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

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Date:   Fri, 27 Oct 2023 14:22:36 -0500
From:   Rob Herring <robh@...nel.org>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.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 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).

Rob