[U-Boot] [PATCH V2] AT91: Small fix on AT91 USART initialization code

[Hong Xu <hong.xu@atmel.com>]

Hi Reinhard,

On 08/01/2011 10:39 PM, Reinhard Meyer wrote:
> Dear Hong Xu,
>  > Before reset dbgu transmitter, we just wait TXEMPTY to drain the
>  > transmitter register. If not doing this, we may sometimes see several
>  > weird characters from DBGU.
>  >
>  > A short delay is also added to make sure the new serial settings are
>  > settled.
>  >
>  > Signed-off-by: Hong Xu<hong.xu@atmel.com>
>  > ---
>  > Change since V1:
>  > - Fix comment for easy reading
>  >
>  > drivers/serial/atmel_usart.c | 8 +++++++-
>  > 1 files changed, 7 insertions(+), 1 deletions(-)
>  >
>  > diff --git a/drivers/serial/atmel_usart.c b/drivers/serial/atmel_usart.c
>  > index e326b2b..6f9c2de 100644
>  > --- a/drivers/serial/atmel_usart.c
>  > +++ b/drivers/serial/atmel_usart.c
>  > @@ -49,17 +49,23 @@ int serial_init(void)
>  > {
>  > atmel_usart3_t *usart = (atmel_usart3_t *)CONFIG_USART_BASE;
>  >
>  > + /* Just in case: drain transmitter register */
>  > + while (!(readl(&usart->csr)& USART3_BIT(TXEMPTY)))
>  > + ;
>  > +
>
> You still have not addressed my concern about a possible hang situation
> here!
> I rather have _some_ weird characters than an apparently dead board...
> When do we have the possibility of weird characters anyway?
> Only if a preloader makes output and transfers to u-boot before its output
> has been flushed. Any other situations?

This is the main cause. I monitored some versions of bootstraps check 
TXRDY before sending data to THR (Transmitter holding register). And for 
the last character, they won't wait till the character has been sent 
out. (And maybe there are similar places in U-Boot before calling 
serial_init, I'm not sure)

The possibility of hang, I don't think so. Reason,
TXEMPTY means "There are no characters in DBGU_THR and there are no 
characters being processed by the transmitter."

Theoretically this situation shall be met here. But of course a timeout 
can be added defensively (chip goes mad?  :)

What's your opinion?

BR,
Eric

>
> Best Regards,
> Reinhard
>