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

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Date:   Mon, 9 Jan 2023 00:06:13 -0800
From:   William Zhang <william.zhang@...adcom.com>
To:     Rob Herring <robh+dt@...nel.org>
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

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@...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.
> 
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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