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 18:04:08 +0100
From: Mark Brown <broonie@...nel.org>
To: David Lechner <dlechner@...libre.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, 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 Wed, Jun 05, 2024 at 11:37:48AM -0500, David Lechner wrote:
> On 6/5/24 7:24 AM, Mark Brown wrote:
> > 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.

> I think this description is missing a key detail that the tx data line
> needs to be high just before and also during the CS assertion at the start
> of each message.

> And it would be helpful to have this more detailed description in the
> source code somewhere and not just in the commit message.

Yes, there's no way anyone could infer this from any aspect of the
series.  I think the properties also need a clearer name since someone
might want the accelerator functionality at some point.

> > 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.

> I could see something like this working for controllers that leave the
> tx data line in the state of the last bit of a write transfer. I.e. do a
> write transfer of 0xff (using the smallest number of bits per word
> supported by the controller) with CS not asserted, then assert CS, then
> do the rest of actual the transfers requested by the peripheral.

> But it doesn't seem like it would work for controllers that always
> return the tx data line to a low state after a write since this would
> mean that the data line would still be low during the CS assertion
> which is what we need to prevent.

With the additional requirement it's not emulatable, but we'd still need
the checks in the core.

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