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

[Graeme Russ <graeme.russ@gmail.com>]

Hi Simon,

On Wed, Mar 21, 2012 at 11:02 AM, Simon Glass <sjg@chromium.org> wrote:
> Hi Graeme,
>
> On Tue, Mar 20, 2012 at 4:52 PM, Graeme Russ <graeme.russ@gmail.com> wrote:
>> Hi Simon,
>>
>> On Wed, Mar 21, 2012 at 10:33 AM, Simon Glass <sjg@chromium.org> wrote:

>>> We cannot select the UART via CONFIG - remember that all of these
>>> boards have the same U-Boot binary. Please read that again :-) The
>>> device tree is the only thing that distinguishes them. All of the
>>> CONFIG options are identical for all boards.
>>
>> But I don't get it - In your Seaboard patch, you only use UARTD so in this
>> case we could CONFIG_ it?
>
> Yes, Stephen specifically asked for this so I changed it. See the
> other ongoing discussion on this.
>
>>
>> And it's sounding like for other scenarios you are going to resign
>> yourself to there not being a common UART so you will send the pre-console
>> (panic) message to multiple UARTs - something that should be avoided at
>> all costs...
>
> Of course - we cannot require the board to use a particular UART. The
> SOCs have various options and different people will make different
> decisions. Honestly, if we can't deal with UART selection in the
> device tree, we aren't going to solve the more difficult problems.

But aren't we dealing in a case where the device tree is probably not
available anyway?

And we are talking about one board vendor taking a SoC and using UARTA
for the panic output and another board vendor deciding to use UARTB - But
surely these vendors will create a separate config file for their boards.

>> I know we are dealing with a corner case abnormal situation here, but
>> something does not smell right... Maybe I'm not understanding something
>> obvious yet...
>
> I'm not sure. I suspect it could be easily explained with an hour at
> the whiteboard, but it's hard by email.
>
> The requirement is to output a message that the user can see, and we
> have a selection of UARTs which *might* be the console UART. For now
> we don't know exactly which one it is (see my SPL config comment
> though which might eventually solve this). So we send output to
> several of them. To protect against any danger, we permit the board
> file to select which are permitted.

Again, we are going back to per-board configurations - Why can't this
selection be CONFIG_'d? Surely we could define a bitmap of available UARTs
in a SoC header (and reserve space for board specific UARTs) which can
then be used in the board config.

I'm not really seeing an example of where two boards use exactly the same
configuration file and yet one has 'UARTx' plumbed and the other does not

Sorry, I'm not trying to be difficult, just poking all the corners to see
what squishes ;)

Regards,

Graeme