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Subject[PATCH v8 3/3] printk: fix double printing with earlycon
Date
If a console was specified by ACPI SPCR table _and_ command line
parameters like "console=ttyAMA0" _and_ "earlycon" were specified,
then log messages appear twice.

The root cause is that the code traverses the list of specified
consoles (the `console_cmdline` array) and stops at the first match.
But it may happen that the same console is referred by the elements
of this array twice:

pl011,mmio,0x87e024000000,115200 -- from SPCR
ttyAMA0 -- from command line

but in this case `preferred_console` points to the second entry and
the flag CON_CONSDEV is not set, so bootconsole is not deregistered.

To fix that, introduce an invariant "The last non-braille console
is always the preferred one" on the entries of the console_cmdline
array. Then traverse it in reverse order to be sure that if
the console is preferred then it will be the first matching entry.

Reported-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Aleksey Makarov <aleksey.makarov@linaro.org>
---

v7 -> v8:
- add an explanation to the comment how console_cmdline can contain entries
referring to the same console
- move the body of the function introduced in the previous version to cycle
- don't panic() (Sergey Senozhatsky). Don't check this condition because
the loop condition guarantees that it holds.
- use swap() (Sergey Senozhatsky)

v6 -> v7:
- return back to v5
- leave the check for already registered entries and add a function that
maintains the invariant.

v5 -> v6:
- drop v5 and continue to work on v4:
- introduce _braille_is_braille_console(). It helps to split original loop
into three parts: 1) search for braille console, 2) check for
preferred_console, 3) match other entries so that these three parts do not
intersect.
- introduce for_each_console_cmdline() macros to traverse console_cmdline
(Petr Mladek)

kernel/printk/printk.c | 49 ++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 42 insertions(+), 7 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index fd752f0c8ef1..462036e7a767 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1909,8 +1909,28 @@ static int __add_preferred_console(char *name, int idx, char *options,
i < MAX_CMDLINECONSOLES && c->name[0];
i++, c++) {
if (strcmp(c->name, name) == 0 && c->index == idx) {
- if (!brl_options)
- preferred_console = i;
+ int last;
+
+ if (brl_options)
+ return 0;
+
+ /*
+ * Maintain an invariant that will help to find if
+ * the matching console is preferred, see
+ * register_console():
+ *
+ * The last non-braille console is always
+ * the preferred one.
+ */
+ for (last = MAX_CMDLINECONSOLES - 1;
+ last >= 0 && !console_cmdline[last].name[0];
+ last--)
+ ;
+
+ if (i != last)
+ swap(console_cmdline[i], console_cmdline[last]);
+
+ preferred_console = last;
return 0;
}
}
@@ -2457,12 +2477,27 @@ void register_console(struct console *newcon)
}

/*
- * See if this console matches one we selected on
- * the command line.
+ * See if this console matches one we selected on the command line.
+ *
+ * There may be several entries in the console_cmdline array matching
+ * with the same console, one with newcon->match(), another by
+ * name/index:
+ *
+ * pl011,mmio,0x87e024000000,115200 -- added from SPCR
+ * ttyAMA0 -- added from command line
+ *
+ * Traverse the console_cmdline array in reverse order to be
+ * sure that if this console is preferred then it will be the first
+ * matching entry. We use the invariant that is maintained in
+ * __add_preferred_console().
*/
- for (i = 0, c = console_cmdline;
- i < MAX_CMDLINECONSOLES && c->name[0];
- i++, c++) {
+ for (i = MAX_CMDLINECONSOLES - 1; i >= 0; i--) {
+
+ if (!console_cmdline[i].name[0])
+ continue;
+
+ c = console_cmdline + i;
+
if (!newcon->match ||
newcon->match(newcon, c->name, c->index, c->options) != 0) {
/* default matching */
--
2.12.0
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