Re: [PATCH] drm/i915: Fix forcewake counts for gen8

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Ben Widawsky <benjamin.widawsky@intel.com> writes:

> On Tue, Feb 18, 2014 at 07:10:24PM +0200, Mika Kuoppala wrote:
>> 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
>> Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
>> Cc: Ben Widawsky <benjamin.widawsky@intel.com>
>> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
  
Double signed-off slipped in it seems.

> Thanks for finding this. I'd been meaning to track down the extra "Timed
> out" messages. I do wonder how this actually fixes a hang. Do you have
> any idea?

No clear idea. I just suspect that the two writes into FORCEWAKE_MT
without proper put/ack dance in between upsets the gpu.

Interestingly with init it hangs like 1 out of 2 times,
but after a gpu reset, it hangs always. The spot is always
the same: I915_WRITE(mmio, (u32)ring->status_page.gfx_addr);

-Mika
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