linux-kernel - Re: [Intel-gfx] [PATCH v11 3/4] drm/i915: Use new CRC debugfs API

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Message-ID: <87eg2b1pr5.fsf@intel.com>
Date:   Wed, 16 Nov 2016 16:08:30 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:     David Airlie <airlied@...ux.ie>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel\@lists.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [Intel-gfx] [PATCH v11 3/4] drm/i915: Use new CRC debugfs API

On Wed, 16 Nov 2016, Tomeu Vizoso <tomeu.vizoso@...labora.com> wrote:
> On 16 November 2016 at 13:58, Jani Nikula <jani.nikula@...ux.intel.com> wrote:
>> On Wed, 16 Nov 2016, Tomeu Vizoso <tomeu.vizoso@...labora.com> wrote:
>>> On 15 November 2016 at 09:27, Jani Nikula <jani.nikula@...ux.intel.com> wrote:
>>>> On Tue, 15 Nov 2016, David Weinehall <tao@...nel.org> wrote:
>>>>> On Mon, Nov 14, 2016 at 12:44:25PM +0200, Jani Nikula wrote:
>>>>>> On Thu, 06 Oct 2016, Tomeu Vizoso <tomeu.vizoso@...labora.com> wrote:
>>>>>> > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
>>>>>> > index 23a6c7213eca..7412a05fa5d9 100644
>>>>>> > --- a/drivers/gpu/drm/i915/intel_display.c
>>>>>> > +++ b/drivers/gpu/drm/i915/intel_display.c
>>>>>> > @@ -14636,6 +14636,7 @@ static const struct drm_crtc_funcs intel_crtc_funcs = {
>>>>>> >    .page_flip = intel_crtc_page_flip,
>>>>>> >    .atomic_duplicate_state = intel_crtc_duplicate_state,
>>>>>> >    .atomic_destroy_state = intel_crtc_destroy_state,
>>>>>> > +  .set_crc_source = intel_crtc_set_crc_source,
>>>>>> >  };
>>>>>> >
>>>>>> >  /**
>>>>>> > diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
>>>>>> > index 737261b09110..31894b7c6517 100644
>>>>>> > --- a/drivers/gpu/drm/i915/intel_drv.h
>>>>>> > +++ b/drivers/gpu/drm/i915/intel_drv.h
>>>>>> > @@ -1844,6 +1844,14 @@ void intel_color_load_luts(struct drm_crtc_state *crtc_state);
>>>>>> >  /* intel_pipe_crc.c */
>>>>>> >  int intel_pipe_crc_create(struct drm_minor *minor);
>>>>>> >  void intel_pipe_crc_cleanup(struct drm_minor *minor);
>>>>>> > +#ifdef CONFIG_DEBUG_FS
>>>>>> > +int intel_crtc_set_crc_source(struct drm_crtc *crtc, const char *source_name,
>>>>>> > +                        size_t *values_cnt);
>>>>>> > +#else
>>>>>> > +static inline int intel_crtc_set_crc_source(struct drm_crtc *crtc,
>>>>>> > +                                      const char *source_name,
>>>>>> > +                                      size_t *values_cnt) { return 0; }
>>>>>> > +#endif
>>>>>>
>>>>>> "inline" here doesn't work because it's used as a function pointer.
>>>>>>
>>>>>> Is it better to have a function that returns 0 for .set_crc_source, or
>>>>>> to set .set_crc_source to NULL when CONFIG_DEBUG_FS=n?
>>>>>
>>>>> I'd say that whenever we have a function pointer we should have a dummy
>>>>> function without side-effects for this kind of things.
>>>>
>>>> Whoever calls .set_crc_source could do smarter things depending on the
>>>> hook not being there vs. just silently plunging on.
>>>
>>> In this specific case, when CONFIG_DEBUG_FS=n it doesn't make any
>>> sense to call that callback, so I think we should have a dummy
>>> implementation to avoid adding an ifdef to the .c.
>>
>> We don't want the ifdef to the .c file, but we could do
>>
>> #ifdef CONFIG_DEBUG_FS
>> int intel_crtc_set_crc_source(struct drm_crtc *crtc, const char *source_name,
>>                                 size_t *values_cnt);
>> #else
>> #define intel_crtc_set_crc_source NULL
>> #endif
>
> Sounds good to me, and though I don't have any objections, wonder why
> this isn't a common idiom in the DRM subsystem. I was able to find
> only one instance: drm_compat_ioctl.

Heh, and it was I who suggested that too. Maybe get a second opinion. ;)

BR,
Jani.

>
> Regards,
>
> Tomeu

-- 
Jani Nikula, Intel Open Source Technology Center