Re: [PATCH 2/2] drm/i915: Correct the bit number for the MI_FLUSH_ENABLE.

[Ben Widawsky <ben@bwidawsk.net>]

On 01/24/12 18:55, Eric Anholt wrote:
> On Sat, 21 Jan 2012 17:36:13 +0100, Daniel Vetter<daniel@ffwll.ch>  wrote:
>> On Thu, Jan 19, 2012 at 10:50:06AM -0800, Eric Anholt wrote:
>>> Older specs claimed this was bit 11, but newer specs and the actual
>>> simulator code say it was bit 12.  Regardless, we don't use MI_FLUSH,
>>> or try to enable it any more.
>>>
>>> Signed-off-by: Eric Anholt<eric@anholt.net>
>>
>> I'd like to amend this with the following (on this patch instead of the
>> other, so that ppl actually can find it with git blame):
>>
>> "Furthermore actually setting bit12 results in gpu hangs both on snb and
>> ivb. Ben Widawsky discovered a ppt that claims that both bit12 and bit11
>> must be set, but that doesn't help either. And last but not least,
>> MI_FLUSH seems to work regardless of the setting of these bits."
>
> I haven't seen bit12 hanging snb/ivb -- I only knew of it hanging ilk
> (since it doesn't exist there).  On my snb, running xvideo so that
> MI_FLUSHes are generated by the userland (I think -- I haven't caught
> them in cat i915_batchbuffers | intel_dump_decode -), with
> intel_reg_read 0x209c saying 0x1240, things are going fine.  Also with
> 0x209c saying 0x240 (the result of this patch).

Daniel has a failing test on IVB. I haven't tried hard enough to make it 
fail on SNB, so I cannot speak to that.

>
> That 2008 PPT mentioned also said "the bit" and "bit 12", and only in
> one cut-and-paste of a command line did I see it say two bits should be
> set.  I would trust the actual code more than a ppt.
>
> But basically, whatever we do to make this broken code go away, I'm fine
> with.

I'm in the same boat. I think trying to figure out which source to trust 
is a losing game for all, and our best bet is to find out what the 
Windows driver does, and presumably that cut-and-paste is not from the 
Windows driver.

Ben