[RFC PATCH 1/1] drivers: introduce ARM SBSA generic UART driver

Arnd Bergmann
2014-09-02T13:48:06Z

On Tuesday 02 September 2014 08:20:53 Rob Herring wrote:
> >>
> >> This alone is not okay. There is no such implementation of hardware.
> >
> > But the SBSA explicitly allows this. I don't know of any vendor who just
> > implements the subset, but I've been told that this has been asked for.
>
> To use baudrate as an example, that must be configurable somehow
> either with pl011 registers or in a vendor specific way. I suppose you
> could do an actual implementation with all those things hardcoded in
> the design, but that seems unlikely.

Why does the baudrate need to be configurable? I think it's completely
reasonable to specify a console port that has a fixed (as in the
OS must not care) rate, and that can be implemented either as a UART
with a programmable rate or as a set of registers that directly talks
to a remote system management device over whatever hardware protocol
they choose.

> >> The DT must specify the implementation such as pl011.
> >
> > If it is a full featured PL011: sure. Then we don't need this driver at
> > all and just use the SBSA UART spec as a guideline for our earlycon
> > implementation.
> > I will try to learn if there is someone actually implementing only the
> > subset.
>
> I would have assumed you knew someone is. Otherwise, I don't really
> think anything should be implemented at this point. Perhaps adding
> SBSA uart as an explicit earlycon option would be worthwhile. Also, we
> should consider using ttySx instead of ttyAMAx for SBSA compliant
> systems (including ones with pl011).

What about systems that have both a SBSA debug port and a 8250
compatible UART? That sounds like a very realistic hardware design
choice for someone coming from an older x86/powerpc/mips/... chip
that has 8250 and adds the extra port for SBSA compliance.

Arnd