linux-kernel - Re: Regression in 4.9-rc1 for PPC32 - bisected to commit 05fd007e4629

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Message-ID: <ea132710-c23a-35a6-fcb8-d6ae63c496ea@lwfinger.net>
Date:   Mon, 24 Oct 2016 20:30:43 -0500
From:   Larry Finger <Larry.Finger@...inger.net>
To:     Paul Burton <paul.burton@...tec.com>
Cc:     Thorsten Leemhuis <regressions@...mhuis.info>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Regression in 4.9-rc1 for PPC32 - bisected to commit 05fd007e4629

I have a hack that works. Perhaps it will give a bit more understanding so that 
a proper patch can be created. My changes were applied on top of 4.9-rc1 with 
the patch from 
https://www.linux-mips.org/archives/linux-mips/2016-10/msg00130.html, and were 
as follows:

Index: linux/kernel/printk/printk.c
===================================================================
--- linux.orig/kernel/printk/printk.c   2016-10-24 12:29:17.838938604 -0500
+++ linux/kernel/printk/printk.c        2016-10-24 19:31:20.012593000 -0500
@@ -2657,7 +2657,9 @@
          *      didn't select a console we take the first one
          *      that registers here.
          */
-       if (preferred_console < 0 && !of_specified_console) {
+       pr_info("Before: newcon->device %p, newcon->setup %p\n", newcon->device, 
newcon->setup);
+       if ((preferred_console < 0 && !of_specified_console) ||
+           !newcon->setup) {
                 if (newcon->index < 0)
                         newcon->index = 0;
                 if (newcon->setup == NULL ||
@@ -2670,6 +2672,7 @@
                 }
         }

+       pr_info("After: newcon->device %p, newcon->setup %p\n", newcon->device, 
newcon->setup);
         /*
          *      See if this console matches one we selected on
          *      the command line.

The changes in commit 05fd007e4629 prevent the body of the 'if 
((preferred_console < 0 ...' loop from ever being executed; however, the failure 
indicates that we do need to go through this code at least once. I tested 
various quantities, and !newcon->setup was the only one that helped.

The outputs from the pr_info statements above and the changes in console as 
shown by 'dmesg | egrep "console|newcon" are:

[    0.000000] Before: newcon->device   (null), newcon->setup   (null)
	Part of the body of if executed
[    0.000000] After: newcon->device   (null), newcon->setup   (null)
[    0.000000] bootconsole [udbg0] enabled
[    0.001581] Before: newcon->device c0326e34, newcon->setup   (null)
	We now have newcon->device set, Part of body of if executed
[    0.001934] After: newcon->device c0326e34, newcon->setup   (null)
[    0.002285] console [tty0] enabled
[    0.002595] bootconsole [udbg0] disabled
[    0.002913] Before: newcon->device c032cc4c, newcon->setup c032cc80
	This time the body of if is not executed and will not be executed anymore.
[    0.002925] After: newcon->device c032cc4c, newcon->setup c032cc80
[    0.002943] Before: newcon->device c0331ad8, newcon->setup c0331c34
[    0.002956] After: newcon->device c0331ad8, newcon->setup c0331c34
[    0.003060] Before: newcon->device c0331ad8, newcon->setup c068dbc8
[    0.003074] After: newcon->device c0331ad8, newcon->setup c068dbc8
[    0.003322] Before: newcon->device c0331ad8, newcon->setup c068ddc4
[    0.003335] After: newcon->device c0331ad8, newcon->setup c068ddc4
[    1.993661] Before: newcon->device c0331ad8, newcon->setup c068dbc8
[    1.993759] After: newcon->device c0331ad8, newcon->setup c068dbc8
[    1.994395] Before: newcon->device c0331ad8, newcon->setup c068dbc8
[    1.994492] After: newcon->device c0331ad8, newcon->setup c068dbc8

Larry