Re: [PATCH] drm/i915: Update WaFlushCoherentL3CacheLinesAtContextSwitch

[Dave Gordon <david.s.gordon@intel.com>]

On 06/07/15 15:33, Daniel Vetter wrote:
> On Mon, Jul 06, 2015 at 02:16:54PM +0100, Dave Gordon wrote:
>> On 06/07/15 13:38, Daniel Vetter wrote:
>>> On Mon, Jul 06, 2015 at 12:52:51PM +0100, Dave Gordon wrote:
>>>> On 03/07/15 16:42, Chris Wilson wrote:
>>>>> On Fri, Jul 03, 2015 at 02:27:31PM +0100, Arun Siluvery wrote:
>>>>>> In this WA we need to set GEN8_L3SQCREG4[21:21] and reset it after PIPE_CONTROL
>>>>>> instruction but there is a slight complication as this is applied in WA batch
>>>>>> where the values are only initialized once.
>>>>>> Dave identified an issue with the current implementation where the register value
>>>>>> is read once at the beginning and it is reused; this patch corrects this by saving
>>>>>> the register value to memory, update register with the bit of our interest and
>>>>>> restore it back with original value.
>>>>>>
>>>>>> This implementation uses MI_LOAD_REGISTER_MEM which is currently only used
>>>>>> by command parser and was using a default length of 0. This is now updated
>>>>>> with correct length and moved to appropriate place.
>>>>>>
>>>>>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>>>>>> Cc: Dave Gordon <david.s.gordon@intel.com>
>>>>>> Signed-off-by: Arun Siluvery <arun.siluvery@linux.intel.com>
>>>>>> ---
>>>>>>   drivers/gpu/drm/i915/i915_cmd_parser.c |  6 +--
>>>>>>   drivers/gpu/drm/i915/i915_reg.h        |  3 +-
>>>>>>   drivers/gpu/drm/i915/intel_lrc.c       | 72 +++++++++++++++++++++++++---------
>>>>>>   3 files changed, 58 insertions(+), 23 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/i915/i915_cmd_parser.c b/drivers/gpu/drm/i915/i915_cmd_parser.c
>>>>>> index 306d9e4..430571b 100644
>>>>>> --- a/drivers/gpu/drm/i915/i915_cmd_parser.c
>>>>>> +++ b/drivers/gpu/drm/i915/i915_cmd_parser.c
>>
>>>>>> @@ -1021,7 +1021,7 @@ static bool check_cmd(const struct intel_engine_cs *ring,
>>>>>>   			 * only MI_LOAD_REGISTER_IMM commands.
>>>>>>   			 */
>>>>>>   			if (reg_addr == OACONTROL) {
>>>>>> -				if (desc->cmd.value == MI_LOAD_REGISTER_MEM) {
>>>>>> +				if (desc->cmd.value == MI_LOAD_REGISTER_MEM(1)) {
>>>>>
>>>>> I had a double take here, but it all comes out in the wash. For one
>>>>> moment, I thought the cmd matching had changed, but that has the length
>>>>> masked out.
>>>>>
>>>>> Reviewed-by: Chris Wilson <chris@cris-wilson.co.uk>
>>>
>>> Queued for -next, thanks for the patch.
>>>
>>>>> Who will start to complain about all the extra frequent register writes,
>>>>> probably into common power wells....
>>>>> -Chris
>>>>
>>>> Hmm ... that is quite confusing, especially as the actual opcode in the
>>>> instruction stream will be MI_LOAD_REGISTER_MEM(2) on GEN8+. It might almost
>>>> be better to use MI_LOAD_REGISTER_MEM(0) to emphasise that the length field
>>>> is a wildcard and not something that will be matched exactly.
>>>
>>> There's a separate _GEN8 #define to accomodate the differences, so I don't
>>> fully understand your concern. We also don't do any decoding in the kernel
>>> ...
>>> -Daniel
>>
>> In the snippet:
>>
>>>> -	CMD(  MI_LOAD_REGISTER_MEM,         SMI,   !F,  0xFF,   W | B,
>>>> +	CMD(  MI_LOAD_REGISTER_MEM(1),      SMI,   !F,  0xFF,   W | B,
>>
>> the (1) goes in the table but is ignored when matching instructions in the
>> stream being parsed. It could just as well be (2) or (0) or (255).
>>
>> Then, in the test:
>>
>>>> -		if (desc->cmd.value == MI_LOAD_REGISTER_MEM) {
>>>> +		if (desc->cmd.value == MI_LOAD_REGISTER_MEM(1)) {
>>
>> the thing on the left of the == is not the instruction being examined, but
>> the entry in the table that matched that instruction. So here also we're not
>> really using the length field, EXCEPT that it MUST be the same as the
>> (arbitrary) value in the table.
>>
>> So my concern here was not about correctness but comprehensibility and hence
>> maintainability -- after all, if Chris had to look twice it obviously isn't
>> as clear as one would like!
>>
>> My suggestion was that maybe the "ignored" length field should be 0 to make
>> it less likely that a reader would think this matches exactly (and only) an
>> opcode of 0xa400001. Or maybe (255) would be even more obviously
>> not-a-literal-match?
>
> Hm, given that the cmd parser is gen7 only I'm not too concerned about
> this. It is indeed a bit surprising though, and I guess (0) would be less
> surprising. Otoh other commands with a lenght field also use (1) in a
> similar fashion, so at least this is consistent.
>
> tbh no opinion here at all from my side, but happy to merge a fixup on top
> to clarify this, if you can agree on a clear improvement.
> -Daniel

Hi Daniel,

Arun & I have just discussed this and we think we should go with this 
as-is for now, and then fix all the places that have an opcode macro 
with a redundant non-zero length in a separate patch.

Thanks,
.Dave.
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