[U-Boot] [PATCH v2 1/4] ns16550: Add function to drain serial output

Simon Glass sjg at chromium.org
Sun Mar 11 01:52:02 CET 2012

Hi Wolfgang,

On Sat, Mar 10, 2012 at 12:24 PM, Wolfgang Denk <wd at denx.de> wrote:
> Dear Simon Glass,
> In message <CAPnjgZ3wprPDNUbe0fGbsequXJgQ_bwzkDEu6FeuR-8GG1ndQQ at mail.gmail.com> you wrote:
>> > Also I do not understand why it would be needed at all. =A0We did not
>> > have such a requirement for any system before, so I feel you must be
>> > doing something wrong, or at least very exotic.
>> I put a fully explanation in the previous patch. Let me just add here
> Explanations in some other patch are not available when someone reads
> the commit message ...

I did put a motivation in this patch at least:

Sometimes we want to be sure that the output FIFO has fully drained (e.g.
because we are about to reset or the port or reset the board). Add a
function for this.

>> that I see a 100ms delay in the reset code to allow serial output to
>> flush, so there is at least in practice a need for this sort of thing.
> Note that such a delay is hardware independent.  You implement a
> special solution that works only for a minority of boards.

True, and we could just have the board panic function implement a
delay, particularly if we move to the panic idea and away from putc().

>> For most archs the extra code will be removed by the linker.
> Ok, granted.
>> You may also recall a discussion about a platform where the SPI flash
>> and console uart were muxed, and we had to flush the console uart
>> before switching to SPI. I feel that draining the FIFO could/should be
>> a useful function.
> You quote an example of highly specific hardware, which I would not
> hesitate to call a broken design.  I do not really fancy adding code
> to common files for such exceptional situations.

It is certainly broken, yes.


> Best regards,
> Wolfgang Denk
> --
> DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
> HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
> Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
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