[v5,1/2] dt-bindings: i2c: mv64xxx: add bus-reset-gpios property

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@alliedtelesis.co.nz>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
---

Notes:
    Changes in v5:
    - Rename reset-gpios and reset-duration-us to bus-reset-gpios and
      bus-reset-duration-us as requested by Wolfram
    Changes in v4:
    - Add r-by from Krzysztof
    Changes in v3:
    - Rename reset-delay-us to reset-duration-us to better reflect its
      purpose
    - Add default: for reset-duration-us
    - Add description: for reset-gpios
    Changes in v2:
    - Update commit message
    - Add reset-delay-us property

 .../devicetree/bindings/i2c/marvell,mv64xxx-i2c.yaml   | 10 ++++++++++
 1 file changed, 10 insertions(+)

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@alliedtelesis.co.nz>
> Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>

Krzysztof, are you fine with this change?

> ---
> 
> Notes:
>     Changes in v5:
>     - Rename reset-gpios and reset-duration-us to bus-reset-gpios and
>       bus-reset-duration-us as requested by Wolfram
>     Changes in v4:
>     - Add r-by from Krzysztof
>     Changes in v3:
>     - Rename reset-delay-us to reset-duration-us to better reflect its
>       purpose
>     - Add default: for reset-duration-us
>     - Add description: for reset-gpios
>     Changes in v2:
>     - Update commit message
>     - Add reset-delay-us property
> 
>  .../devicetree/bindings/i2c/marvell,mv64xxx-i2c.yaml   | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/i2c/marvell,mv64xxx-i2c.yaml b/Documentation/devicetree/bindings/i2c/marvell,mv64xxx-i2c.yaml
> index 461d1c9ee3f7..b165d1c4f0b1 100644
> --- a/Documentation/devicetree/bindings/i2c/marvell,mv64xxx-i2c.yaml
> +++ b/Documentation/devicetree/bindings/i2c/marvell,mv64xxx-i2c.yaml
> @@ -70,6 +70,16 @@ properties:
>    resets:
>      maxItems: 1
>  
> +  bus-reset-gpios:
> +    description:
> +      GPIO pin providing a common reset for all downstream devices. This GPIO
> +      will be asserted then released before the downstream devices are probed.
> +    maxItems: 1
> +
> +  bus-reset-duration-us:
> +    description: Reset duration in us.
> +    default: 1
> +
>    dmas:
>      items:
>        - description: RX DMA Channel
> -- 
> 2.42.0
>

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@alliedtelesis.co.nz>
>> Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.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)

https://lpc.events/event/17/contributions/1507/

Current solution is heavily limited. What about regulators? What about
buses having 2 reset lines (still the same bus)? What about sequence?

Best regards,
Krzysztof

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@alliedtelesis.co.nz>
> >> Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.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

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@alliedtelesis.co.nz>
>>>> Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.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