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

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Message-ID: <20140701230259.23f2f452@alan.etchedpixels.co.uk>
Date:	Tue, 1 Jul 2014 23:02:59 +0100
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Alex Elder <elder@...aro.org>
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 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).

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 ?

Alan
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