On Wednesday 15 July 2015 12:08:05 Linus Walleij wrote: > On Wed, Jul 15, 2015 at 11:38 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote: > > > 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. > > Isn't that for the case where the clock frequency will never change at runtime? > > The problem (I think) with many systems using PL011 is that they can > actually change the serial port clock at runtime (software controlled clock), > so providing a "clock-frequency" would imply that they cannot, and the clock > frequency is fixed to that value. > > I think it's better to use boot-clock-frequency or so to indicate that a richer > OS will provide more refined clocks. For example that frequency can > typically change if the SoC changes operating point or so, though it would > be awkward to handle in the driver, admittedly and I haven't seen a > piece of code that actually go and change the UART input frequency. > > Still for completion it is better for the UART to tie into the clk framework > as the OS gets up, and just providing clock-frequency seems to imply > that it need not do that or something. The semantical effect of providing both > clock-frequency = and clocks =< &..> must be clarified in any case. > Like the latter overrides the former if clock phandles can be handled by > the environment. (And this should be stated in the binding.) Ah right. I don't think that naming is super critical though, as a lot of properties in the DT just describe how things are at boot time. Arnd -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
