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SubjectRe: [PATCH 03/16] dt-bindings: spi: Add spi peripheral specific property
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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@broadcom.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.
>
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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