linux-kernel - Re: [RFC PATCH 1/1] drivers: introduce ARM SBSA generic UART driver

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Date:	Tue, 02 Sep 2014 15:48:06 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Rob Herring <robherring2@...il.com>,
	Andre Przywara <andre.przywara@....com>,
	Russell King <linux@....linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: Re: [RFC PATCH 1/1] drivers: introduce ARM SBSA generic UART driver

On Tuesday 02 September 2014 08:20:53 Rob Herring wrote:
> >>
> >> This alone is not okay. There is no such implementation of hardware.
> >
> > But the SBSA explicitly allows this. I don't know of any vendor who just
> > implements the subset, but I've been told that this has been asked for.
> 
> To use baudrate as an example, that must be configurable somehow
> either with pl011 registers or in a vendor specific way. I suppose you
> could do an actual implementation with all those things hardcoded in
> the design, but that seems unlikely.

Why does the baudrate need to be configurable? I think it's completely
reasonable to specify a console port that has a fixed (as in the
OS must not care) rate, and that can be implemented either as a UART
with a programmable rate or as a set of registers that directly talks
to a remote system management device over whatever hardware protocol
they choose.

> >> The DT must specify the implementation such as pl011.
> >
> > If it is a full featured PL011: sure. Then we don't need this driver at
> > all and just use the SBSA UART spec as a guideline for our earlycon
> > implementation.
> > I will try to learn if there is someone actually implementing only the
> > subset.
> 
> I would have assumed you knew someone is. Otherwise, I don't really
> think anything should be implemented at this point. Perhaps adding
> SBSA uart as an explicit earlycon option would be worthwhile. Also, we
> should consider using ttySx instead of ttyAMAx for SBSA compliant
> systems (including ones with pl011).

What about systems that have both a SBSA debug port and a 8250
compatible UART? That sounds like a very realistic hardware design
choice for someone coming from an older x86/powerpc/mips/... chip
that has 8250 and adds the extra port for SBSA compliance.

	Arnd
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