Re: [PATCH 07/10] drm/i915: print Gen5+ CPU poison interrupts

[Ben Widawsky <ben@bwidawsk.net>]

On Thu, 14 Feb 2013 18:35:32 -0200
Paulo Zanoni <przanoni@gmail.com> wrote:

> Hi
> 
> 2013/2/9 Ben Widawsky <ben@bwidawsk.net>:
> > On Fri, 8 Feb 2013 11:42:39 -0800
> > Jesse Barnes <jbarnes@virtuousgeek.org> wrote:
> >
> >> On Fri,  8 Feb 2013 17:35:18 -0200
> >> Paulo Zanoni <przanoni@gmail.com> wrote:
> >>
> >> > From: Paulo Zanoni <paulo.r.zanoni@intel.com>
> >> >
> >> > On ILK/SNB all we need to do is to enable the "poison" bit, but
> >> > on IVB/HSW we need to enable the CPU error interrupt register,
> >> > which is responsible not only for poison interrupts, but also
> >> > other things. This includes the "unclaimed register" interrupt,
> >> > so on the IVB irq handler we now need to: (i) check whether the
> >> > interrupt was triggered by an unclaimed register and (ii) mask
> >> > the error interrupt bit so we don't risk generating "unclaimed
> >> > register" interrupts form inside the interrupt handler.
> >> >
> >> > Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
> >> > ---
> >>
> >> OTOH there's nothing the user can do about it... so we might do a
> >> WARN_ONCE or something here instead.  But even then, I'm not sure
> >> there's much *we* can do about these, as they indicate a
> >> corruption in the communication between the CPU and PCH.
> >>
> >
> > I agree with Jesse. I wouldn't bother with these. Even a WARN_ONCE
> > isn't helpful since the backtrace wouldn't really be meaningful.
> 
> Why isn't it helpful? Right now we don't even know whether this
> problem happens or not, we're completely "blind" to a possible problem
> that may be affecting us in some specific cases and we don't even
> know. Knowing that it happens and how often it happens is IMHO
> certainly better than closing our eyes and pretending it doesn't
> exist.
> 

I suppose you're right. I'm strongly of the opinion that we won't
ever see this error because the system will crap out before we'd be
able to get that info - of course I cannot prove that, and I don't
know enough about what exactly poison means. I just think it sucks that
we have yet another gen specific thing which has TBD value. I certainly
won't nak it, and of course if it proves useful, I'll be most
apologetic.

As for the WARN being unhelpful, it's the same problem again. You're
getting the notifications via interrupt, so a backtrace is useless on
IVB/HSW. Perhaps it makes sense on ILK/SNB. Reinventing a "do this
once" macro isn't worthwhile either, so I guess WARN_ON with the
assumption that we ignore the backtrace is fine on IVB/HSW.

> >
> > If OTOH, you wanted to save away this information into error state;
> > I could get behind that.
> 
> 
> 
>