linux-kernel - Re: [PATCH v2 4/4] arm64: dts: rockchip: use cs-gpios for cros_ec

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Message-ID: <20170620004739.GA67314@google.com>
Date:   Mon, 19 Jun 2017 17:47:40 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     Mark Brown <broonie@...nel.org>
Cc:     Jeffy Chen <jeffy.chen@...k-chips.com>,
        linux-kernel@...r.kernel.org, dianders@...omium.org,
        heiko@...ech.de, devicetree@...r.kernel.org,
        linux-rockchip@...ts.infradead.org,
        Rob Herring <robh+dt@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v2 4/4] arm64: dts: rockchip: use cs-gpios for cros_ec_spi

Hi Mark,

Forgot to follow up here:

On Tue, Jun 13, 2017 at 07:22:25PM +0100, Mark Brown wrote:
> On Tue, Jun 13, 2017 at 10:50:44AM -0700, Brian Norris wrote:
> > On Tue, Jun 13, 2017 at 01:25:43PM +0800, Jeffy Chen wrote:
> > > The cros_ec requires CS line to be active after last message. But the CS
> > > would be toggled when powering off/on rockchip spi, which breaks ec xfer.
> > > Use GPIO CS to prevent that.
> 
> > I suppose this change is fine. (At least, I don't have a good reason not
> > to do this.)
> 
> > But I still wonder whether this is something that the SPI core can be
> > expected to handle. drivers/mfd/cros_ec_spi.c already sets the
> > appropriate trans->cs_change bits, to ensure CS remains active in
> > between certain messages (all under spi_bus_lock()). But you're
> > suggesting that your bus controller may deassert CS if you runtime
> > suspend the device (e.g., in between messages).
> 
> > So, is your controller just peculiar? Or should the SPI core avoid
> > autosuspending the bus controller when it's been instructed to keep CS
> > active? Any thoughts Mark?
> 
> This sounds like the controller being unusual - though frankly the
> ChromeOS chip select usage is also odd so it's fairly rare for something
> like this to come up.  I'd not expect a runtime suspend to loose the pin
> state, though possibly through use of pinctrl rather than the
> controller.

I haven't personally verified this behavior (it probably wouldn't be too
hard to rig up a test driver to hold CS low while allowing the
controller to autosuspend? spidev can do this?), but Rockchip folks seem
to have concluded this.

I suppose I'm fine with relying on cs-gpios as a workaround.

Brian