linux-kernel - Re: [PATCH v5 1/2] dt-bindings: i2c: mv64xxx: add bus-reset-gpios property

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Date:   Sat, 28 Oct 2023 09:51:58 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Wolfram Sang <wsa@...nel.org>,
        Chris Packham <chris.packham@...iedtelesis.co.nz>,
        krzysztof.kozlowski+dt@...aro.org, gregory.clement@...tlin.com,
        andi.shyti@...nel.org, conor+dt@...nel.org,
        linux-i2c@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Abel Vesa <abel.vesa@...aro.org>
Subject: Re: [PATCH v5 1/2] dt-bindings: i2c: mv64xxx: add bus-reset-gpios
 property

On 27/10/2023 21:22, Rob Herring wrote:
> On Fri, Oct 27, 2023 at 01:25:56PM +0200, Krzysztof Kozlowski wrote:
>> On 27/10/2023 11:09, Wolfram Sang wrote:
>>> On Fri, Oct 27, 2023 at 04:31:03PM +1300, Chris Packham wrote:
>>>> Add bus-reset-gpios and bus-reset-duration-us properties to the
>>>> marvell,mv64xxx-i2c binding. These can be used to describe hardware
>>>> where a common reset GPIO is connected to all downstream devices on and
>>>> I2C bus. This reset will be asserted then released before the downstream
>>>> devices on the bus are probed.
>>>>
>>>> Signed-off-by: Chris Packham <chris.packham@...iedtelesis.co.nz>
>>>> Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
>>>
>>> Krzysztof, are you fine with this change?
>>
>> Actually no. NAK.
>>
>> Not because of the naming, but because the new name triggered some new
>> paths in my brain which brought the point - this is old problem of power
>> sequencing of children.
>>
>> I believe this must be solved in more generic way. First - generic for
>> all I2C devices. Second - generic also matching other buses/subsystems,
>> which have similar problem. We did it for USB (onboard USB), MMC
>> (unloved MMC power sequence) and now we are doing it for PCIe and few
>> others (Cc: Abel)
> 
> Unlike the others I2C doesn't expect to access the bus/device before 
> devices probe, right?
> 
>> https://lpc.events/event/17/contributions/1507/
> 
> Oh, good!
> 
>> Current solution is heavily limited. What about regulators? What about
>> buses having 2 reset lines (still the same bus)? What about sequence?
> 
> A more complicated case should be handled by the device's driver. If the 
> GPIO reset was not shared we'd be handling it there too. I think what's 
> needed is to solve the shared aspect. That's already done with reset 
> subsys, so I think making 'reset-gpios' handled by it too is the way 
> forward. That would handle the QCA WiFi/BT case I think.
> 
> I'm not sure waiting for that or something else to happen is worth 
> holding up this simple case. It's not the only case of a common reset 
> for a bus (MDIO).

I argue also that this bus-reset-gpios is not a property of this I2C
controller. IIUC, the I2C controller does not have a line to reset all
children. It's the children who have reset lines and it happens it is
shared. Just like my WSA884x case:
https://lore.kernel.org/alsa-devel/84f9f1c4-0627-4986-8160-b4ab99469b81@linaro.org/


Best regards,
Krzysztof