linux-kernel - Re: [PATCH 4/7] ASoC: soc-dai: define TDM idle behaviour modes

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Message-ID: <aTjZc3mgu7CQOkDU@sirena.co.uk>
Date: Wed, 10 Dec 2025 11:22:43 +0900
From: Mark Brown <broonie@...nel.org>
To: James Calligeros <jcalligeros99@...il.com>
Cc: Liam Girdwood <lgirdwood@...il.com>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, Shawn Guo <shawnguo@...nel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Pengutronix Kernel Team <kernel@...gutronix.de>,
	Fabio Estevam <festevam@...il.com>,
	Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>,
	Shengjiu Wang <shengjiu.wang@....com>,
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
	Shenghao Ding <shenghao-ding@...com>, Kevin Lu <kevin-lu@...com>,
	Baojun Xu <baojun.xu@...com>, linux-sound@...r.kernel.org,
	devicetree@...r.kernel.org, imx@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	asahi@...ts.linux.dev
Subject: Re: [PATCH 4/7] ASoC: soc-dai: define TDM idle behaviour modes

On Tue, Dec 09, 2025 at 07:31:17PM +1000, James Calligeros wrote:

> Imagine for example two codecs sharing a bus. When one codec is
> transmitting, the other must ensure that it is holding its side
> to 0, or data from the transmitting codec will be corrupted. We
> can trust the "idle" codec to simply do this itself, however
> this is undefined behaviour. Some devices may leave the line
> floating, others still may pull the line high. We need a way to
> control this behaviour.

I'm finding this explanation quite confusing - you appear to be
describing the situation where two devices simultaneously drive the same
signal which would be extremely unusual and I can't see how it would
work electrically.

> Thus, we define five possible bus-keeping modes that a device can
> be in: NONE (UB/as initialised), OFF (explicitly disabled), ZERO
> (actively transmit a 0), PULLDOWN, and HIZ (floating).

What is "explicitly disabled" in ths context?  Why aren't pull up or
drive high options?

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