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

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Date: Fri, 7 Jun 2024 14:50:00 +0100
From: Mark Brown <broonie@...nel.org>
To: Marcelo Schmitt <marcelo.schmitt1@...il.com>
Cc: Marcelo Schmitt <marcelo.schmitt@...log.com>, 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,
	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 Thu, Jun 06, 2024 at 04:57:52PM -0300, Marcelo Schmitt wrote:

> As far as I searched, the definitions for SPI protocol usually don't specify any
> behavior for the MOSI line when the controller is not clocking out data.
> So, I think SPI controllers that are not capable of implementing any type
> of MOSI idle configuration are anyway compliant to what is usual SPI.
> For those that can implement such feature, I thought peripherals could request
> it by setting SPI mode bits.

The issue here is the one that Richard highlighted with it not being
clear exactly what the intended behaviour is.

> But yeah, it's not that evident what this patch set is all about and why this is
> wanted so I made a wiki page to explain the reasoning for this set.
> https://wiki.analog.com/software/linux/docs/spi/spi_copi_idle?rev=1717699755
> Hopefully the figures with timing diagrams and transfer captures there will 
> provide quicker understanding of this rather than I try to explain it with
> only text.

It needs to be apparent to someone looking at the kernel what the code
is intended to do.

> If you still think we need feature detection for MOSI idle capability just let
> me know, I'll implement what be needed.

If the devices actually require this mode then we can't just randomly
ignore them when they request it.

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