Re: [Xen-devel] [PATCH v2] arm/acpi: Add Server Base System Architecture UART support

[Julien Grall <julien.grall@xxxxxxx>]

Hi Andre,

On 01/06/16 16:56, Andre Przywara wrote:
> > > I would like to support both the interfaces
> > > (ACPI_DBG2_SBSA_32/ACPI_DBG2_SBSA) If you are okay.
> > I am a bit puzzled, so your UART is only supporting 32-bit access (i.e
> > no 16-bit and 8-bit access)?
> Just to clarify: the SBSA spec is not very clear in this respect, as it
> only speaks of "a set of 32-bit registers". But this has been
> interpreted as "supports 32-bit accesses". So there was a patch lately
> in Linux to change all accesses to SBSA UARTs to 32-bit accessors
> (writel/readl), because there is at least this one mentioned platform
> that requires this, while all the other relevant platforms we could get
> hold of can also cope with 32-bit accesses. This may not be true for all
> legacy PL011 implementations out there, but for the SBSA subset this is
> deemed a safe assumption.
Thank you for the explanation.

> So if possible we should switch to 32-bit accessors for the SBSA UART.
The driver use 32-bit accessors exclusively.

> > If so your platform is based on SBSA v2.3, and therefore the PL011
> > driver needs more modification to support properly your platform. For
> > instance, the register UARTMIS is not present in v2.3 but used by the
> > driver.
> I think this has been changed in the spec lately, it was present in
> earlier revisions of the spec.
If so, I would replace the read to UARTMIS by a combination UARTIMSC and 
UARTRIS to avoid any issue with the UART based on SBSA v2.3.
Cheers,

--
Julien Grall

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