[Intel-gfx] [PATCH] drm/i915: fix overlay on i830M

[Chris Wilson]

On Mon, 22 Oct 2012 12:44:18 +0200, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> The overlay on the i830M has a peculiar failure mode: It works the
> first time around after boot-up, but consistenly hangs the second time
> it's used.
> 
> Chris Wilson has dug out a nice errata:
> 
> "1.5.12 Clock Gating Disable for Display Register
> Address Offset:	06200h–06203h
> 
> "Bit 3
> Ovrunit Clock Gating Disable.
> 0 = Clock gating controlled by unit enabling logic
> 1 = Disable clock gating function
> DevALM Errata ALM049: Overlay Clock Gating Must be Disabled:  Overlay
> & L2 Cache clock gating must be disabled in order to prevent device
> hangs when turning off overlay.SW must turn off Ovrunit clock gating
> (6200h) and L2 Cache clock gating (C8h)."
> 
> Now I've nowhere found that 0xc8 register and hence couldn't apply the
> l2 cache workaround. But I've remembered that part of the magic that
> the OVERLAY_ON/OFF commands are supposed to do is to rearrange cache
> allocations so that the overlay scaler has some scratch space.
> 
> And while pondering how that could explain the hang the 2nd time we
> enable the overlay, I've remembered that the old ums overlay code did
> _not_ issue the OVERLAY_OFF cmd.
> 
> And indeed, disabling the OFF cmd results in the overlay working
> flawlessly, so I guess we can workaround the lack of the above
> workaround by simply never disabling the overlay engine once it's
> enabled.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=47827
> Cc: stable at vger.kernel.org
> Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>

That looks fun, and indeed will probably work. So if we never issue the
OFF command, do we still need i830_init_clock_gating()?

And if we do still need i830_init_clock_gating() wouldn't it be better
applied as a quirk to intel_overlay_on()?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre