Re: mysterious crashes on OMAP5 uevm

[Tony Lindgren <tony@atomide.com>]

* Grazvydas Ignotas <notasas@gmail.com> [150908 13:44]:
> On Tue, Sep 8, 2015 at 4:38 PM, Tony Lindgren <tony@atomide.com> wrote:
> > * Grazvydas Ignotas <notasas@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,.

OK nice to hear you found it. Yeah looks like some runtime
capability check is needed.
 
> > Do you have some easy way to reproduce this issue?
> 
> Just moving a browser window around with mouse usually triggers it
> within a minute.

OK good to know.

Regards,

Tony