Re: [alsa-devel] [PATCH 12/15] drm/edid: Add API to help find connection type

[Thierry Reding <thierry.reding@gmail.com>]

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On Thu, Dec 03, 2015 at 01:09:16PM +0200, Jani Nikula wrote:
> On Thu, 03 Dec 2015, "Subhransu S. Prusty" <subhransu.s.prusty@intel.com> wrote:
> > On Wed, Dec 02, 2015 at 06:07:01PM +0100, Thierry Reding wrote:
> >> On Wed, Dec 02, 2015 at 11:53:02AM +0200, Jani Nikula wrote:
> >> > On Tue, 01 Dec 2015, "Subhransu S. Prusty" <subhransu.s.prusty@intel.com> wrote:
> >> > > To fill the audio infoframe it is required to identify the connection type
> >> > > as DP or HDMI. So parse the required bits in ELD to find the connection
> >> > > type.
> >> > >
> >> > > Signed-off-by: Subhransu S. Prusty <subhransu.s.prusty@intel.com>
> >> > > Signed-off-by: Vinod Koul <vinod.koul@intel.com>
> >> > > Cc: David Airlie <airlied@linux.ie>
> >> > > Cc: dri-devel@lists.freedesktop.org
> >> > > Cc: Daniel Vetter <daniel.vetter@intel.com>
> >> > > ---
> >> > >  include/drm/drm_edid.h | 10 ++++++++++
> >> > >  1 file changed, 10 insertions(+)
> >> > >
> >> > > diff --git a/include/drm/drm_edid.h b/include/drm/drm_edid.h
> >> > > index 2af9769..c7595a5 100644
> >> > > --- a/include/drm/drm_edid.h
> >> > > +++ b/include/drm/drm_edid.h
> >> > > @@ -403,6 +403,16 @@ static inline int drm_eld_size(const uint8_t *eld)
> >> > >  	return DRM_ELD_HEADER_BLOCK_SIZE + eld[DRM_ELD_BASELINE_ELD_LEN] * 4;
> >> > >  }
> >> > >  
> >> > > +/**
> >> > > + * drm_eld_get_conn_type - Get device type hdmi/dp connected
> >> > > + * @eld: pointer to an eld memory structure
> >> > > + */
> >> > > +static inline int drm_eld_get_conn_type(const uint8_t *eld)
> >> > > +{
> >> > > +	return (eld[DRM_ELD_SAD_COUNT_CONN_TYPE] & DRM_ELD_CONN_TYPE_MASK) >>
> >> > > +		DRM_ELD_CONN_TYPE_SHIFT;
> >> > > +}
> >> > 
> >> > I'm not sure how much this helps when the caller still needs to
> >> > magically know what the return value means...  Indeed the next patch
> >> > with /* 0 is hdmi and 1 is DP */ and "conn_type == 0" is a bit ugly.
> >> > 
> >> > How about just not shifting the return value, and using
> >> > DRM_ELD_CONN_TYPE_HDMI and DRM_ELD_CONN_TYPE_DP in the caller? Bonus
> >> > points for referencing those in the kernel-doc above.
> >> 
> >> We already have a similar function for detecting HDMI vs. DVI (see the
> >> drm_detect_hdmi_monitor()), so perhaps adhering to that convention might
> >> be preferable. This could be:
> >> 
> >> 	static inline bool drm_eld_detect_dp(const u8 *eld)
> >> 	{
> >> 		u8 type = eld[DRM_ELD_SAD_COUNT_CONN_TYPE] & DRM_ELD_CONN_TYPE_MASK;
> >> 
> >> 		return type == DRM_ELD_CONN_TYPE_DP;
> >> 	}
> >
> > With this approach it needs two APIs to be added for HDMI or DP
> > detection. So I prefer what Jani suggested and caller compares
> > whether it is HDMI/DP connection type. Will updae the kernel doc
> > for the same as well.
> 
> I presume Thierry means you'd assume HDMI if drm_eld_detect_dp() returns
> false.

Yes, that's what I was implying. This is probably highly subjective, but
I personally find boolean return values much easier to deal with because
of the limited set of values. With drm_eld_get_conn_type() you'd need to
explicitly check == DRM_ELD_CONN_TYPE_HDMI and == DRM_ELD_CONN_TYPE_DP
and then still have special code to reject all other values. Unless of
course if you consider != DRM_ELD_CONN_TYPE_DP as being HDMI, in which
case a boolean is much more concise.

But like I said, this is just my opinion, and I don't feel strongly
enough to object to the current patch.

Thierry

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