Re: [PATCH] ARM: dts: da850: Add basic DTS for the L138/C6748 Dev Kit

[Karl Beldan <kbeldan@baylibre.com>]

On Thu, Aug 04, 2016 at 04:13:10PM -0500, Rob Herring wrote:
> +Peter H
> 
> On Thu, Aug 4, 2016 at 4:03 PM, Karl Beldan <kbeldan@baylibre.com> wrote:
> > On Thu, Aug 04, 2016 at 01:29:02PM -0500, Rob Herring wrote:
> >> On Thu, Aug 04, 2016 at 02:43:32PM +0000, Karl Beldan wrote:
> >> > The LCDK is the successor to the late Hawkboard and has the same machine
> >> > number.
> >> > Among the differences are the flash (16bits vs 8bits) and some pins
> >> > (MMC, LEDs, buttons, some external connectors), however the main
> >> > components remain the same (eth. PHY, Audio Codec, Video decoder and
> >> > DAC) except for the main PMIC, different and hard-wired on the LCDK (the
> >> > LDOs and DCDCs are always ON).
> >> > A DT-only boot with this addition gives functional uart, reboot via
> >> > watchdog, rtc, ethernet and MMC (I added the CD gpio for the MMC
> >> > although davinci_mmc doesn't call yet the mmc core OF facilities).
> 
> [...]
> 
> >> > +   aliases {
> >> > +           serial2 = &serial2;
> >> > +   };
> >> > +
> >> > +   chosen {
> >> > +           bootargs = "console=ttyS2,115200n8 earlycon";
> >>
> >> You don't need console with stdout-path set. And really, earlycon should
> >> not be the default.
> >>
> > Ok for earlycon.
> >
> > For the console, I thought likewise .. until I _struggled_ to understand
> > why the console was disappearing under my feet.
> > It seemed even stranger when I started looking at the codepath because
> > drivers/of/base.c would properly add_preferred_console as expected after
> > the corresponding UART was added. But the reality was there, without it
> > the console was not like stdout-path.
> > Among the commits that could instill doubt as to whether stdout-path
> > rules is f64255b5072d:
> > "The assumption that at least 1 preferred console will be registered
> > when the stdout-path property is set is invalid, which can result in
> > _no_ consoles.", though it didn't answer my question.
> > I started looking whether tty0 was not getting in the way and in the end
> > I came up with setting console.
> > But you seem affirmative.
> 
> There have been some issues in this area, but I thought they were in
> conjunction with earlycon and solved now. This is a fairly common
> review request for dts files, so I'm surprised if it doesn't work
> right. Maybe having tty0 or not has some influence. Or people just
> accept review comments without testing or caring...
> 
I can assure you I tested without early{con,printk} ;).

Now I have just rechecked the codepaths and something caught my
attention in kernel/printk/printk.c:__add_preferred_console(): if
console_cmdline is empty selected_console gets unconditionally set to
zero ! Knowing this I confirmed by replacing with idx (works), and also
by using ttyS0 (i.e. alias serial0 = &uart2) (works), this would not
affect DTs using serial0. It's late here so I'll stop there.
 
Karl