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

["Siluvery, Arun" <arun.siluvery@linux.intel.com>]

On 23/10/2014 14:41, Ville Syrjälä wrote:
> On Thu, Oct 23, 2014 at 01:50:23PM +0100, Chris Wilson wrote:
>> 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).
>
> The maximum length seems to be 0xff on gen6+ and 0x3f before that,
> which would mean at most 128 or 32 registers.
>
> Also the context image is full of these multi register LRIs. Based on a
> quick glance the longest LRI in there is 0x5f on IVB, 0xcf on HSW, and
> 0xdf on BDW, which translate to 48, 104, and 108 registers per LRI. So
> we know at least those must work or context restore would not work.
> Before gen7 the context doesn't seem to resemble a batch, so I can't
> tell anything about those platforms based on the context image.
>

w->count is already checked against max workarounds which is 16 now so 
we are well within the limit; I think additional check would be 
redundant here and it is unlikely to have more than 128 workarounds.

regards
Arun