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

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Message-ID: <20121110210118.GA26809@avionic-0098.mockup.avionic-design.de>
Date:	Sat, 10 Nov 2012 22:01:18 +0100
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Christian König <deathsimple@...afone.de>
Cc:	Rafał Miłecki <zajec5@...il.com>,
	linux-tegra@...r.kernel.org, Dave Airlie <airlied@...hat.com>,
	devicetree-discuss@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 2/2] drm: tegra: Add HDMI support

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?

Thierry

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