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

[Julien Grall <julien.grall@xxxxxxx>]

On 21/06/16 13:30, Dirk Behme wrote:
> > > 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);
>
> ?
Yes. However I don't have any spec in hand so I am not sure if it is 
correct.
Regards,

--
Julien Grall

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