <tushar.behera@...aro.org>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	linux-samsung-soc@...r.kernel.org, jslaby@...e.cz,
	ben.dooks@...ethink.co.uk, broonie@...nel.org
Subject: Re: [PATCH 2/2] serial: pl011: Move uart_register_driver call to
	device probe

On Thu, Feb 13, 2014 at 10:27:01AM -0800, Greg KH wrote:
> On Thu, Feb 13, 2014 at 06:15:59PM +0000, Russell King - ARM Linux wrote:
> > On Thu, Feb 13, 2014 at 10:12:16AM -0800, Greg KH wrote:
> > > On Mon, Jan 20, 2014 at 10:04:15AM +0000, Russell King - ARM Linux wrote:
> > > > On Mon, Jan 20, 2014 at 02:32:35PM +0530, Tushar Behera wrote:
> > > > > uart_register_driver call binds the driver to a specific device
> > > > > node through tty_register_driver call. This should typically happen
> > > > > during device probe call.
> > > > > 
> > > > > In a multiplatform scenario, it is possible that multiple serial
> > > > > drivers are part of the kernel. Currently the driver registration fails
> > > > > if multiple serial drivers with same default major/minor numbers are
> > > > > included in the kernel.
> > > > > 
> > > > > A typical case is observed with amba-pl011 and samsung-uart drivers.
> > > > 
> > > > NAK.  There should not be any other driver using amba-pl011's device numbers.
> > > 
> > > I agree, but there is.  And because of that, moving the registration to
> > > the probe call fixes the issue with building a kernel with all of the
> > > drivers built into them, so I'm going to take both of these patches, as
> > > it does solve that problem, while still allowing the device number
> > > collision to happen.
> > 
> > So what happens when two _devices_ are probed by this driver at the same
> > time?
> 
> The bus that the driver is on will not allow that to happen, I thought
> we went through this before...
> 
> And yes, devices on different busses could cause problems, but is that
> the case here for these devices?  At first glance, I don't think that
> can happen for these drivers.

We went through this before, and I stated the paths, and no one disagreed
with that.

It /is/ racy.

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