[PATCH v4 1/3] serial: sh-sci: Add OF support

[lethal@linux-sh.org (Paul Mundt)]

On Tue, Mar 05, 2013 at 07:26:25PM +0000, Arnd Bergmann wrote:
> On Tuesday 05 March 2013, Bastian Hecht wrote:
> > >> +- renesas,scbrr-algo-id : Algorithm ID for the Bit Rate Register
> > >> +  1 = SCBRR_ALGO_1 ((clk + 16 * bps) / (16 * bps) - 1)
> > >> +  2 = SCBRR_ALGO_2 ((clk + 16 * bps) / (32 * bps) - 1)
> > >> +  3 = SCBRR_ALGO_3 (((clk * 2) + 16 * bps) / (16 * bps) - 1)
> > >> +  4 = SCBRR_ALGO_4 (((clk * 2) + 16 * bps) / (32 * bps) - 1)
> > >> +  5 = SCBRR_ALGO_5 (((clk * 1000 / 32) / bps) - 1)
> > >
> > > Maybe replace this with a "clock-frequency" property? This may
> > > be what the registers contain, but it is not very readable.
> > 
> > Hmm... do you want a frequency in absolute terms? And then calculate
> > the best SCBRR value for it?
> > I'm unsure about this, either you must exactly hit it or accept
> > tolerances? As something in between I renamed the property to
> > "renesas,clock-algorithm", but I don't know if this is what you want.
> 
> I meant the absolute frequency in HZ, since that is what we do
> for the 8250 uart and other devices, but only if that is sufficient
> for your device.
> 
No, we still need to figure out how to generate that baud rate, whether
it needs an internal or external clock for driving the rate, etc. The
frequency in and of itself doesn't provide this information, and various
parts use different algorithms for factoring the baud rate generator,
even varying across otherwise identical ports. It's unfortunately not
possible to infer anything about the SCBRR algorithm from port type or
specified baud rate.

The algorithm IDs here are wholly arbitrary anyways, but are the
variations I came up with from roughly 60-70 different CPUs.