linux-kernel - Re: [PATCH 2/2] drm: tegra: Add HDMI support

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Message-ID: <20121112072410.GA925@avionic-0098.mockup.avionic-design.de>
Date:	Mon, 12 Nov 2012 08:24:10 +0100
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Christian König <deathsimple@...afone.de>,
	devicetree-discuss@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org,
	Dave Airlie <airlied@...hat.com>
Subject: Re: [PATCH 2/2] drm: tegra: Add HDMI support

On Sun, Nov 11, 2012 at 03:46:44PM +0100, Daniel Vetter wrote:
> On Sat, Nov 10, 2012 at 10:01:18PM +0100, Thierry Reding wrote:
> > On Fri, Nov 09, 2012 at 05:00:54PM +0100, Christian König wrote:
> > > On 09.11.2012 16:45, Rafał Miłecki wrote:
> > > >2012/11/9 Thierry Reding <thierry.reding@...onic-design.de>:
> > > >>+/* all fields little endian */
> > > >>+struct hdmi_audio_infoframe {
> > > >>+       /* PB0 */
> > > >>+       u8 csum;
> > > >>+
> > > >>+       /* PB1 */
> > > >>+       unsigned cc:3; /* channel count */
> > > >>+       unsigned res1:1;
> > > >>+       unsigned ct:4; /* coding type */
> > > >>+
> > > >>+       /* PB2 */
> > > >>+       unsigned ss:2; /* sample size */
> > > >>+       unsigned sf:3; /* sample frequency */
> > > >>+       unsigned res2:3;
> > > >>+
> > > >>+       /* PB3 */
> > > >>+       unsigned cxt:5; /* coding extention type */
> > > >>+       unsigned res3:3;
> > > >>+
> > > >>+       /* PB4 */
> > > >>+       u8 ca; /* channel/speaker allocation */
> > > >>+
> > > >>+       /* PB5 */
> > > >>+       unsigned res5:3;
> > > >>+       unsigned lsv:4; /* level shift value */
> > > >>+       unsigned dm_inh:1; /* downmix inhibit */
> > > >>+
> > > >>+       /* PB6-10 reserved */
> > > >>+       u8 res6;
> > > >>+       u8 res7;
> > > >>+       u8 res8;
> > > >>+       u8 res9;
> > > >>+       u8 res10;
> > > >>+} __packed;
> > > >I was told it won't work on different endian devices. See
> > > >[RFC][PATCH] drm/radeon/hdmi: define struct for AVI infoframe
> > > >http://lists.freedesktop.org/archives/dri-devel/2012-May/022544.html
> > > 
> > > Yeah, that's indeed true. And honestly adding just another
> > > implementation of the HDMI info frames sounds like somebody should
> > > finally sit down and implement it in a common drm_hdmi.c
> > 
> > So I've been looking at what most other implementations do and it seems
> > a lot just fill the AVI infoframe with zeroes while only a few actually
> > try to put useful information in them. Still in order to plan for a
> > generic solution, I thought maybe something like the below set of
> > structures and functions could work:
> > 
> > /*
> >  * Structure that contains the infoframe fields in a form that allows them to
> >  * be easily accessed from C code.
> >  */
> > struct hdmi_avi_infoframe;
> > 
> > /*
> >  * DRM helper to fill a struct hdmi_avi_infoframe with information taken from
> >  * a struct drm_display_mode. Fields that cannot automatically be derived by
> >  * looking at a struct drm_display_mode are set to the default values.
> >  */
> > int drm_hdmi_avi_infoframe_from_display_mode(struct hdmi_avi_infoframe *frame,
> > 					     struct drm_display_mode *mode);
> > 
> > /*
> >  * Packs the struct hdmi_avi_infoframe into a binary buffer that can be
> >  * programmed to the hardware-specific registers.
> >  */
> > ssize_t hdmi_avi_infoframe_pack(struct hdmi_avi_infoframe *frame,
> > 				void *buffer, size_t size);
> > 
> > Such a scheme would allow DRM drivers to call the helper and tweak the
> > fields in the structure if the want or need to and call the packing
> > function to obtain a buffer that they can write to the controller.
> > 
> > Does that sound at all reasonable?
> 
> Sounds good, especially the disdinction between the infoframe creation and
> packing. E.g. on intel sdvo outputs we may not put in one of the ECC bytes
> (since the hw creates it), so we need our own packing code there.

Actually what I had in mind was a packed binary representation of
infoframes as specified by HDMI 1.3a (I don't have access to 1.4, but I
would think it doesn't differ in this respect) in section 5.3 and 5.3.5
more specifically. According to the specification, the ECC bytes only
come into play at a later stage, when data is actually transmitted on
the TMDS link (Section 5.2.3). Tegra, nouveau and radeon also seem to be
doing the checksumming in hardware, so I guess we don't need to compute
the ECC bytes in software at all (for now).

Once we have this for the AVI infoframes I guess the same concept can be
used for audio infoframes and for vendor-specific infoframes (for HDMI
1.4 3D).

Thierry

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