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

From: Andre Przywara <andre.przywara@arm.com>
To: Rob Herring <robherring2@gmail.com>, Arnd Bergmann <arnd@arndb.de>
Cc: "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	Russell King <linux@arm.linux.org.uk>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"linux-serial@vger.kernel.org" <linux-serial@vger.kernel.org>,
	Jiri Slaby <jslaby@suse.cz>
Subject: Re: [RFC PATCH 1/1] drivers: introduce ARM SBSA generic UART driver
Date: Fri, 05 Sep 2014 15:37:18 +0100	[thread overview]
Message-ID: <5409CA9E.2060807@arm.com> (raw)
In-Reply-To: <CAL_JsqKGbSd5BymnFhT8KBjbBCPO8wsrHhG9Qn20c7mrrxTDDA@mail.gmail.com>

Hi Rob,

On 02/09/14 18:38, Rob Herring wrote:
> On Tue, Sep 2, 2014 at 8:48 AM, Arnd Bergmann <arnd@arndb.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.

But this would be a different driver for a different hardware then and
not covered by the SBSA version - so to some degree not our problem ;-)
I don't see how we could really cover this problem for every possibly
upcoming implementation.

If I got the spec correctly, then exposing the baudrate setting for SBSA
hardware is not meaningful in the sense of the spec.
If you want to go with a configurable UART complying to the spec, you'd
probably use a full featured PL011.

> IIRC, the enabling and disabling bits are not
> specified either.

Correct, also the FIFO is always on and the word format is fixed to 8n1.

Regards,
Andre.

> 
> Not having configurability is simply one variation on possible implementations.
> 
> Rob
> 