linux-kernel - Re: [PATCH 1/4] dt-bindings: reset: Add PDC reset binding for SDM845 SoCs

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Date:   Wed, 8 Aug 2018 21:14:05 +0530
From:   Sibi Sankar <sibis@...eaurora.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Philipp Zabel <p.zabel@...gutronix.de>, bjorn.andersson@...aro.org,
        linux-remoteproc@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, ohad@...ery.com, mark.rutland@....com,
        sricharan@...eaurora.org, akdwived@...eaurora.org,
        linux-arm-msm@...r.kernel.org, tsoni@...eaurora.org,
        ilina@...eaurora.org, jcrouse@...eaurora.org
Subject: Re: [PATCH 1/4] dt-bindings: reset: Add PDC reset binding for SDM845
 SoCs

Hi Rob,
Thanks for the review

On 08/07/2018 11:46 PM, Rob Herring wrote:
> On Tue, Jul 31, 2018 at 06:27:24PM +0530, Sibi S wrote:
>> Hi Philipp,
>> Thanks for the review!
>>
>> On 07/31/2018 02:12 PM, Philipp Zabel wrote:
>>> Hi Sibi,
>>>
>>> On Fri, 2018-07-27 at 20:58 +0530, Sibi Sankar wrote:
>>>> Add SDM845 PDC (Power Domain Controller) reset controller binding
>>>>
>>>> Signed-off-by: Sibi Sankar <sibis@...eaurora.org>
>>>> ---
>>>>    .../bindings/reset/qcom,pdc-reset.txt         | 52 +++++++++++++++++++
>>>>    include/dt-bindings/reset/qcom,sdm845-pdc.h   | 20 +++++++
>>>>    2 files changed, 72 insertions(+)
>>>>    create mode 100644 Documentation/devicetree/bindings/reset/qcom,pdc-reset.txt
>>>>    create mode 100644 include/dt-bindings/reset/qcom,sdm845-pdc.h
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/reset/qcom,pdc-reset.txt b/Documentation/devicetree/bindings/reset/qcom,pdc-reset.txt
>>>> new file mode 100644
>>>> index 000000000000..85e159962e08
>>>> --- /dev/null
>>>> +++ b/Documentation/devicetree/bindings/reset/qcom,pdc-reset.txt
>>>> @@ -0,0 +1,52 @@
>>>> +PDC Reset Controller
>>>> +======================================
>>>> +
>>>> +This binding describes a reset-controller found on PDC-Global(Power Domain
>>>> +Controller) block for Qualcomm Technologies Inc SDM845 SoCs.
>>>> +
>>>> +Required properties:
>>>> +- compatible:
>>>> +	Usage: required
>>>> +	Value type: <string>
>>>> +	Definition: must be:
>>>> +		    "qcom,sdm845-pdc-global"
>>>> +
>>>> +- reg:
>>>> +	Usage: required
>>>> +	Value type: <prop-encoded-array>
>>>> +	Definition: must specify the base address and size of the register
>>>> +	            space.
>>>> +
>>>> +- #reset-cells:
>>>> +	Usage: required
>>>> +	Value type: <uint>
>>>> +	Definition: must be 1; cell entry represents the reset index.
>>>> +
>>>> +Example:
>>>> +
>>>> +pdc_reset: reset-controller@...0000 {
>>>
>>> Is this really just a reset controller?
>>>
>>> The name makes it sound like a driver binding to this should also
>>> provide pm_genpd and the binding should probably call this a power-
>>> controller: Documentation/devicetree/bindings/power/power_domain.txt.
>>>
>>
>> The PDC-global reg space which is a part of PDC-wrapper reg space seems
>> to be only used for the reset lines.
>>
>> Couple of other drivers use other parts of the PDC-wrapper reg space:
>> https://patchwork.kernel.org/patch/10223701/ (PDC-Interrupt controller)
>> https://patchwork.kernel.org/patch/10255767/ (GMU-PDC incorrectly tries
>> to occupy the entire pdc-wrapper reg space)
>>
>> since it couldn't be logically mapped into pdc-interrupt driver, it had
>> to be included as a separate reset driver.
> 
> You can't have overlapping regions in DT (well, you can because we have
> to work-around existing DTs that do, but you shouldn't).
> 
> A single node can be multiple providers such as interrupt controller and
> reset controller. It's an OS problem to split that into multiple
> drivers.
>   

There will be no overlaps. Jordan will be changing the dt binding of
gmu_pdc so that there is no overlap I guess. What I meant to say is that
pdc-global is a separate reg-space and currently has no other
functionality other than exposing the reset lines.

>>>> +	compatible = "qcom,sdm845-pdc-global";
>>>> +	reg = <0xb2e0000 0x20000>;
>>>
>>> This looks like this is the register space of the complete PDC, not just
>>> the reset register?
>>>
>>
>> The entire register space was chosen because it is only used for its
>> reset lines (had a good look at the downstream kernel and had a conversation
>> with Lina) and to ensure break backward compatibility for
>> the for the dt entry if the reg-space was used for other purposes in
>> the future.
> 
> Why do you want to ensure breaking backwards compatibility?
>

Similar to the AOSS reset driver which had a unused clock part, this
driver also exposes a reg space of which only reset lines are used.

> Rob
> 

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