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

[William Zhang]

Hi Rob,

On 01/07/2023 07:38 AM, Rob Herring wrote:
> On Fri, Jan 6, 2023 at 9:27 PM William Zhang <william.zhang@xxxxxxxxxxxx> 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.
This applies only to each chip select. There is only one device under 
each chip select.  So won't impact any other devices under other cs.

> 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.
That is true but spi host controller driver reads and parses these slave 
device flag directly.
> Rob

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