Attempting to upgrade audio hardware. Creating new/modified machine driver.

**Robert Murch** · 10-11-2018

Hey there, so I am not an experienced C programmer just to get that out of the way. However, I am attempting to both learn C and the Linux ASoC system. I am working on replacing some old hardware (codec) with a new one. Specifically, I am attempting to write/adapt a new machine driver for the WM8750 audio codec based off of the existing machine driver that I have for the WM8753 audio codec (deprecated).

I have a few questions:

1) The following define:

Code:

#define WM8753_SSI MASTER        1

What does the 1 mean after the initial deceleration?

2)

Code:

#if WM8753_SSI_MASTER
    dai_format |= SND_SOC_DAIFMT_CBM_CFM;

Bit-wise OR operation I think it is equivalent to:
dai_format = dai_format | SND_SOC_DAIFMT_CBM_CFM
is that right?

3)

Code:

ssi_mode->sync_mode = 1;

Equivalent to: (*xxi_mode).sync_mode = 1... where xxi = ssi (wont let me type what I want to... keeps replacing it with ..........)

I believe, this means: assign the member sync_mode that ssi_mode points to and set its value to 1?

4) :

Code:

channels == 1 ? 0xfffffffe : 0xfffffffc,

I believe this means: if channels == 1 then 0xfffffffe else 0xfffffffc

5) :

Code:

.name = "WM8753"

-What does it mean when a variable has a . in front of it?

6) :

Code:

plat = priv->pdev->dev.platform_data;

plat = (*priv).(*pdev).(*dev.platform_data)??? -not sure what's happening here.

Well that is plenty for now I would say. Hey if anyone wants to help me with the project I can post the old machine driver and the codecs, just let me know!

**Salem** · 10-12-2018

> What does the 1 mean after the initial deceleration?
It means that wherever the pre-processor sees WM8753_SSI_MASTER, it will substitute the value 1

> dai_format = dai_format | SND_SOC_DAIFMT_CBM_CFM
> is that right?
Yes.

> I believe, this means: assign the member sync_mode that ssi_mode points to and set its value to 1?
Yes,
(*ptr).member is the same as ptr->member.

> I believe this means: if channels == 1 then 0xfffffffe else 0xfffffffc
Yes it does.

> -What does it mean when a variable has a . in front of it?
It means assign that member within the structure.
Historically, member assignment is in declaration order, so

Code:

struct foo {
    int a;
    int b;
} = { 0, 1 };

Which is a bit fragile if you want to add more members to the structure.

Whereas

Code:

struct foo {
    int a;
    int b;
} = { .a = 0, .b = 1 };

Will always do the right thing even if the structure changes to say

Code:

struct foo {
    int x;
    int a;
    int y;
    int b;
} = { .a = 0, .b = 1 };

> plat = priv->pdev->dev.platform_data;
> -not sure what's happening here.
Well it's as above.
priv is a pointer pointing to a structure which contains a member called pdev, which is itself a pointer to a dev.

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