From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Hurley Subject: Re: [PATCH -next 00/13] Extensible console matching & direct earlycon Date: Thu, 26 Feb 2015 09:48:44 -0500 Message-ID: <54EF324C.2080205@hurleysoftware.com> References: <1424795830-31223-1-git-send-email-peter@hurleysoftware.com> <54ECD6BA.9070009@hurleysoftware.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Rob Herring Cc: Greg Kroah-Hartman , Andrew Morton , Arnd Bergmann , Jiri Slaby , "linux-serial@vger.kernel.org" , "linux-kernel@vger.kernel.org" List-Id: linux-serial@vger.kernel.org Hi Rob, On 02/24/2015 03:20 PM, Rob Herring wrote: > On Tue, Feb 24, 2015 at 1:53 PM, Peter Hurley 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. 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). 3. omap doesn't support ioremap() at early param parsing :( 4. the ARM arch doesn't support fixmap hacking at early param parsing :( Regards, Peter Hurley