[U-Boot] ns16550: tegra: Specify debugging serial port at boot.

[Stephen Warren <swarren@wwwdotorg.org>]

On 03/08/2012 11:39 AM, Wolfgang Denk wrote:
> Dear Stephen Warren,
> 
> In message <4F58F5B8.6070402@wwwdotorg.org> you wrote:
>>
>> I noticed this patch isn't applied yet that I can find. Are there any
>> comments on it; can it be applied? Thanks.
>>
>> For reference, it's in patchwork at:
>> http://patchwork.ozlabs.org/patch/134712/
>>
>>> ---
>>> drivers/serial/ns16550.c |    7 +++++++
>>>  1 files changed, 7 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/drivers/serial/ns16550.c b/drivers/serial/ns16550.c
>>> index 0c23955..19a28cd 100644
>>> --- a/drivers/serial/ns16550.c
>>> +++ b/drivers/serial/ns16550.c
>>> @@ -62,6 +62,13 @@ void NS16550_init(NS16550_t com_port, int baud_divisor)
>>>  	serial_out(0, &com_port->mdr1);
>>>  #endif
>>>  #endif /* CONFIG_OMAP */
>>> +#if defined(CONFIG_TEGRA2)
>>> +	/*
>>> +	 * Put a 'D' in the scratchpad to let the kernel know which UART
>>> +	 * for earlyprintk [D]ebugging.
>>> +	 */
>>> +	serial_out('D', &com_port->spr);
>>> +#endif
>>>  }
> 
> I don't like to see such highly architecture specific stuff in common
> code, especially if it's such a dirty hack like this.

Are there any hooks where we can do the same thing in SoC-specific code?

> I don't really understand the arguments for the need of this patch
> either.  There are standard ways for passing hardware consifuration to
> the kernel, and the comment shows that you are aware of these.
> 
> Inventing yet another hackish method seems not a good idea to me.

The point of this information is to enable the kernel's earlyprintk
support, which runs well before the device tree, or other mechanisms,
are available.

As soon as the regular console, as set by the kernel command-line etc.,
is initialized by the regular "higher level" mechanisms, it takes over
from this earlyprintk code.