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. > 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. > 3. omap doesn't support ioremap() at early param parsing :( ARM in general does not. > 4. the ARM arch doesn't support fixmap hacking at early param parsing :( There's a patch on the list to enable it. It's been slow as fixmap has been a moving target. Rob -- 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
