Re: [Xen-devel] [PATCH 3/3] xen/arm: drivers: scif: Add clock auto detection

[Dirk Behme <dirk.behme@xxxxxxxxxxxx>]

Hi Julien,

On 21.06.2016 14:20, Julien Grall wrote:
> Hello Dirk,
>
> On 21/06/16 10:15, Dirk Behme wrote:
> > Besides the 14MHz external clock, the SCIF might be clocked by an
> > internal 66MHz clock. Detect this clock based on the SCIF_DL register
> > being 0 (internal clock) or != 0 (external clock).
> Do you have a public link to the specification?

I have to check this ;)


> > Signed-off-by: Dirk Behme <dirk.behme@xxxxxxxxxxxx>
> > ---
> >   xen/drivers/char/scif-uart.c | 15 +++++++++++++--
> >   1 file changed, 13 insertions(+), 2 deletions(-)
> >
> > diff --git a/xen/drivers/char/scif-uart.c b/xen/drivers/char/scif-uart.c
> > index bc157fe..678f46b 100644
> > --- a/xen/drivers/char/scif-uart.c
> > +++ b/xen/drivers/char/scif-uart.c
> > @@ -107,8 +107,19 @@ static void __init scif_uart_init_preirq(struct
> > serial_port *port)
> >       scif_readw(uart, SCIF_SCLSR);
> >       scif_writew(uart, SCIF_SCLSR, 0);
> >
> > -    /* Select Baud rate generator output as a clock source */
> > -    scif_writew(uart, SCIF_SCSCR, SCSCR_CKE10);
> > +    /*
> > +     * Select Baud rate generator output as a clock source
> > +     * The clock source can be an internal or external clock.
> > +     * Depending on this the DL register is either 0 or contains
> > +     * the divisor. I.e. we can use this to detect the clock
> > +     * source and based on this can configure the CKE[1:0] bits
> > +     * of the SCSCR register.
> > +     */
> > +    if ( scif_readw(uart, SCIF_DL) )
> > +        scif_writew(uart, SCIF_SCSCR, SCSCR_CKE10); /* External clk */
> > +    else
> > +        scif_writew(uart, SCIF_SCSCR, SCSCR_CKE00); /* Internal clk */
> Why would we need to select the baud rate generator if the baud has been
> configured by the firmware?

Just to get the correct understanding: The proposal is to just remove 
the code which (wrongly) overwrites the correct settings done by the 
firmware?
I.e. instead of doing the same thing the firmware is already doing, 
again (the if .. else ), the proposal is simply dropping the

 -    /* Select Baud rate generator output as a clock source */
 -    scif_writew(uart, SCIF_SCSCR, SCSCR_CKE10);

?


Best regards

Dirk



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