Re: [PATCH 1/5] drm/edid: convert drm_parse_hdmi_vsdb_video to use struct cea_db *

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Oct 28, 2024 at 03:45:07PM +0200, Jani Nikula wrote:
> On Sun, 27 Oct 2024, Vamsi Krishna Brahmajosyula <vamsikrishna.brahmajosyula@gmail.com> wrote:
> > @@ -6320,19 +6321,20 @@ static void drm_parse_hdmi_deep_color_info(struct drm_connector *connector,
> >  
> >  /* HDMI Vendor-Specific Data Block (HDMI VSDB, H14b-VSDB) */
> >  static void
> > -drm_parse_hdmi_vsdb_video(struct drm_connector *connector, const u8 *db)
> > +drm_parse_hdmi_vsdb_video(struct drm_connector *connector, const struct cea_db *db)
> >  {
> >  	struct drm_display_info *info = &connector->display_info;
> >  	u8 len = cea_db_payload_len(db);
> > +	const u8 *data = cea_db_data(db);
> >  
> >  	info->is_hdmi = true;
> >  
> > -	info->source_physical_address = (db[4] << 8) | db[5];
> > +	info->source_physical_address = (data[3] << 8) | data[4];
> >  
> >  	if (len >= 6)
> > -		info->dvi_dual = db[6] & 1;
> > +		info->dvi_dual = data[5] & 1;
> 
> Just commenting on one hunk, because it's a good example of the whole
> series I think.
> 
> The above is nice, because it improves the offset vs. length
> comparisons. Many of the old checks like above look like off-by-ones,
> when indexing from the beginning of the data block, not from the
> beginning of payload, and cea_db_payload_len() excludes the first byte.
> 
> The main problem is that the specs are written with indexing from the
> beginning of the data block. For example, HDMI 1.4 table 8-16 defining
> the HDMI VSDB says source physical address is at byte offsets 4 and 5,
> and dvi dual flag at byte offset 6. That will no longer be the case in
> code. It gets tricky to review when you have to keep adjusting the
> offsets in your head. (I don't remember if there are specs that specify
> the offsets starting from the "actual" payload after all the meta stuff
> has been removed.)

IIRC there was some off-by-one indexing difference between
some of the specs. But I don't remember which ones use what.

> 
> Now, if we accept having to do that mental acrobatics, why stop there?
> You also have extended tags (first payload byte is the tag), as well as
> vendor tags (first three payload bytes are the OUI). It begs the
> question whether there should be higher level data and length helpers
> that identify and remove the tags (including extended tags and OUI
> stuff). For example, the actual data for HDMI VSDB starts at payload
> offset 3, as the first three bytes are the HDMI OUI.
> 
> What to do? Ville, thoughts?

So just different *_{data,len}() for the different indexing variants
(as defined by the relevant spec)? That seems like a reasonable
apporach as then the len vs. index checks might actually make sense.

-- 
Ville Syrjälä
Intel