Re: [PATCH] drm/i915: Emit even number of dwords when emitting LRIs

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

Chris Wilson <chris@chris-wilson.co.uk> writes:

> On Thu, Oct 23, 2014 at 01:42:38PM +0100, Damien Lespiau wrote:
>> On Thu, Oct 23, 2014 at 02:21:02PM +0200, Daniel Vetter wrote:
>> > On Wed, Oct 22, 2014 at 06:59:52PM +0100, Arun Siluvery wrote:
>> > > The number of DWords should be even when doing ring emits as
>> > > command sequences require QWord alignment.
>> > > 
>> > > v2: user LRI variant that can write multiple regs in one go (Damien).
>> > > We can simply insert one NOP at the end instead of one per register write.
>> > > 
>> > > Cc: Mika Kuoppala <mika.kuoppala@intel.com>
>> > > Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
>> > > ---
>> > >  drivers/gpu/drm/i915/intel_ringbuffer.c | 5 +++--
>> > >  1 file changed, 3 insertions(+), 2 deletions(-)
>> > > 
>> > > diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
>> > > index 497b836..a8f72e8 100644
>> > > --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
>> > > +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
>> > > @@ -680,15 +680,16 @@ static int intel_ring_workarounds_emit(struct intel_engine_cs *ring)
>> > >  	if (ret)
>> > >  		return ret;
>> > >  
>> > > -	ret = intel_ring_begin(ring, w->count * 3);
>> > > +	ret = intel_ring_begin(ring, (w->count * 2 + 2));
>> > >  	if (ret)
>> > >  		return ret;
>> > >  
>> > > +	intel_ring_emit(ring, MI_LOAD_REGISTER_IMM(w->count));
>> > 
>> > Afaik there's a limit to the size of an MI_LRI. Where's the check for
>> > that (probably with a WARN_ON for now to avoid unecessary complexity)?
>> 
>> I guess there's always the size of the length field, I don't see any
>> other indication. Note that I can find the documentation of the
>> multi-registers version of LRI either. So, well, we probably should
>> double check it does work.
>
> It does work. The max is around 60 iirc (the max length of the
> command).
> -Chris
>

I did some test with bdw:

The maximum is 128 writes, resulting the 8 bit length 
field of the command being 0xff, thus following the spec.
The 128'th write went through.

Perhaps the max command length is then less in older gens?

Perhaps WARN_ON(x > 128) in MI_LOAD_REGISTER_IMM would be in place
but one needs minor tweak to command parser a bit also then.

#define I915_MAX_WA_REGS 16

keeps us safe for now atleast.

-Mika

> -- 
> Chris Wilson, Intel Open Source Technology Centre