linux-kernel - RE: [PATCH] tty/serial: add support for Xilinx PS UART

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Message-ID: <bba0eb69-c082-48a9-9d8e-3034e66bc9d4@VA3EHSMHS018.ehs.local>
Date:	Tue, 19 Apr 2011 17:47:24 -0600
From:	John Linn <John.Linn@...inx.com>
To:	Greg KH <greg@...ah.com>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	<linux-kernel@...r.kernel.org>, <linux-serial@...r.kernel.org>
Subject: RE: [PATCH] tty/serial: add support for Xilinx PS UART

> -----Original Message-----
> From: Greg KH [mailto:greg@...ah.com]
> Sent: Tuesday, April 19, 2011 5:43 PM
> To: John Linn
> Cc: Alan Cox; linux-kernel@...r.kernel.org; linux-
> serial@...r.kernel.org
> Subject: Re: [PATCH] tty/serial: add support for Xilinx PS UART
> 
> On Tue, Apr 19, 2011 at 05:39:30PM -0600, John Linn wrote:
> > > -----Original Message-----
> > > From: Greg KH [mailto:greg@...ah.com]
> > > Sent: Tuesday, April 19, 2011 5:22 PM
> > > To: John Linn
> > > Cc: Alan Cox; linux-kernel@...r.kernel.org; linux-
> > > serial@...r.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@...rguk.ukuu.org.uk]
> > > > > Sent: Tuesday, April 19, 2011 3:16 PM
> > > > > To: John Linn
> > > > > Cc: linux-kernel@...r.kernel.org; linux-serial@...r.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@...inx.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.
> 

Great, I will dig into it more and give it a test.  Learn something new 
everyday in Linux.

Thanks for clarifying.
John

> thanks,
> 
> greg k-h


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