Re: [PATCH V3 2/3] dt-bindings: gpio: zynqmp: Add binding documentation for modepin

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

On 18.08.21 11:38, Michal Simek wrote:
> Hi Ahmad,
> 
> On 8/18/21 11:00 AM, Ahmad Fatoum wrote:
>> On 18.08.21 10:10, Piyush Mehta wrote:
>>> This patch adds DT binding document for zynqmp modepin GPIO controller.
>>> Modepin GPIO controller has four GPIO pins which can be configurable
>>> as input or output.
>>>
>>> Modepin driver is a bridge between the peripheral driver and GPIO pins.
>>> It has set and get APIs for accessing GPIO pins, based on the device-tree
>>> entry of reset-gpio property in the peripheral driver, every pin can be
>>> configured as input/output and trigger GPIO pin.
>>>
>>> For more information please refer zynqMp TRM link:
>>> Link: https://www.xilinx.com/support/documentation/user_guides/ug1085-zynq-ultrascale-trm.pdf
>>> Chapter 2: Signals, Interfaces, and Pins
>>> Table 2-2: Clock, Reset, and Configuration Pins - PS_MODE
>>>
>>> Signed-off-by: Piyush Mehta <piyush.mehta@xilinx.com>
>>> Acked-by: Michal Simek <michal.simek@xilinx.com>
>>> ---
>>> Changes in v2:
>>> - Addressed review comments: Update commit message
>>>
>>> Review Comments:
>>> https://lore.kernel.org/linux-arm-kernel/20210615080553.2021061-2-piyush.mehta@xilinx.com/T/#mbd1fbda813e33b19397b350bde75747c92a0d7e1
>>> https://lore.kernel.org/linux-arm-kernel/20210615080553.2021061-2-piyush.mehta@xilinx.com/T/#me82b1444ab3776162cdb0077dfc9256365c7e736
>>>
>>> Changes in v3:
>>> - Addressed Rob and Michal review comments:
>>>   - Update DT example. 
>>>
>>> Review Comments:
>>> https://lore.kernel.org/linux-arm-kernel/YRbBnRS0VosXcZWz@robh.at.kernel.org/
>>> https://lore.kernel.org/linux-arm-kernel/d71ad7f9-6972-8cc0-6dfb-b5306c9900d0@xilinx.com/
>>> ---
>>>  .../bindings/gpio/xlnx,zynqmp-gpio-modepin.yaml    | 41 ++++++++++++++++++++++
>>>  .../bindings/gpio/xlnx,zynqmp-gpio-modepin.yaml    | 43 ++++++++++++++++++++++
>>>  1 file changed, 43 insertions(+)
>>>  create mode 100644 Documentation/devicetree/bindings/gpio/xlnx,zynqmp-gpio-modepin.yaml
>>>
>>> diff --git a/Documentation/devicetree/bindings/gpio/xlnx,zynqmp-gpio-modepin.yaml b/Documentation/devicetree/bindings/gpio/xlnx,zynqmp-gpio-modepin.yaml
>>> new file mode 100644
>>> index 0000000..1442815
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/gpio/xlnx,zynqmp-gpio-modepin.yaml
>>> @@ -0,0 +1,43 @@
>>> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
>>> +%YAML 1.2
>>> +---
>>> +$id: "http://devicetree.org/schemas/gpio/xlnx,zynqmp-gpio-modepin.yaml#"
>>> +$schema: "http://devicetree.org/meta-schemas/core.yaml#"
>>> +
>>> +title: ZynqMP Mode Pin GPIO controller
>>> +
>>> +description:
>>> +  PS_MODE is 4-bits boot mode pins sampled on POR deassertion. Mode Pin
>>> +  GPIO controller with configurable from numbers of pins (from 0 to 3 per
>>> +  PS_MODE). Every pin can be configured as input/output.
>> So, at Linux runtime, someone decides to boot the system into e.g. a USB
>> recovery mode and then toggles the appropriate GPIOs and does a system
>> reset?
>>
>> If so, are you aware of the reboot mode[1] infrastructure?
>>
>> A reboot-mode-gpio driver on top of this GPIO controller would allow you
>> to describe the supported reboot modes in the device tree and instead of
>> exporting GPIOs to userspace, users can then just do
>>
>> 	systemctl restart recovery
>>
>> to toggle the appropriate bits.
>>
>> Also to be sure: PS_MODE are actual GPIO pins that you could toggle
>> board level components with, right? i.e. it's not just a register that
>> overrides the values read from the boot mode pins? (In the latter case
>> a syscon-reboot-mode without GPIO controller would be the correct
>> abstraction).
>>
>> [1]: drivers/power/reset/reboot-mode.c
> 
> Thanks for these links. I wasn't aware about it.
> But this device/IP is not working like this. Changing gpios to certain
> state won't ensure that on reboot/reset (done in whatever way) won't
> stay on values you chose.

Ah, the "PS_MODE is 4-bits boot mode pins sampled on POR deassertion" part
misled me. These pins are sampled on startup, but can afterwards be reused 
via talking to firmware. Thanks for clearing this up.
> modepin gpio driver is at BOOT_PIN_CTRL 	0xFF5E0250
> 
> (To be fair if you add additional external chip it could work like this
> but I have never seen it).

Ye, that would've been strange, that's why I asked. :)

> But when you bring this up. Xilinx ZynqMP is providing a way how to
> setup alternative boot mode which is done via
> BOOT_MODE_USER 	0xFF5E0200
> Bit 8 and 15-12.
> Then you can setup any bootmode.
> 
> ZynqMP supports couple of modes listed here
> https://source.denx.de/u-boot/u-boot/-/blob/master/arch/arm/mach-zynqmp/include/mach/hardware.h#L73
> 
> but again routing to this register needs to be done via firmware
> interface but it should be done via separate driver.

Yes.

> Is there an option to setup whatever modes you like?
> 
> I mean to simply cover all modes like this?
> 
> mode-jtag = <0>;
> mode-sd = <3>;
> mode-sd1 = <5>;

Yes, you can define the supported modes in the SoC dtsi
and boards inherit that and can extend it as necessary.

> And then users/customers can say what normal/recovery/test modes are.

Yes, that would be nice. But after your clarification, I see that it's
unrelated to this patch series. Binding is fine. Question on driver
is still applicable.

Cheers,
Ahmad

> 
> Thanks,
> Michal
> 


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