linux-kernel - Re: [PATCH 1/2] serial: samsung: Move uart_register

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Message-ID: <20140120231603.GL15937@n2100.arm.linux.org.uk>
Date:	Mon, 20 Jan 2014 23:16:03 +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 1/2] serial: samsung: Move uart_register_driver call to
	device probe

On Mon, Jan 20, 2014 at 03:11:41PM -0800, Greg KH wrote:
> On Mon, Jan 20, 2014 at 09:32:06PM +0000, Russell King - ARM Linux wrote:
> > On Mon, Jan 20, 2014 at 01:16:01PM -0800, Greg KH wrote:
> > > On Mon, Jan 20, 2014 at 10:05:30AM +0000, Russell King - ARM Linux wrote:
> > > > On Mon, Jan 20, 2014 at 02:32:34PM +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.
> > > > 
> > > > The samsung-uart driver is at fault here - the major/minor numbers were
> > > > officially registered to amba-pl011.  Samsung needs to be fixed properly.
> > > 
> > > I agree, the Samsung driver is "broken" here, but that's no reason why
> > > these two drivers can't register with the tty layer _after_ the hardware
> > > is detected, and not before.
> > > 
> > > That saves resources on systems that build the drivers in, yet do not
> > > have the hardware present, which is always a good thing.
> > 
> > Great, so what you're saying is that we need to wait until the first
> > device calls into the probe function.  What about removal... how does
> > a driver know when it's last device has been removed to de-register
> > that?
> 
> The "bus" that the device is on handles that, right?
> 
> > I guess it needs the driver model to provide some way to know when a
> > driver is completely unbound - but isn't that racy?
> 
> How is it racy?  That's how the driver model works...

Think about what happens when the last device unregisters, but a new
device comes along and is probed.

I don't believe the driver model has any locking to prevent a drivers
->probe function running concurrently with it's ->remove function for
two (or more) devices.

The locking against this is done on a per-device basis, not a per-driver
basis.

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