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

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Date:   Wed, 16 Nov 2016 14:58:03 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:     David Weinehall <tao@...nel.org>, 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 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

BR,
Jani.

-- 
Jani Nikula, Intel Open Source Technology Center