Re: Is MI_FLUSH_ENABLE bit 12?

[Eric Anholt <eric@anholt.net>]

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On Mon, 5 Dec 2011 14:57:09 -0800, Jesse Barnes <jbarnes@virtuousgeek.org> wrote:
> On Wed, 30 Nov 2011 13:54:47 -0800
> Eric Anholt <eric@anholt.net> wrote:
> 
> > On Wed, 30 Nov 2011 03:42:00 +0000, Ben Widawsky <ben@bwidawsk.net> wrote:
> > Non-text part: multipart/signed
> > > On Tue, Nov 29, 2011 at 04:47:57PM -0800, Eric Anholt wrote:
> > > > On Mon, 28 Nov 2011 18:48:04 -0800, "Keith Packard" <keithp@keithp.com> wrote:
> > > > Non-text part: multipart/mixed
> > > > Non-text part: multipart/signed
> > > > > 
> > > > > Just reading through vol1c.4 of the bspec  this evening and found something odd.
> > > > > 
> > > > > Bit 11 of MI_MODE is "Invalidate UHPTR enable".
> > > > > Bit 12 of MI_MODE is "MI_FLUSH Enable"
> > > > > 
> > > > > And, yet, in i915_reg.h:
> > > > > 
> > > > > #define MI_MODE		0x0209c
> > > > > # define VS_TIMER_DISPATCH				(1 << 6)
> > > > > # define MI_FLUSH_ENABLE				(1 << 11)
> > > > > 
> > > > > Are we off-by-one on MI_FLUSH_ENABLE? Seems like this would cause
> > > > > serious problems...
> > > > 
> > > > I think we are.  On the other hand, based on actual behavior plus
> > > > reading of simulator, I believe that the bit does nothing, regardless.
> > > 
> > > I do not think so. We've (Chris, I, and perhaps Jesse?) been through
> > > this excercise at least twice before, and both times resulted in hangs
> > > when we switched to bit 12 on Ironlake, not sure about other platforms.
> > 
> > There is no MI_FLUSH enable bit on Ironlake in my copy of the docs.  Bit
> > 12 is MBZ on that hardware.
> 
> It's one of those "been there forever" bits.  A comment wouldn't hurt,
> but we should also put in a doc change request (in fact I think I did
> that but it went into a black hole).

Well, by "been there forever", you mean "introduced in:

commit a69ffdbfcba8eabf2ca9d384b578e6f28b339c61
Author: Zhenyu Wang <zhenyuw@linux.intel.com>
Date:   Mon Aug 30 16:12:42 2010 +0800

    drm/i915: Enable MI_FLUSH on Sandybridge
    
    MI_FLUSH is being deprecated, but still available on Sandybridge.
    Make sure it's enabled as userspace still uses MI_FLUSH.
    
    Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
    Cc: stable@kernel.org
    Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>"

It should be removed on Ironlake, and fixed to be 12 on SNB or just
removed.

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