Re: [PATCH] drm/i915/dmc: Add debugfs for dc6 counter

[Gustavo Sousa <gustavo.sousa@intel.com>]

Quoting Imre Deak (2025-02-03 09:43:38-03:00)
>On Mon, Feb 03, 2025 at 02:26:13PM +0530, Mohammed Thasleem wrote:
>> Starting from MTl we don't have a platform agnostic way to validate DC6 state
>> due to dc6 counter has been removed to validate DC state.
>> Adding dc6_entry_counter at display dirver to validate dc6 state.
>> 
>> Signed-off-by: Mohammed Thasleem <mohammed.thasleem@intel.com>
>> ---
>>  drivers/gpu/drm/i915/display/intel_display_core.h       | 1 +
>>  drivers/gpu/drm/i915/display/intel_display_power_well.c | 2 ++
>>  drivers/gpu/drm/i915/display/intel_dmc.c                | 1 +
>>  3 files changed, 4 insertions(+)
>> 
>> diff --git a/drivers/gpu/drm/i915/display/intel_display_core.h b/drivers/gpu/drm/i915/display/intel_display_core.h
>> index 554870d2494b..cc244617011f 100644
>> --- a/drivers/gpu/drm/i915/display/intel_display_core.h
>> +++ b/drivers/gpu/drm/i915/display/intel_display_core.h
>> @@ -376,6 +376,7 @@ struct intel_display {
>>          struct {
>>                  struct intel_dmc *dmc;
>>                  intel_wakeref_t wakeref;
>> +                u32 dc6_entry_counter;
>>          } dmc;
>>  
>>          struct {
>> diff --git a/drivers/gpu/drm/i915/display/intel_display_power_well.c b/drivers/gpu/drm/i915/display/intel_display_power_well.c
>> index f45a4f9ba23c..0eb178aa618d 100644
>> --- a/drivers/gpu/drm/i915/display/intel_display_power_well.c
>> +++ b/drivers/gpu/drm/i915/display/intel_display_power_well.c
>> @@ -869,6 +869,8 @@ void skl_enable_dc6(struct intel_display *display)
>>          intel_dmc_wl_enable(display, DC_STATE_EN_UPTO_DC6);
>>  
>>          gen9_set_dc_state(display, DC_STATE_EN_UPTO_DC6);
>> +
>> +        display->dmc.dc6_entry_counter++;
>
>AFAIU the goal is to validate that the display HW can reach the DC6
>power state. There is no HW DC6 residency counter (and there wasn't such
>a counter earlier either), so an alternative way is required. According
>to the HW team the display driver has programmed everything correctly in
>order to allow the DC6 power state if the DC5 power state is reached
>(indicated by the HW DC5 residency counter incrementing) and DC6 is
>enabled by the driver.

Yep. That's what I learned as well when looking into this stuff a while
ago.

>
>Based on the above, we'd need a DC6 residency counter maintained by the
>driver which is incremented if the DC5 residency counter increments
>while DC6 is enabled. The dc6_entry_counter in this patch is not enough
>for this, since it doesn't take into account the DC5 residency. While
>user space could check both dc6_entry_counter and the DC5 residency,
>that check would be racy wrt. the driver enabling/disabling the DC6
>state asynchronously.

I'm not sure doing a driver-maintained dc6 entry counter would be
something worth implementing. Even if we have successfully entered DC5
and, in theory, DC6 would follow if enabled, this would be a synthetic
counter and it could be masking some hardware bug that could be
preventing DC6.

On the IGT side, we could just skip if we are on a platform that does
not support DC6 counters, at least while we do not have a reliable
alternative way of checking for DC6.

It would be good if we could detect that PG0 was in fact disabled, which
I believe is a stronger indication of DC6.

--
Gustavo Sousa

>
>I suppose the driver could take a snapshot of the DC5 residency counter
>right after it enables DC6 (dc5_residency_start) and increment the SW
>DC6 residency counter right before it disables DC6 or when user space
>reads the DC6 counter. So the driver would update the counter at these
>two points in the following way:
>dc6_residency += dc5_residency_current - dc5_residency_start
>
>The commit log would need a justification, something along the above
>lines.
>
>>  }
>>  
>>  void bxt_enable_dc9(struct intel_display *display)
>> diff --git a/drivers/gpu/drm/i915/display/intel_dmc.c b/drivers/gpu/drm/i915/display/intel_dmc.c
>> index 221d3abda791..f51bd8e6011d 100644
>> --- a/drivers/gpu/drm/i915/display/intel_dmc.c
>> +++ b/drivers/gpu/drm/i915/display/intel_dmc.c
>> @@ -1293,6 +1293,7 @@ static int intel_dmc_debugfs_status_show(struct seq_file *m, void *unused)
>>          if (i915_mmio_reg_valid(dc6_reg))
>>                  seq_printf(m, "DC5 -> DC6 count: %d\n",
>>                             intel_de_read(display, dc6_reg));
>> +        seq_printf(m, "DC6 entry count: %d\n", display->dmc.dc6_entry_counter);
>>  
>>          seq_printf(m, "program base: 0x%08x\n",
>>                     intel_de_read(display, DMC_PROGRAM(dmc->dmc_info[DMC_FW_MAIN].start_mmioaddr, 0)));
>> -- 
>> 2.43.0
>>