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

[Craig McQueen]

> 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.

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.

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.

On what BeagleBone list do you suggest this should be brought up?

-- 
Craig McQueen