[U-Boot] [PATCH] At start of autoboot check, flush any pending RX bytes

[Robert Nelson]

On Tue, Jan 19, 2016 at 10:35 PM, Simon Glass <sjg at chromium.org> wrote:
> Hi,
>
> On 18 January 2016 at 23:46, Craig McQueen <craig.mcqueen at innerrange.com> wrote:
>>> Tom Rini wrote:
>>>
>>> On Mon, Jan 11, 2016 at 09:59:18AM -0700, Simon Glass wrote:
>>> > Hi Craig,
>>> >
>>> > On 20 December 2015 at 19:07, Craig McQueen
>>> > <craig.mcqueen at innerrange.com> wrote:
>>> > > This is to avoid the boot sequence halting due to initial "junk"
>>> > > received on serial input.
>>> > >
>>> > > Signed-off-by: Craig McQueen <craig.mcqueen at innerrange.com>
>>> > > ---
>>> > > I have found that on the BeagleBone Black, U-Boot occasionally halts
>>> > > at the U-Boot prompt at boot-up, whether power-up, warm external
>>> > > reset or software reset. I have seen other people report the same issue.
>>> > >
>>> > > This seems to be due to U-Boot receiving "junk" data on the serial
>>> > > console. The BeagleBone Black has a pull-down resistor which was
>>> > > apparently added to try to mitigate this issue, but it doesn't
>>> > > entirely fix it.
>>> >
>>> > I wonder if this can be fixed for that board only?
>>>
>>> No, and it's not -exactly- a that board only problem.  It's a HW design issue
>>> that can show up elsewhere too.  I _think_ however in this case the answer
>>> would be to migrate to perhaps CONFIG_AUTOBOOT_KEYED_CTRLC as that's
>>> one of the reason various other boards use that set of options.
>>>
>>> I would suggest bringing this up on the beaglebone list to see what the
>>> various people that ship and support these boards think, thanks!
>>
>> Other options such as CONFIG_AUTOBOOT_KEYED_CTRLC sound like a work-around of the problem, rather than a fix of the root-cause. But maybe that's just my opinion. Simon Glass' suggestion (clear out the UART input buffer when the driver is probed) sounded reasonable, but I haven't tried it.
>
> Since it seems like a hardware bug, we can only have a workaround. A
> real fix would involve fixing the root cause, i.e. fixing the
> hardware.
>
>>
>> I think BeagleBone Black definitely needs _some_ sort of fix -- currently BBB can randomly fail to boot, which isn't good. If my patch isn't wanted, then please implement a suitable alternative.
>
> How about what Tom suggested? Ctrl-C is not likely to happen by accident.

This should be good enough for everyone, it'll look for an exact match
over serial "<space>" otherwise it'll ignore everything else:

http://lists.denx.de/pipermail/u-boot/2016-January/241286.html

Regards,

-- 
Robert Nelson
https://rcn-ee.com/