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

[Julien Grall <julien.grall@xxxxxxx>]

On 30/05/2016 13:08, Julien Grall wrote:
> 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.
Actually, looking a bit more at the code. The code to handle a fixed 
baud rate (baud != BAUD_AUTO) is likely buggy because the clock 
frequency (clk_hz) is hardcoded in Xen. Based on the commit log, the 
value is versatile express specific.
Anyway, the driver is always setting the baud rate to auto. So I will 
send a patch to drop completely the selection of the baud rate.
Note, that this may not be sufficient to support the SBSA UART.

Cheers,

--
Julien Grall

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