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

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Message-ID: <ZW4uWBpEc2_4JR2b@dell-precision-5540>
Date:   Mon, 4 Dec 2023 14:54:00 -0500
From:   Ben Wolsieffer <ben.wolsieffer@...ring.com>
To:     Mark Brown <broonie@...nel.org>
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 Mon, Dec 04, 2023 at 12:43:42PM +0000, Mark Brown wrote:
> 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.

I'm working on a new patch that uses runtime PM and will submit it soon.

> > > 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?

Yes, I should have been more specific in the patch that only MOSI and
CLK float when the controller is disabled and that CS remains driven.