Re: [alsa-devel] [PATCH v2 3/3] ASoC: add generic dt-card support
From: Lars-Peter Clausen
Date: Sat Jan 24 2015 - 06:27:44 EST
On 01/23/2015 07:34 PM, Jean-Francois Moine wrote:
On Fri, 23 Jan 2015 14:56:04 +0100
Lars-Peter Clausen <lars@xxxxxxxxxx> wrote:
On 01/23/2015 01:15 PM, Jean-Francois Moine wrote:
[...]
The DT should describe the hardware, and the simple-card mixes hardware
and software.
For example, the kirkwood controller may create 2 CPU DAIs. With the
simple-card, the DT contains a number to reference these DAIs (for
example, implicitly, <audio1 0> references the I2S output). So, what if
the controller creates only one DAI, or, what if the FreeBSD/OpenBSD/..
driver does not set the same references to these DAIs?
The graph of port fixes this problem.
Even with the simple-card bindings there are no software specific bits. The
DAI that is referenced in your example is the physical DAI as it exists in
the hardware. Which DAI maps to which specifier is defined in the devicetree
bindings definition for the hardware unit.
There is no first or second output in the kirkwood controller; there
are I2S and S/PDIF outputs. Their hardware controls are done by bits in
the different registers. Then, the driver may define its DAIs in any
order and assign any DAI ID to these DAIs. So, for a given DAI, there
is no relation between its type (I2s or S/PDIF), index in the dai
driver (= asoc component index) and ID.
In the simple-card DT, the CPU DAI (as the CODEC DAI) is referenced by
a phandle (the audio-controller) and a number which should identify the
DAI. This number is translated to a DAI name thanks to the function
snd_soc_get_dai_name(). This function scans the components and returns
the name of the DAI (component) supported by the audio controller with
the searched number equal to the component index (there is no 'xlate'
function).
So, with the simple-card, the CPU or CODEC DAI binding (phandle +
number) identifies what the software has put in its table, but not a
hardware entity.
Are you talking about the kirkwood driver or about things in general?
What you are saying is not true in general, if it is true for the kirkwood
driver than the driver is broken in regards to DT bindings.
The specifier that follows after the phandle is hardware specific. This can
be any number of integers. And the meaning of this is defined in the
bindings specification for that hardware.
Many specifications define a binding in which either no specifier needs to
be supplied, since there is only a single DAI, or one integer specifier
which denotes which DAI is referenced for a multi-DAI controller. In the
later case the bindings specification has to state which number maps to
which physical DAI. Out of convenience many drivers use the same order as
given in the bindings specification for their ASoC DAIs. Which is why the
ASoC core provides a standard implementation for translating phandle +
specifier to a ASoC DAI. But it is important to remember that the
authoritative entity here is the bindings specification and not the driver.
If the bindings specification uses a different scheme for referencing the
DAIs a custom xlate function needs to be provided by the driver confirms to
this specification.
More: a simple audio card may easily be created from a graph of ports
as the simple-card does, but by the audio-controller (sorry, I also
forgot the kirkwood patch for this in my previous patch request).
In case of complex cards, the links and properties of this graph may
also be used by board specific card devices.
One issue is how to deal with multi-point-to-multi-point links. I2S/TDM is a
bus and can have more than one reader/writer.
The second issue is how to describe the clock and frame master
relationships. Multiple different buses can share the same clock and frame
generator. E.g. typically the capture and playback stream are linked in this
way.
The ports and endpoints may contain properties to describe these
configurations. Complex cases should be handled by specific card
builders.
Could you describe in detail what a card builder is and how to decide when
and how a card builder is executed?
A card builder is a device which
- scans the graph of ports,
- fills the struct snd_soc_card according to the links between the
ports and their properties,
- and, eventually, calls snd_soc_register_card().
The simple card builder, 'dt-card' (maybe a better name would have been
'graph-card'), acts just like the simple-card except that it does not
appear in the DT. Its creation is done by an audio controller.
For complex cards, a card builder would be the same as an actual card
driver, but with the capability to know about the board thanks to the
graph of ports. Its creation could be done by a compatible device in
the DT (as actually) or by the unique or a 'master' audio controller.
A card builder is subject to a PROBE_DEFER after calling
snd_soc_register_card(), so, when it is created by an audio controller,
this last one should have created its own resources.
An audio controller does the following:
- initialize the hardware and software,
- create the DAIs from:
- the graph of ports or
- default values
- register these DAIs and
- if the DAIs where created by a graph of port,
create the card builder.
I don't understand how invoking the card builder from the CPU side audio
controller driver will work. The audio controller driver does not know what
card it is part of, so how does it decide which builder to call, and what
happens in the case of multiple CPU side audio controllers involved in the
same card. In rare cases there might not even be a CPU side audio controller
involved at all.
I think this is something that needs to be done in the ASoC/ALSA core
itself. Create the graph, wait until all endpoints of the graph have been
registered and then create the card. Or something similar.
- Lars
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