[U-Boot] [PATCH 11/20] dm: serial: Update binding for PL01x serial UART

[Arnd Bergmann]

On Wednesday 15 July 2015 11:00:59 Linus Walleij wrote:
> On Wed, Jul 8, 2015 at 4:53 AM, Simon Glass <sjg at chromium.org> wrote:
> 
> > This binding differs from that of Linux. Update it and change existing
> > users.
> >
> > Signed-off-by: Simon Glass <sjg at chromium.org>
> 
> I'm confused by this. Isn't devicetree at vger.kernel.org the place to discuss
> device tree bindings? This is sent only to the U-Boot list, does this mean
> that there will now be *two* ontologies for DT bindings? That sounds like
> a recepie for ... something not good.
> 
> > +- clock-frequency:
> > +          Input clock frequency for UART.
> 
> This is the real trick is it not?
> 
> Is the real commit blurb for this patch something like:
> 
> "we want a simple way to look up the clock frequency for our serial
> ports during early boot, and following clocks = <&...> phandles
> is too much trouble and might imply having to implement a whole
> clock framework in U-Boot so we just see a lot of trouble, instead
> provide a default frequency here"
> 
> I suggest in that case, instead do this:
> 
> - boot-frequency:
>     input clock frequency at boot time, to be used by boot loaders
>     and other early access.
> 
> This would be good for early Linux stuff as well I believe and I
> do not think it's very controversial.

The CHRP ISA binding defines that a 8250 compatible UART must have this
property:

  "clock-frequency" S

  Standard property, encoded as with encode-int, that shall be the baud-rate
  generator's clock input frequency (in hertz).
  Typically, the clock frequency is nominally 1,843,200 Hz. Some devices
  generate the baud rate input clock by dividing a higher-frequency clock 
  source that is not an exact multiple of 1,843,200 Hz, resulting in a
  small but acceptable error in the nominal clock frequency. In such cases,
  the "clock-frequency" shall report the actual nominal frequency. For
  example, if the baud rate input clock is generated by dividing a 24
  MHz clock by 13, the value of the "clock-frequency" property shall be 1846153.
	
	Arnd