[alsa-devel] [PATCH 3/3] ASoC: rt5645: Wait for 400msec before concluding on value of RT5645_VENDOR_ID2

[Mark Brown]

On Fri, Nov 03, 2017 at 04:35:45PM -0400, Alex Deucher wrote:

> Minimum time required between power On of codec and read
> of RT5645_VENDOR_ID2 is 400msec. We should wait and attempt
> before erroring out.

So the description says we have to wait 400ms before attempting a
read...

> BUG=b:66978383

What does this mean?

> @@ -3786,6 +3789,15 @@ static int rt5645_i2c_probe(struct i2c_client *i2c,
>  	}
>  	regmap_read(regmap, RT5645_VENDOR_ID2, &val);
>  
> +	/*
> +	 * Read for 400msec, as it is the interval required between
> +	 * read and power On.
> +	 */
> +	while (val != RT5645_DEVICE_ID && val != RT5650_DEVICE_ID && --timeout) {
> +		msleep(1);
> +		regmap_read(regmap, RT5645_VENDOR_ID2, &val);
> +	}
> +

...but what we actually do is try to read up to 400 times starting well
before that 400ms is up.  This directly contradicts what the commit
message said we needed, may take a lot longer if the chip misbehaves on
the I2C bus while it's not ready (which wouldn't be that much of a
surprise), might lead to us reporting before the chip is really stable
(if the read happens to work while the chip isn't yet stable) and could
cause lots of noise on the console if the I2C controller gets upset.
What are we actually waiting for here?

If we really need 400ms of dead reckoning time (which is a lot for a
modern chip, that feels more like a VMID ramp) then it's going to be
safer to just do that.
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