linux-kernel - Re: [PATCH] ASoC: Add support for multi register mux

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Message-ID: <20140327012913.GC30768@sirena.org.uk>
Date:	Thu, 27 Mar 2014 01:29:13 +0000
From:	Mark Brown <broonie@...nel.org>
To:	Arun Shamanna Lakshmi <aruns@...dia.com>
Cc:	lgirdwood@...il.com, swarren@...dotorg.org, perex@...ex.cz,
	tiwai@...e.de, alsa-devel@...a-project.org,
	linux-kernel@...r.kernel.org, Songhee Baek <sbaek@...dia.com>
Subject: Re: [PATCH] ASoC: Add support for multi register mux

On Tue, Mar 25, 2014 at 05:02:35PM -0700, Arun Shamanna Lakshmi wrote:

> +	}
> +	if (!match) {
> +		dev_err(codec->dev, "ASoC: Failed to find matched enum value\n");
> +		return -EINVAL;
> +	} else
> +		ucontrol->value.enumerated.item[0] = i;

Coding style nit: if one side of the if has braces both should.  Most of
this code could also use more blank lines.

> +	for (reg_idx = 0; reg_idx < e->num_regs; reg_idx++) {
> +		val = e->values[item * e->num_regs + reg_idx];
> +		ret = snd_soc_update_bits_locked(codec, e->reg[reg_idx],
> +						e->mask[reg_idx], val);
> +		if (ret)
> +			return ret;
> +	}

So, this is a bit interesting.  It will update one register at a time
which means that we are likely to transiently set an invalid value
sometimes which might not make the hardware happy or may cause us to
write a valid value with undesirable consequences.  I'd expect to see
some handling of this, some combination of providing a safe value that
the hardware could be reset to prior to change and doing a bulk write to
all the registers simultaneously if we can (I know sometimes hardware
has special handling for atomic updates of multi-register values in a
single block transfer).

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