[alsa-devel] [PATCH v2 3/3] ASoC: add generic dt-card support

[Lars-Peter Clausen]

On 01/23/2015 07:34 PM, Jean-Francois Moine wrote:
> On Fri, 23 Jan 2015 14:56:04 +0100
> Lars-Peter Clausen <lars at metafoo.de> 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