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

[Simon Glass]

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.

>
> It could be worth checking the UART's error flags (break, framing error, parity error, etc), and ignoring a byte with any error. But it looks as though the U-Boot code would need more extensive changes to support that.

I'm not sure. You could add a Kconfig option to flush the UART on probe().

>
> On what BeagleBone list do you suggest this should be brought up?
>
> --
> Craig McQueen
>

Regads,
Simon