Re: drivers/video/hdmi.c: Source Product Description (SPD) Information frame logging

[Thierry Reding <treding@nvidia.com>]

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On Tue, Aug 26, 2014 at 03:31:02PM +0200, Damien Lespiau wrote:
> On Tue, Aug 26, 2014 at 12:46:32PM +0200, Martin Bugge (marbugge) wrote:
> > Hello Damien
> > 
> > I'm writing to you as you seems to be one of latest maintainers of:
> > 
> > include/linux/hdmi.h
> > drivers/video/hdmi.c
> > 
> > I wanted to add "Source Product Description information frame"
> > logging for the drivers in
> > 
> > ./drivers/media/i2c/adv7604.c
> > ./drivers/media/i2c/adv7842.c
> > 
> > But was advised to contact you and use some of the defines in
> > include/linux/hdmi.h
> > 
> > As you can see in:
> > 
> > http://www.spinics.net/lists/linux-media/msg74727.html
> > 
> > Would you consider the addition of a hdmi_spd_infoframe_log function
> > in hdmi.c a good idea ?
> > 
> > And the same goes for other HDMI Information frames.
> 
> I don't see why not. Currently the code there is somewhat geared for
> writing HDMI info frames, not really reading back/debug.
> 
> The various structures are not a 1:1 mapping with how the fields are
> actually represented in an info frame packet, so you get a _pack()
> function to serialize the info frame structures. Something to
> consider, then, for your logging API would be to either:
> 
>   - Give a buffer/size to your log() function and decode a raw buffer
>   - make an _unpack() function that takes a raw buffer and translate it
>     into one of the various infoframe structure and then have a _log
>     function that operates on the "unpacked" structure.
> 
> Another thing to keep in mind is that those "unpacked" versions of the
> infoframes don't actually have all the fields. eg.
> 
>        /*
>          * Data byte 1, bit 4 has to be set if we provide the active format
>          * aspect ratio
>          */
>         if (frame->active_aspect & 0xf)
>                 ptr[0] |= BIT(4);
> 
> 	[...]
> 
>         ptr[1] = ((frame->colorimetry & 0x3) << 6) |
>                  ((frame->picture_aspect & 0x3) << 4) |
>                  (frame->active_aspect & 0xf);
> 
> You can see that active_aspect needs two fields set, a "active aspect valid"
> bit (data byte 1, bit 4) and the actual active_aspect value. To prevent the
> user of the API to generate invalid infoframes (ie have a active_aspect value
> but forgetting to set the valid bit), I decided to "hide" the valid bit and set
> it automatically if active_aspect is set. This isn't just theoritical, we've
> had the bug that forgot the valid bit before.
> 
> That's the details I can think about atm, I don't seen any reason to not
> improve that code to be able to dump info frames. I have a slight preference to
> have an _unpack() version and have the _log() functions operate on the
> "expoded" structures, maybe Thierry/Paulo (in Cc.) have some input as well.

I do have a preference for adding an _unpack() variant, too. Although as
you pointed out there could be difficulties in reconstructing that from
the raw data given that we don't have a 1:1 encoding. For example, what
does the _unpack() do if the active aspect is set but not the valid bit?

One advantage perhaps would be that _unpack()/_pack() could act as a
sanitizer.

Thierry

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