mysterious crashes on OMAP5 uevm

[Grazvydas Ignotas]

On Tue, Sep 8, 2015 at 4:38 PM, Tony Lindgren <tony at atomide.com> wrote:
> * Grazvydas Ignotas <notasas at gmail.com> [150908 05:50]:
>> Hi,
>>
>> this is a longstanding problem I'm seeing since the very beginning,
>> which was around 3.12 or so (when I've first got the hardware) and it
>> seems 4.2 is affected by it still. Basically what happens is Xorg
>> randomly segfaults at some "impossible" location. I don't have the
>> details at the moment (could get them is needed), but from what I
>> examined with gdb some time ago the situation did not make any sense.
>>
>> There are 2 workarounds that I know which make the problem go away
>> (one is enough):
>> - recompile Xorg with -marm (I'm using Debian armhf so it's thumb2 by default)
>> - disable ARCH_MULTI_V6 in the kernel config
>>
>> Because of the above workarounds I have forgotten about it several
>> times, but it regularly comes back and bites again. It would look like
>> some missing erratum workaround, but I have all of them enabled in the
>> kernel.
>>
>> Does anyone know about this? Perhaps some missing erratum workaround
>> in the bootloader? u-boot isn't too old here (2015.07).
>
> Seems like some incorrect handling with CONFIG_CPU_V6 compiled in..
> Maybe try to narrow it down by commenting out some CONFIG_CPU_V6 and
> __LINUX_ARM_ARCH__ = 6 ifdefs in the git grep CONFIG_CPU_V6
> places ignoring uncompress and davinci code.

ok with that it was quite easy to find. On a kernel with ARCH_MULTI_V6
disabled, it is enough to just do this:

--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -340,13 +340,13 @@ setup_return(struct pt_regs *regs, struct ksignal *ksig,
                /*
                 * The LSB of the handler determines if we're going to
                 * be using THUMB or ARM mode for this signal handler.
                 */
                thumb = handler & 1;

-#if __LINUX_ARM_ARCH__ >= 7
+#if 0 //__LINUX_ARM_ARCH__ >= 7
                /*
                 * Clear the If-Then Thumb-2 execution state
                 * ARM spec requires this to be all 000s in ARM mode
                 * Snapdragon S4/Krait misbehaves on a Thumb=>ARM
                 * signal transition without this.
                 */

... and the problem appears, so I guess this needs some real
multiplatform handling,.

> Do you have some easy way to reproduce this issue?

Just moving a browser window around with mouse usually triggers it
within a minute.

>
> Regards,
>
> Tony

Gražvydas