[ +cc devicetree, Mark Rutland, Grant Likely ] On 02/26/2015 09:58 AM, Rob Herring wrote: > On Thu, Feb 26, 2015 at 8:48 AM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: >> Hi Rob, >> >> On 02/24/2015 03:20 PM, Rob Herring wrote: >>> On Tue, Feb 24, 2015 at 1:53 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: >> >> [...] >> >>>>>> Direct earlycon >>>>>> >>>>>> This feature enables arches and proms to start an earlycon directly, >>>>>> rather than requiring an "earlycon=" command line parameter. >>>>>> Devicetree can already do this via the 'linux,stdout-path' property, >>>>>> but arch and prom code requires direct coupling to the serial driver. >>>>>> >>>>>> This support is implemented by judicious refactoring and the same >>>>>> construct that devicetree and early_param use: a link table containing >>>>>> the necessary information (name and setup() function) to find and >>>>>> bind the appropriate earlycon "driver". >>>>> >>>>> I've skimmed thru this and it looks like a great improvement. >>>>> >>>>> One problem we have currently with DT stdout-path and earlycon is a >>>>> preferred console does not get registered, so the console will get >>>>> switched to tty0 and you lose your console. The problem is DT does not >>>>> know the console name to register a preferred console. It looks like >>>>> this series may help this problem, but I'm not sure and wanted your >>>>> thoughts. >>>> >>>> I thought that of_alias_scan() + of_console_check() caused DT stdout-path >>>> to add_preferred_console() the driver console @ port registration time >>>> via uart_add_one_port() -> of_console_check(). >>>> >>>> Is that not how that works? >>> >>> Yes, I believe that is how it works with earlycon not enabled. This >>> doesn't work when earlycon is enabled with just "earlycon" on the >>> command line. The fix I have is here[1], but I don't like putting DT >>> specifics into the console code. >> >> After much gnashing of teeth and pulling of hair yesterday, I managed >> to mock up the situation you describe, but I need to study it in more >> detail. Some things I did learn: >> >> 1. The serial console _does_ come back up when using stdout-path but the >> line settings don't match, because the serial core sets them to the >> default of 9600n8 if unspecified. > > That may have been what I saw as I tested on QEMU which ignores the > baud rate. But it does stop between the time tty0 is enabled and the > "real" serial console which is a time period we really want the > console. The bug is that of_alias_scan() does not emit a preferred console, but rather defers it until of_console_check(). In many setups, this means there is no preferred console. So when the dummy vt console loads (which happens via console_init()), all the boot consoles are disabled. The trivial fix is to emit a preferred console from of_alias_scan(), even if that console is never matched. I've sent you a patch to do just that. >> 2. The line settings can now be set with stdout-path like, >> stdout-path = "serial0:115200n8" >> but this breaks DT earlycon (as I wrote in the other email you were >> cc'd on). > > Right. We should fix libfdt. Ok. Regards, Peter Hurley -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
