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

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Message-ID: <2856634.LN9gbfSP9v@wuerfel>
Date:	Tue, 02 Sep 2014 21:34:43 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Rob Herring <robherring2@...il.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	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 12:38:23 Rob Herring wrote:
> On Tue, Sep 2, 2014 at 8:48 AM, Arnd Bergmann <arnd@...db.de> wrote:
> > 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.
> 
> Sure. It is also completely reasonable that baudrate is configurable
> and vendors can implement it however they choose since the SBSA does
> not specify it. IIRC, the enabling and disabling bits are not
> specified either.
> 
> Not having configurability is simply one variation on possible
> implementations.

It's not obvious to me though that we are served better by a
pl011 driver that allows any possible subset of the features,
rather than having the existing driver for pl011, and a new driver
for the sbsa subset, which then won't allow any of the optional
features.

Yes, there is some duplication, but a driver for this kind of
dumb console port should be doable in very little code, at
least less than the proposed implementation.

	Arnd
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