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

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Message-ID: <CAAObsKA4OKw8qtJJ6++QaSw_4bwKXMP4z2HS2OKmBHqbAoj9JQ@mail.gmail.com>
Date:   Wed, 16 Nov 2016 15:04:20 +0100
From:   Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     David Airlie <airlied@...ux.ie>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.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 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.

Regards,

Tomeu