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

[Julien Grall <julien.grall@xxxxxxx>]

On 30/05/2016 13:00, Stefano Stabellini wrote:
> On Fri, 27 May 2016, Julien Grall wrote:
> > Hello Shanker,
> >
> > On 27/05/16 15:01, Shanker Donthineni wrote:
> > > On 05/27/2016 08:04 AM, Julien Grall wrote:
> > > > On 27/05/16 01:28, Shanker Donthineni wrote:
> > > > > The ARM Server Base System Architecture describes a generic UART
> > > > > interface. It doesn't support clock control registers to set
> > > > > baudrate. So, extend the driver probe() to handle SBSA interface
> > > > > types and set the baudrate to 115200 for SBSA interfaces.
> > > > I cannot find any mention of the baudrate in the SBSA. Where does it come
> > > > from?
> > > Yes, no where mentioned about the baudrate in SBSA document. I used 115200
> > > based on the  the Linux  PL011 driver.
> > Looking at the Linux code, it is a default when there is no valid
> > configuration (which may not be suitable for any platform?).
> >
> > Whilst Linux userspace cares about the baudrate, Xen only use it to configure
> > the hardware. Given that the UART should have been configured by the
> > hardware-specific software, the proper value should be BAUD_AUTO.
> >
> > If you look at the code, pl011_init_preirq will read the baud rate when
> > uart->baud == BAUD_AUTO but will never be used after.
> The problem is that IBRD and FBRD are not part of the SBSA doc, so to
> make the driver compliant, we would have to skip reading the baudrate
> too, which shouldn't be a problem.
Well, we are reading the baud rate, but it is never used within the 
driver except. So I would drop the code rather than skip it.
Cheers,

--
Julien Grall

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