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

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

Hi Reinhard,

On 08/02/2011 05:54 PM, Reinhard Meyer wrote:
> Dear Hong Xu,
>  > > > 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)
>
> There should be no output made by u-boot BEFORE serial_init() is called!
>
>  >
>  > 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."
>
> What if there is no Baud Clock?
> What if the USART is unconfigured or misconfigured?
>
>  >
>  > Theoretically this situation shall be met here. But of course a timeout
>  > can be added defensively (chip goes mad? :)
>  >
>  > What's your opinion?
>
> Timeout is complicated at the pre-relocation level.
> What is MOST IMPORTANT at this point is that u-boot must made be able to
> print messages. Anything that could perhaps impair that should not be
> done.
> Keep the code simple and live with occasional weird characters.
> End users of a system will usually not look at the serial debug output.
> Or just use a __udelay(1000) - that should give time to drain any char
> at 9600 bit/s and above.

Agree. Actually I tried __udelay to overcome the weird character issue 
firstly. And later found the TXEMPTY was not set when serial_init is 
called. I'll send another patch soon (by using __udelay, simple but 
useful enough :-).

Thanks

BR,
Eric

> Best Regards,
> Reinhard
>
>