[PATCH 19/19] Documentation: ACPI for ARM64

Arnd Bergmann arnd at arndb.de
Tue Jul 29 03:07:05 PDT 2014

On Tuesday 29 July 2014 10:17:22 Graeme Gregory wrote:
> On 28/07/2014 17:14, Andre Przywara wrote:
> >
> > On 28/07/14 16:23, Arnd Bergmann wrote:
> >> On Monday 28 July 2014 15:20:06 Andre Przywara wrote:
> >>> On 28/07/14 11:46, Arnd Bergmann wrote:
> >>>> On Monday 28 July 2014 10:23:57 Graeme Gregory wrote:
> >>>>> The PL011 UART is the use-case I keep hitting, that IP block has a
> >>>>> variable input clock on pretty much everything I have seen in the wild.
> >>>> Ok, I see. What does ACPI-5.1 say about pl011?
> >>>>
> >>>> Interestingly, the subset of pl011 that is specified by SBSA does not
> >>>> contain the IBRD/FBRD registers, effectively making it a fixed-rated
> >>>> UART (I guess that would be a ART, without the U then), and you
> >>>> consequently don't even need to know the clock rate.
> >>> The idea of this was probably to let the baudrate set by some firmware
> >>> code to the "right" value and the spec just didn't want to expose the
> >>> details for the generic UART:
> >>> "This specification does not cover registers needed to configure the
> >>> UART as these are considered hardware-specific and will be set up by
> >>> hardware-specific software."
> >>> To me that reads like the SBSA UART is just for debugging, and you are
> >>> expected just to access the data register.
> >> Right, makes sense. It also avoids the case where Linux for some reason
> >> ends up using a different line rate than the firmware, which can
> >> cause a lot of unnecessary pain.
> I must be suffering snow blindness reading specs, I totally missed that 
> the pl011 subset does not allow baud setting. This means that my current 
> test hardware "Juno" does not actually need any clocks defined in DSDT 
> at this stage (given that this new driver is created).
> I may then return to my original opinion of not defining clocks in the 
> DSDT at all.

Ok, that certainly simplifies it a lot, thanks!


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