On Monday 29 December 2014 12:24:56 Eddie Huang wrote:

Hi Rob,

On Fri, 2014-12-26 at 15:24 -0600, Rob Herring wrote:

quoted

On Thu, Dec 18, 2014 at 2:33 AM, Eddie Huang [off-list ref] wrote:

quoted

Mediatek UART has highspeed register, but 8250_early.c doesn't
support this, so add earlycon in 8250_mtk.c

I don't see any highspeed register setup here. More generically,
aren't you just skipping any UART setup? That may be useful on other
platforms with 8250s, too. With the kernel command line version, you
could perhaps add a "noinit" flag. The DT case is harder, and I'm not
sure how we should handle that. We could perhaps add a
"stdout-path-initialized" flag to chosen.

Yes, I skipped UART setup same as msm_serial.c and amba-pl011.c
(although they are standalone serial driver). Just like earlyprintk, I
think earlycon should reuse UART setting in loader. Since some other
platforms with 8250 already depend on this, it's ok to add flags to
distinguish whether 8250 earlycon driver should init hw or not. As you
said, add "noinit" flag is simple, but "stdout-path-initialized" need
more discussion. 

It would definitely be nice to have consistent behavior, and I also
think it would be good to default to noinit for the console, because that
is the more likely case. Basically anything that can do debug_ll on ARM
already depends on this behavior because the debug_ll infrastructure
does not initialize the console before using it. I believe the same
is true for powerpc udbg.

	Arnd