linux-kernel - Re: [PATCH] spi: stm32: enable controller before asserting CS

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Message-ID: <9aa5e049-bd1c-41a6-b9b8-037ebb4f54b8@sirena.org.uk>
Date:   Mon, 4 Dec 2023 12:43:42 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Ben Wolsieffer <ben.wolsieffer@...ring.com>
Cc:     linux-spi@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Alain Volmat <alain.volmat@...s.st.com>
Subject: Re: [PATCH] spi: stm32: enable controller before asserting CS

On Fri, Dec 01, 2023 at 06:11:36PM -0500, Ben Wolsieffer wrote:
> On Fri, Dec 01, 2023 at 09:50:33PM +0000, Mark Brown wrote:
> > On Fri, Dec 01, 2023 at 04:40:14PM -0500, Ben Wolsieffer wrote:

> > This feels like it'd be a good fit for moving to runtime PM - that way
> > we avoid bouncing the controller on and off between messages which is
> > probably better anyway.  The driver already does pinctrl management for
> > the device there.

> Yes, that probably makes sense. There are a few bits that can only be
> configured while the controller is disabled, but it doesn't look like
> that applies to any of the ones set in stm32_spi_prepare_msg().

> I'm a little hesitant to make big changes to the driver since I can only
> test them on an STM32F7 though.

It doesn't seem much more complex than what you're already proposing.

> > It also occurs to me that this isn't going to work for devices which
> > chip select inverted - for them we can't stop driving chip select at all
> > since they need it held high when idle.  There aren't that many such
> > devices and it'd loose us the PM which is rather awkward...  I guess
> > that's an incremental issue with a more invasive fix though.

> The driver only supports GPIO chip select rather than native, so I don't
> think this is a problem. Also, I don't think there's any difference

So mentioning the drive seems a bit confusing then?

> between inverted or uninverted here. They both either need to be driven
> all the time or have pull-up/downs.

It's a lot more likely you'll get away with things one way or another
for a missing pull down.

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