Date: Tue, 19 Apr 2011 16:42:30 -0700
From: Greg KH <greg@kroah.com>
To: John Linn <John.Linn@xilinx.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, linux-kernel@vger.kernel.org,
        linux-serial@vger.kernel.org
Subject: Re: [PATCH] tty/serial: add support for Xilinx PS UART
Message-ID: <20110419234230.GA21209@kroah.com>
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On Tue, Apr 19, 2011 at 05:39:30PM -0600, John Linn wrote:
> > -----Original Message-----
> > From: Greg KH [mailto:greg@kroah.com]
> > Sent: Tuesday, April 19, 2011 5:22 PM
> > To: John Linn
> > Cc: Alan Cox; linux-kernel@vger.kernel.org; linux-
> > serial@vger.kernel.org
> > Subject: Re: [PATCH] tty/serial: add support for Xilinx PS UART
> > 
> > On Tue, Apr 19, 2011 at 04:22:00PM -0600, John Linn wrote:
> > > > -----Original Message-----
> > > > From: Alan Cox [mailto:alan@lxorguk.ukuu.org.uk]
> > > > Sent: Tuesday, April 19, 2011 3:16 PM
> > > > To: John Linn
> > > > Cc: linux-kernel@vger.kernel.org; linux-serial@vger.kernel.org
> > > > Subject: Re: [PATCH] tty/serial: add support for Xilinx PS UART
> > > >
> > > > On Tue, 19 Apr 2011 14:14:52 -0600
> > > > John Linn <john.linn@xilinx.com> wrote:
> > > >
> > > > > The Xilinx PS Uart is used on the new ARM based SoC. This
> > > > > UART is not compatible with others such that a seperate
> > > > > driver is required.
> > > >
> > > > Joyous. I wish people would standardise.
> > > >
> > > > > +		 213 = /dev/ttyPS0		Xilinx PS serial
> port 0
> > > > > +		 214 = /dev/ttyPS1		Xilinx PS serial
> port 1
> > > > > +		 215 = /dev/ttyPS2		Xilinx PS serial
> port 2
> > > > > +		 216 = /dev/ttyPS3		Xilinx PS serial
> port 3
> > > >
> > > > Is there a specific reason you need fixed minor numbers ? If not
> > > please
> > > > use a dynamic range and keep Linus happy.
> > >
> > > Hi Alan,
> > >
> > > I hope you don't mind me asking a bit more to better understand.
> > Here's
> > > my
> > > concerns (maybe not valid).
> > >
> > > It seems like since this is a console it can get hard to debug with
> > > dynamic
> > > nodes for this driver.
> > >
> > > This driver is for an embedded device where we don't want to require
> > > udev
> > > or mdev to assign nodes.
> > 
> > Why not use devtmpfs?  There's no need to use udev or mdev at all.
> > 
> 
> Hi Greg,
> 
> Reading up on it a bit as I'm not an expert on it (like you). 
> It's not clear to me what the device driver uses for it's major/minor in
> this case.  
> 
> Maybe it doesn't matter, the driver can use ttyS0 and then it gets
> deleted later if you want to change it.
> 
> The only reason we didn't just use ttyS0 was that it's not a 8250 really
> and since we have are an FPGA people can add real 8250s in soft logic
> and then the system would get confusing.

No, the point is that you don't care about the major/minor number, as
userspace can open the proper named device node, with no need for udev
at all.  And you can dynamically allocate your major/minor number for
your device and it all "just works".

So please follow what Alan said to do, there's no excuse that you have
to use udev to take advantage of dynamice device numbers on embedded
systems anymore.

thanks,

greg k-h
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