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
>
>
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