linux-kernel - Re: [PATCH v3 1/6] spi: Add SPI mode bit for MOSI idle state configuration

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Date: Wed, 5 Jun 2024 13:24:33 +0100
From: Mark Brown <broonie@...nel.org>
To: Marcelo Schmitt <marcelo.schmitt@...log.com>
Cc: lars@...afoo.de, Michael.Hennerich@...log.com, jic23@...nel.org,
	robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
	conor+dt@...nel.org, nuno.sa@...log.com, dlechner@...libre.com,
	marcelo.schmitt1@...il.com, linux-iio@...r.kernel.org,
	devicetree@...r.kernel.org, linux-spi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/6] spi: Add SPI mode bit for MOSI idle state
 configuration

On Tue, Jun 04, 2024 at 07:41:47PM -0300, Marcelo Schmitt wrote:

> The behavior of an SPI controller data output line (SDO or MOSI or COPI
> (Controller Output Peripheral Input) for disambiguation) is not specified
> when the controller is not clocking out data on SCLK edges. However, there
> exist SPI peripherals that require specific COPI line state when data is
> not being clocked out of the controller.

This is an optimisation for accelerating devices that need a specific
value, really if these devices need a value they should send it.

>  #define SPI_MOSI_IDLE_LOW	_BITUL(17)	/* leave mosi line low when idle */
> +#define SPI_MOSI_IDLE_HIGH	_BITUL(18)	/* leave mosi line high when idle */

Realistically we'll have a large set of drivers that are expecting the
line to be held low so I'm not sure we need that option.  I would also
expect to have an implementation of these options in the core which
supplies buffers with the relevant data for use with controllers that
don't have the feature (similar to how _MUST_TX and _MUST_RX are done).
Even without that we'd need feature detection so that drivers that try
to use this aren't just buggy when used with a controller that doesn't
implement it, but once you're detecting you may as well just make things
work.

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