[Intel-gfx] [PATCH 1/5] drm/i915: Fix forcewake counts for gen8

[Ben Widawsky]

On Fri, Feb 21, 2014 at 11:38:48AM -0800, Ben Widawsky wrote:
> On Fri, Feb 21, 2014 at 05:31:59PM +0200, mika.kuoppala at intel.com wrote:
> > From: Mika Kuoppala <mika.kuoppala at intel.com>
> > 
> > Sometimes generic driver code gets forcewake explicitly by
> > gen6_gt_force_wake_get(), which check forcewake_count before accessing
> > hardware. However the register access with gen8_write function access
> > low level hw accessors directly, ignoring the forcewake_count. This
> > leads to nested forcewake get from hardware, in ring init and possibly
> > elsewhere, causing forcewake ack clear errors and/or hangs.
> > 
> > Fix this by checking the forcewake count also in gen8_write
> > 
> > v2: Read side doesn't care about shadowed registers,
> >     Remove __needs_put funkiness from gen8_write. (Ville)
> >     Improved commit message.
> > 
> > References: https://bugs.freedesktop.org/show_bug.cgi?id=74007
> > Cc: Ben Widawsky <benjamin.widawsky at intel.com>
> > Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > Signed-off-by: Mika Kuoppala <mika.kuoppala at intel.com>
> > Reviewed-by: Ben Widawsky <ben at bwidawsk.net>
> 
> For those concerned with the performance implication of the extra if
> (if anyone at all cares, it's Chris) - I suppose we could also just add
> the lock to gen6_gt_force_wake_get/put.
> 
> [snip]

I should have also said, we have very few cases where it's actually a
problem (ie. not just done at init). And in fact, I am wondering now how
one could hit this at all without enabling rps.

-- 
Ben Widawsky, Intel Open Source Technology Center