Re: [PATCH 4/4] drm/i915/dmc_wl: Enable the debugfs only with enable_dmc_wl_debugfs=1

[Jani Nikula <jani.nikula@linux.intel.com>]

On Mon, 27 Jan 2025, Gustavo Sousa <gustavo.sousa@intel.com> wrote:
> Quoting Jani Nikula (2025-01-27 09:01:39-03:00)
>>On Fri, 17 Jan 2025, Gustavo Sousa <gustavo.sousa@intel.com> wrote:
>>> We use a spinlock to protect DMC wakelock debugfs data, since it is also
>>> accessed by the core DMC wakelock logic. Taking the spinlock when the
>>> debugfs is not in use introduces a small but unnecessary penalty.
>>>
>>> Since the debugfs functionality is only expected to be used for, uh,
>>> debugging sessions, let's protect it behind a module parameter
>>> enable_dmc_wl_debugfs. That way, we only take the lock if the feature
>>> was enabled in the first place.
>>
>>If the debug struct were an opaque pointer, you could check for that
>>being != NULL. Register the debugfs always, and have that initialize
>>everything as needed?
>
> Hm... I'm failing to see how this would keep us from having to take the
> spinlock once we have the pointer being non-NULL.
>
> The idea of the parameter is to protect us from taking the spinlock when
> we are not debugging DMC wakelock offsets.

If you only allocate and assign the pointer when you enable the feature
via debugfs, wouldn't that achieve the goal?

BR,
Jani.

>
> --
> Gustavo Sousa
>
>>
>>BR,
>>Jani.
>>
>>>
>>> Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
>>> ---
>>>  .../gpu/drm/i915/display/intel_display_params.c  |  5 +++++
>>>  .../gpu/drm/i915/display/intel_display_params.h  |  1 +
>>>  .../gpu/drm/i915/display/intel_dmc_wl_debugfs.c  | 16 +++++++++++++++-
>>>  3 files changed, 21 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/display/intel_display_params.c b/drivers/gpu/drm/i915/display/intel_display_params.c
>>> index c4f1ab43fc0c..bc36d1b0ef87 100644
>>> --- a/drivers/gpu/drm/i915/display/intel_display_params.c
>>> +++ b/drivers/gpu/drm/i915/display/intel_display_params.c
>>> @@ -133,6 +133,11 @@ intel_display_param_named_unsafe(enable_dmc_wl, int, 0400,
>>>          "(-1=use per-chip default, 0=disabled, 1=enabled, 2=match any register, 3=always locked) "
>>>          "Default: -1");
>>>  
>>> +intel_display_param_named_unsafe(enable_dmc_wl_debugfs, bool, 0400,
>>> +        "Enable DMC wakelock debugfs"
>>> +        "(0=disabled, 1=enabled) "
>>> +        "Default: 0");
>>> +
>>>  __maybe_unused
>>>  static void _param_print_bool(struct drm_printer *p, const char *driver_name,
>>>                                const char *name, bool val)
>>> diff --git a/drivers/gpu/drm/i915/display/intel_display_params.h b/drivers/gpu/drm/i915/display/intel_display_params.h
>>> index 5317138e6044..cb7dc1bc6846 100644
>>> --- a/drivers/gpu/drm/i915/display/intel_display_params.h
>>> +++ b/drivers/gpu/drm/i915/display/intel_display_params.h
>>> @@ -48,6 +48,7 @@ struct drm_printer;
>>>          param(bool, psr_safest_params, false, 0400) \
>>>          param(bool, enable_psr2_sel_fetch, true, 0400) \
>>>          param(int, enable_dmc_wl, -1, 0400) \
>>> +        param(bool, enable_dmc_wl_debugfs, false, 0400) \
>>>  
>>>  #define MEMBER(T, member, ...) T member;
>>>  struct intel_display_params {
>>> diff --git a/drivers/gpu/drm/i915/display/intel_dmc_wl_debugfs.c b/drivers/gpu/drm/i915/display/intel_dmc_wl_debugfs.c
>>> index 1493d296ac98..f4e4c7a5a730 100644
>>> --- a/drivers/gpu/drm/i915/display/intel_dmc_wl_debugfs.c
>>> +++ b/drivers/gpu/drm/i915/display/intel_dmc_wl_debugfs.c
>>> @@ -37,6 +37,9 @@
>>>   * which exports a buffer of untracked register offsets and also allows extra
>>>   * register offsets to be tracked by the driver.
>>>   *
>>> + * The debugfs directory is only exported if the module parameter
>>> + * enable_dmc_wl_debugfs=1 is passed.
>>> + *
>>>   * Untracked offsets
>>>   * -----------------
>>>   *
>>> @@ -411,6 +414,9 @@ void intel_dmc_wl_debugfs_register(struct intel_display *display)
>>>  {
>>>          struct dentry *dir;
>>>  
>>> +        if (!display->params.enable_dmc_wl_debugfs)
>>> +                return;
>>> +
>>>          if (!HAS_DMC_WAKELOCK(display))
>>>                  return;
>>>  
>>> @@ -453,6 +459,9 @@ void intel_dmc_wl_debugfs_log_untracked(struct intel_display *display, u32 offse
>>>          struct intel_dmc_wl_dbg *dbg = &display->wl.dbg;
>>>          unsigned long flags;
>>>  
>>> +        if (!display->params.enable_dmc_wl_debugfs)
>>> +                return;
>>> +
>>>          spin_lock_irqsave(&dbg->lock, flags);
>>>  
>>>          if (!dbg->untracked.size)
>>> @@ -479,9 +488,14 @@ void intel_dmc_wl_debugfs_log_untracked(struct intel_display *display, u32 offse
>>>  bool intel_dmc_wl_debugfs_offset_in_extra_ranges(struct intel_display *display, u32 offset)
>>>  {
>>>          struct intel_dmc_wl_dbg *dbg = &display->wl.dbg;
>>> -        bool ret = false;
>>> +        bool ret;
>>>          unsigned long flags;
>>>  
>>> +        if (!display->params.enable_dmc_wl_debugfs)
>>> +                return false;
>>> +
>>> +        ret = false;
>>> +
>>>          spin_lock_irqsave(&dbg->lock, flags);
>>>  
>>>          if (!dbg->extra_ranges)
>>
>>-- 
>>Jani Nikula, Intel

-- 
Jani Nikula, Intel