[Mesa-dev] [PATCH] i965: Disable guardband clipping on SNB unless workaround is present.

Eric Anholt eric at anholt.net
Mon Oct 8 08:17:10 PDT 2012

Kenneth Graunke <kenneth at whitecape.org> writes:

> On 10/07/2012 06:34 PM, Kenneth Graunke wrote:
>> On 10/07/2012 06:06 PM, Paul Berry wrote:
>>> Can we make a kernel patch that adds 3D_CHICKEN3 to the whitelist?  That
>>> would at least allow guardband clipping to work with future kernels.
>> That would definitely work.  It's a bit ironic though: assuming the
>> kernel fix to apply the workaround went in before the whitelist change,
>> we wouldn't even need to check the value returned.  We could simply
>> check whether drm_intel_reg_read returned -EINVAL or 0, as the mere
>> ability to read the register would prove that the kernel was new enough
>> that we could safely use guardband clipping.  (One could argue that we
>> *should* check the value to guard against future kernel bugs, but...)
>> What do you think, Daniel?  Should we whitelist 3D_CHICKEN3 and check
>> for bit 5 in Mesa?  Do you have a better idea?
> On second thought, I don't know why I felt compelled to do anything 
> about it in Mesa at all.  We've had much worse problems (GPU hangs in 
> Google MapsGL) that have required kernel-side workarounds where we could 
> have done Mesa-side workaround detection and disabled features 
> accordingly (HiZ/separate stencil), and chose not to.
> I guess we didn't have the register read mechanism then, but without the 
> whitelist, we kind of don't now either.
> Perhaps the best course of action is to simply:
> 1. Cc: stable at kernel.org the kernel fix and get it backported as far 
> back as anybody cares.
> 2. Leave GB clipping disabled in 9.0, but keep it for 9.1.  That way, by 
> the time it ships and non-developers/enthusiasts run it---in roughly 6 
> months---people will likely also have received the updated kernel.
> So far I like this plan best.  Just treat it as a kernel bug and 
> backport the patch.  It's trivial to do anyway...one register write.

I agree -- if the next Mesa release is that far off, the kernel fix
should trickle down to everyone who would be updating Mesa, so I'm not
worried about that little chance of regression.
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