linux-kernel - [PATCH] serial: kgdboc: Allow earlycon initialization to be deferred

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Message-Id: <20200429170804.880720-1-daniel.thompson@linaro.org>
Date:   Wed, 29 Apr 2020 18:08:04 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...aro.org
Subject: [PATCH] serial: kgdboc: Allow earlycon initialization to be deferred

As described in the big comment in the patch, earlycon initialization
can be deferred if, a) earlycon was supplied without arguments and, b)
the ACPI SPCR table hasn't yet been parsed.

Unfortunately, if deferred, then the earlycon is not ready during early
parameter parsing so kgdboc cannot use it. This patch mitigates the
problem by giving kgdboc_earlycon a second chance during
dbg_late_init(). Adding a special purpose interface slightly increase
the intimacy between kgdboc and debug-core but this seems better than
adding kgdb specific hooks into the arch code (and much, much better
than faking non-intimacy with function pointers).

Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
---

Notes:
    Hi Doug,
    
    This patch extends your patch set to make it easier to deploy on ACPI
    systems[1]:
      earlycon kgdboc_earlycon kgdboc=ttyAMA0
    
    I have mixed feeling about it because it adds calls from debug-core
    into kgdboc and I don't think there are other examples of this.
    However earlycon auto-configuration is so awesome I'd like to
    be able to keep using it and this is the best I have come up with
    so far ;-).
    
    
    Daniel.
    
    
    [1] And also on DT based arm64 systems that have ACPI support
        enabled at compile time because such systems don't decide
        whether to adopt DT or ACPI until after early parameter
        parsing.

 drivers/tty/serial/kgdboc.c | 26 +++++++++++++++++++++++++-
 include/linux/kgdb.h        |  2 ++
 kernel/debug/debug_core.c   |  4 ++++
 3 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
index 7aca0a67fc0b..a7a079ce2c5d 100644
--- a/drivers/tty/serial/kgdboc.c
+++ b/drivers/tty/serial/kgdboc.c
@@ -509,6 +509,8 @@ static struct kgdb_io kgdboc_earlycon_io_ops = {
 	.is_console		= true,
 };

+static bool kgdboc_earlycon_late_enable __initdata;
+
 static int __init kgdboc_earlycon_init(char *opt)
 {
 	struct console *con;
@@ -529,7 +531,23 @@ static int __init kgdboc_earlycon_init(char *opt)
 	console_unlock();

 	if (!con) {
-		pr_info("Couldn't find kgdb earlycon\n");
+		/*
+		 * If earlycon deferred its initialization then we also need to
+		 * do that since there is no console at this point. We will
+		 * only defer ourselves when kgdboc_earlycon has no arguments.
+		 * This is because earlycon init is only deferred if there are
+		 * no arguments to earlycon (we assume that a user who doesn't
+		 * specify an earlycon driver won't know the right console name
+		 * to put into kgdboc_earlycon and will let that auto-configure
+		 * too).
+		 */
+		if (!kgdboc_earlycon_late_enable &&
+		    earlycon_acpi_spcr_enable && (!opt || !opt[0])) {
+			earlycon_kgdboc_late_enable = true;
+			pr_info("No suitable earlycon yet, will try later\n");
+		} else {
+			pr_info("Couldn't find kgdb earlycon\n");
+		}
 		return 0;
 	}

@@ -545,6 +563,12 @@ static int __init kgdboc_earlycon_init(char *opt)
 }

 early_param("kgdboc_earlycon", kgdboc_earlycon_init);
+
+void __init kgdb_earlycon_late_init(void)
+{
+	if (kgdboc_earlycon_late_enable)
+		earlycon_kgdboc_init(NULL);
+}
 #endif /* CONFIG_KGDB_SERIAL_CONSOLE */

 module_init(init_kgdboc);
diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
index 77a3c519478a..02867a2f0eb4 100644
--- a/include/linux/kgdb.h
+++ b/include/linux/kgdb.h
@@ -227,6 +227,8 @@ extern int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt);
 extern void kgdb_arch_late(void);


+extern void __init kgdb_earlycon_late_init(void);
+
 /**
  * struct kgdb_arch - Describe architecture specific values.
  * @gdb_bpt_instr: The instruction to trigger a breakpoint.
diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index 2d74dcbca477..f066ef2bc615 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -963,11 +963,15 @@ void __weak kgdb_arch_late(void)
 {
 }

+void __init __weak kgdb_earlycon_late_init(void)
+
 void __init dbg_late_init(void)
 {
 	dbg_is_early = false;
 	if (kgdb_io_module_registered)
 		kgdb_arch_late();
+	else
+		kgdb_earlycon_late_init();
 	kdb_init(KDB_INIT_FULL);

 	if (kgdb_io_module_registered && kgdb_break_asap)

base-commit: 6a8b55ed4056ea5559ebe4f6a4b247f627870d4c
prerequisite-patch-id: cbaa70eb783f1f34aec7f5839d1fecbc7616a9f6
prerequisite-patch-id: d7543cdd19fb194ded3361d52818970083efdb06
prerequisite-patch-id: 2238d976451dac9e3ee1bf02a077d633e342aa0c
prerequisite-patch-id: 9e4296261b608ee172060d04b3de431a5e370096
prerequisite-patch-id: 2b008e0e14a212072874ecb483d9c6844d161b08
prerequisite-patch-id: f5b692b89c997d828832e3ab27fffb8f770d7b6f
prerequisite-patch-id: 851d6f4874aa24540db9d765275ae736e8b2955b
prerequisite-patch-id: d3969c2fb7cd320eafebe63d7da270dac5a82fc9
prerequisite-patch-id: e1fc1478b7f75094d263ffc64a9f3528151831cf
prerequisite-patch-id: 45fb53996a9f5993e03673c10eebf2834c58307f
prerequisite-patch-id: 50ac1ddb52c3cce8b712036f212fdd67d7493112
--
2.25.1