[Intel-gfx] [PATCH 10/15] drm/i915: check the power well when capturing error state

[Daniel Vetter]

On Thu, Mar 21, 2013 at 11:12 PM, Paulo Zanoni <przanoni at gmail.com> wrote:
> I don't understand your way of thinking: your argument is that "this
> is difficult to maintain", but then you're proposing the addition of a
> new i915_read, which IMHO is way much more complex and feels like a
> hackish solution to the problem. Every patch to the "normal"
> i915_read#x will have to also patch i915_read_nocheck#x. And on patch
> 0014, which you also suggested to use the "I915_READ_NOCHECK"
> function, our code will be relying on the _coincidence_ that when we
> read a register that does not exist, we read 0. I really don't think
> this is a sane approach.

I915_READ_NOCHECK already exists, it's called I915_READ_NOTRACE ;-)
We'd lose the tracepoint when just using that, but imo that's no too
horrible for debug checks.

Wrt the assert_pipe check relying on the fact that the hw returns 0,
I'm totally ok on relying on that. If that ever changes in a future
product, we can fix it.

> Our hardware is getting much more complex with respect to these reads,
> and I think we should never read ever read from a register that does
> not exist or is powered off. I'm open to more suggestions on how to
> write this patch, but I really think we shouldn't read registers that
> don't exist or are powered off.

The "much more complex" part is actually why I've looked into
different options for the _debug_ functions. I agree with you for all
the real display code. But debug code (especially the gpu hang stuff)
tends to not be run too often, hence I'm leaning more towards a "just
do it, I know what I'm doing" kind of approach. Currently it's fairly
simple, but with future hw we'll have more chances to screw up and not
dump a few regs we actually want. So I just want to avoid getting a
bug report where the first thing we have to do is patch up the reg
dumper (and we can't really test this any other way than by human
inspection of a dump).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch