linux-kernel - Re: [PATCH 03/16] dt-bindings: spi: Add spi peripheral specific property

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Date:   Sat, 7 Jan 2023 09:38:24 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     William Zhang <william.zhang@...adcom.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Linux SPI List <linux-spi@...r.kernel.org>,
        Broadcom Kernel List <bcm-kernel-feedback-list@...adcom.com>,
        anand.gore@...adcom.com, tomer.yacoby@...adcom.com,
        dan.beygelman@...adcom.com, joel.peshkin@...adcom.com,
        f.fainelli@...il.com, jonas.gorski@...il.com,
        kursad.oney@...adcom.com, dregan@...l.com,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/16] dt-bindings: spi: Add spi peripheral specific property

On Fri, Jan 6, 2023 at 9:27 PM William Zhang <william.zhang@...adcom.com> wrote:
>
> Hi Mark,
>
> On 01/06/2023 01:14 PM, Mark Brown wrote:
> > On Fri, Jan 06, 2023 at 12:07:55PM -0800, William Zhang wrote:
> >
> >> brcm,no-clk-gate is a Broadcom Broadband HS SPI controller specific
> >> property for certain SPI device such as Broadcom ISI voice daughtercard
> >> to work properly. It disables the clock gating feature when the chip
> >> select is deasserted for any device that wants to keep the clock
> >> running.
> >
> > Why would this property be Broadcom specific?  Other devices could in
> > theory implement this.
> >
> It does not need to be Broadcom specific if other SoC's SPI bus
> controller support such function. I am not aware of such case but
> certainly I am no expert on other chips. I can put it in the generic
> spi-peripheral-props.yaml if that is what you suggest.
>
> >> +properties:
> >> +  brcm,no-clk-gate:
> >> +    $ref: /schemas/types.yaml#/definitions/flag
> >> +    description:
> >> +      Some SPI device such as Broadcom ISI based voice daughtercard requires SPI
> >> +      clock running even when chip select is deasserted. By default the
> >> +      controller turns off or gate the clock when cs is not active to save
> >> +      power. This flag tells the controller driver to keep the clock running
> >> +      when chip select is not active.
> >
> > This seems problematic with any host controlled chip select support...
> >
> Yes those ISI chip based voice cards do need such strange requirement
> and will not work with other controller.  That is one of the reason I
> put this as Broadcom specific option.

Keeping the clock on or not would affect all devices unless you have a
per device clock you can gate, so making this a per device flag
doesn't make sense.

If this is a requirement of the slave device, then the device's
compatible string can imply the need for this and its driver can tell
the host controller in some way.

Rob