Re: [PATCH 1/2] drm/i915/bdw: Apply workarounds using the golden render state

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

Ville Syrjälä <ville.syrjala@linux.intel.com> writes:

> On Wed, Aug 20, 2014 at 03:19:17PM +0100, Arun Siluvery wrote:
>> Workarounds for bdw are currently applied in init_clock_gating() but they
>> are lost following a gpu reset. Some of the WA registers are part of register
>> state context and they are restored with every context switch so initializing
>> them in golden render state ensures that they are applied even when we start
>> with an uninitialized context or during hw initlialization followed by a reset.
>> 
>> v2: Add comments corresponding to WAs in golden render state (Chris).
>> 
>> The generation of render state is not a straighforward process, it would
>> be ideal to augment WA values from during the setup state as opposed to
>> using a tool but that would be a follow up patch.
>
> I'd still prefer just emitting the LRIs from code rather tha mucking
> about with null batch. Less hoops to jump through when adding a new w/a.

I agree with this. We should aim to keep null state as per
gen. Workaround set is different for gtX inside particular
gen so we would need then multiple null states per gen. 

After brief chat with Ville, I think that the correct
spot to init the context specific workarounds is after MI_SET_CONTEXT
to default and right before null batch is run. If we do these
with emitting LRIs to ring, we should be safe as they are then saved
with default ctx.

The default ctx is then used as a 'parent' for newly created
contexts. Ofcource if registers get globbered, then we inherit
crap.

If we have the per gen null state and the ring is initializing
workarounds for the default context, then in future we can
save this state as 'read only golden context'. And use it as the
initial state for all newly created contexts.

Then the full plan how to init would look like this:

#1 reset the gpu (on driver load, on resume or on hang recovery)
#2 if we have 'read only golden context', copy it to default ctx
#3 switch to default context
#4 if we had 'read only golden context' we are done with the init.

---

#5 if this is driver load thus there is no 'read only golden context' yet.
#6 init workarounds through ring LRIs
#7 run null/golden state batch
#8 save this state as a 'read only golden context'

---

#9 for each new context, initialize ctx obj with 'read only golden
 context' (either by memcpy or restoring from it when switching to new)

-Mika
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