Re: [Intel-gfx] [PATCH 5/6] drm/i915/gsc: Disable GSC engine and power well if FW is not selected

["Ceraolo Spurio, Daniele" <daniele.ceraolospurio@intel.com>]

On 11/22/2022 12:52 PM, Rodrigo Vivi wrote:
> On Mon, Nov 21, 2022 at 03:16:16PM -0800, Daniele Ceraolo Spurio wrote:
>> From: Jonathan Cavitt <jonathan.cavitt@intel.com>
>>
>> The GSC CS is only used for communicating with the GSC FW, so no need to
>> initialize it if we're not going to use the FW. If we're not using
>> neither the engine nor the microcontoller, then we can also disable the
>> power well.
>>
>> IMPORTANT: lack of GSC FW breaks media C6 due to opposing requirements
>> between CS setup and forcewake idleness. See in-code comment for detail.
>>
>> Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
>> Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
>> Cc: Matt Roper <matthew.d.roper@intel.com>
>> Cc: John C Harrison <John.C.Harrison@intel.com>
>> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
>> Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
>> ---
>>   drivers/gpu/drm/i915/gt/intel_engine_cs.c | 18 ++++++++++++++++++
>>   drivers/gpu/drm/i915/intel_uncore.c       |  3 +++
>>   2 files changed, 21 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/i915/gt/intel_engine_cs.c b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
>> index c33e0d72d670..99c4b866addd 100644
>> --- a/drivers/gpu/drm/i915/gt/intel_engine_cs.c
>> +++ b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
>> @@ -894,6 +894,24 @@ static intel_engine_mask_t init_engine_mask(struct intel_gt *gt)
>>   	engine_mask_apply_compute_fuses(gt);
>>   	engine_mask_apply_copy_fuses(gt);
>>   
>> +	/*
>> +	 * The only use of the GSC CS is to load and communicate with the GSC
>> +	 * FW, so we have no use for it if we don't have the FW.
>> +	 *
>> +	 * IMPORTANT: in cases where we don't have the GSC FW, we have a
>> +	 * catch-22 situation that breaks media C6 due to 2 requirements:
>> +	 * 1) once turned on, the GSC power well will not go to sleep unless the
>> +	 *    GSC FW is loaded.
>> +	 * 2) to enable idling (which is required for media C6) we need to
>> +	 *    initialize the IDLE_MSG register for the GSC CS and do at least 1
>> +	 *    submission, which will wake up the GSC power well.
>> +	 */
>> +	if (__HAS_ENGINE(info->engine_mask, GSC0) && !intel_uc_wants_gsc_uc(&gt->uc)) {
>> +		drm_notice(&gt->i915->drm,
>> +			   "No GSC FW selected, disabling GSC CS and media C6\n");
>> +		info->engine_mask &= ~BIT(GSC0);
>> +	}
>> +
>>   	return info->engine_mask;
>>   }
>>   
>> diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
>> index c1befa33ff59..e63d957b59eb 100644
>> --- a/drivers/gpu/drm/i915/intel_uncore.c
>> +++ b/drivers/gpu/drm/i915/intel_uncore.c
>> @@ -2701,6 +2701,9 @@ void intel_uncore_prune_engine_fw_domains(struct intel_uncore *uncore,
>>   		if (fw_domains & BIT(domain_id))
>>   			fw_domain_fini(uncore, domain_id);
>>   	}
>> +
>> +	if ((fw_domains & BIT(FW_DOMAIN_ID_GSC)) && !HAS_ENGINE(gt, GSC0))
>> +		fw_domain_fini(uncore, FW_DOMAIN_ID_GSC);
> On a quick glace I was asking "why do you need this since it doesn't have the gsc0?
> Then I remember that fw_domain got initialized and it will be skipped, right?
> Then I though about at least have a comment here, but finally I got myself wondering
> why we don't do this already in the if above, while we are cleaning the engine mask?

I've followed the existing code flows that we have in place for fused 
off VCS/VECS. Basically the existing code goes like this:

1) All FW domains for the platform are initialized
2) We read the fuses and adjust the engine mask accordingly
3) We go back and prune the FW domains that are not applicable anymore 
due to the updated mask.

The idea is to have a single gt-level function doing all the mask 
adjusting and an uncore-level one doing all the domain pruning. I'm not 
against changing this approach, but in that case we should update the 
behavior for VCS/VECS as well (which might be complicated, because 
VCS/VECS engines share FW domains, so the pruning logic is ugly).

Daniele

>
>>   }
>>   
>>   static void driver_flr(struct intel_uncore *uncore)
>> -- 
>> 2.37.3
>>