linux-kernel - Re: [PATCH] serial: 8250_dw: support high baudrates if possible

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Message-ID: <53B34F19.9060608@linaro.org>
Date:	Tue, 01 Jul 2014 19:15:21 -0500
From:	Alex Elder <elder@...aro.org>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
CC:	gregkh@...uxfoundation.org, heikki.krogerus@...ux.intel.com,
	david.daney@...ium.com, loic.poulain@...el.com,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] serial: 8250_dw: support high baudrates if possible

On 07/01/2014 05:02 PM, One Thousand Gnomes wrote:
> On Sat, 28 Jun 2014 15:15:56 -0500
> Alex Elder <elder@...aro.org> wrote:
> 
>> On 06/28/2014 10:36 AM, One Thousand Gnomes wrote:
>>> On Fri, 27 Jun 2014 12:25:20 -0500
>>>> +		rate = 16 * max(115200U, (unsigned int)baud);
>>>> +
>>>
>>> This assumes an arbitarily configurable clock, which is not I think the
>>> usual case. 
>>
>> If the clock's rate can't change, this will return an error,
>> and the recorded rate (p->uartclk) will not be changed.
> 
> Which assumes an arbitrarily configurable clock, whereas you want to find
> the correct clock and multiplier combination for the baud rate.
> 
> Most of these ports are wired to fixed clocks (which is fine) or clocks
> with limited numbers of supported frequencies (which is not).

Yes, I acknowledged that in my earlier response.  But over
the weekend I decided to abandon hope that I'd be able to
verify there are no problems for affected machines, and
re-formulated my patch.
    https://lkml.org/lkml/2014/7/1/323

> Your patch is an improvement but doesn't really fix the overall problem.
> If we have enough devices with variable clocks for it to be useful then
> fine - but can we merge it with a big FIXME note so that whoever comes
> along wondering why their clock doesn't work or behaves very oddly can
> figure it out and fix that case ?

What I've done now is define a dw8250_adjustable_clk() predicate
that indicates the UART baud can be adjusted by changing the
clock rate.  Anyone who wants to activate this functionality
can just modify that function to recognize their device.

							-Alex

> Alan
> 

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