linux-kernel - Re: [PATCH v4 1/2] Documentation: Documentation for qcom, llcc

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Date:   Wed, 18 Apr 2018 09:52:59 -0500
From:   Rob Herring <robh@...nel.org>
To:     Rishabh Bhatnagar <rishabhb@...eaurora.org>
Cc:     "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        devicetree@...r.kernel.org, linux-arm@...ts.infradead.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Trilok Soni <tsoni@...eaurora.org>,
        Kyle Yan <kyan@...eaurora.org>, ckadabi@...eaurora.org,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Evan Green <evgreen@...omium.org>
Subject: Re: [PATCH v4 1/2] Documentation: Documentation for qcom, llcc

On Tue, Apr 17, 2018 at 5:12 PM,  <rishabhb@...eaurora.org> wrote:
> On 2018-04-17 10:43, rishabhb@...eaurora.org wrote:
>>
>> On 2018-04-16 07:59, Rob Herring wrote:
>>>
>>> On Tue, Apr 10, 2018 at 01:08:12PM -0700, Rishabh Bhatnagar wrote:
>>>>
>>>> Documentation for last level cache controller device tree bindings,
>>>> client bindings usage examples.
>>>
>>>
>>> "Documentation: Documentation ..."? That wastes a lot of the subject
>>> line... The preferred prefix is "dt-bindings: ..."
>>>
>>>>
>>>> Signed-off-by: Channagoud Kadabi <ckadabi@...eaurora.org>
>>>> Signed-off-by: Rishabh Bhatnagar <rishabhb@...eaurora.org>
>>>> ---
>>>>  .../devicetree/bindings/arm/msm/qcom,llcc.txt      | 58
>>>> ++++++++++++++++++++++
>>>>  1 file changed, 58 insertions(+)
>>>>  create mode 100644
>>>> Documentation/devicetree/bindings/arm/msm/qcom,llcc.txt
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/arm/msm/qcom,llcc.txt
>>>> b/Documentation/devicetree/bindings/arm/msm/qcom,llcc.txt
>>>> new file mode 100644
>>>> index 0000000..497cf0f
>>>> --- /dev/null
>>>> +++ b/Documentation/devicetree/bindings/arm/msm/qcom,llcc.txt
>>>> @@ -0,0 +1,58 @@
>>>> +== Introduction==
>>>> +
>>>> +LLCC (Last Level Cache Controller) provides last level of cache memory
>>>> in SOC,
>>>> +that can be shared by multiple clients. Clients here are different
>>>> cores in the
>>>> +SOC, the idea is to minimize the local caches at the clients and
>>>> migrate to
>>>> +common pool of memory
>>>> +
>>>> +Properties:
>>>> +- compatible:
>>>> +        Usage: required
>>>> +        Value type: <string>
>>>> +        Definition: must be "qcom,sdm845-llcc"
>>>> +
>>>> +- reg:
>>>> +        Usage: required
>>>> +        Value Type: <prop-encoded-array>
>>>> +        Definition: must be addresses and sizes of the LLCC registers
>>>
>>>
>>> How many address ranges?
>>>
>> It consists of just one address range. I'll edit the definition to make
>> it more clear.
>>>>
>>>> +
>>>> +- #cache-cells:
>>>
>>>
>>> This is all written as it is a common binding, but it is not one.
>>>
>>> You already have most of the configuration data for each client in the
>>> driver, I think I'd just put the client connection there too. Is there
>>> any variation of this for a given SoC?
>>>
>> #cache-cells and max-slices won't change for a given SOC. So you want me
>> to hard-code in the driver itself?
>>
> I can use of_parse_phandle_with_fixed_args function and fix the number of
> args as 1 instead of keeping #cache-cells here in DT. Does that look fine?

No, I'm saying why even put cache-slices properties in DT to begin
with? You could just define client id's within the kernel and clients
can use those instead of getting the id from the DT.

I have a couple of hesitations with putting this into the DT. First, I
think a cache is just one aspect of describing the interconnect
between masters and memory (and there's been discussions on
interconnect bindings too) and any binding needs to consider all of
the aspects of the interconnect. Second, I'd expect this cache
architecture will change SoC to SoC and the binding here is pretty
closely tied to the current cache implementation (e.g. slices). If
there were a bunch of SoCs with the same design and just different
client IDs (like interrupt IDs), then I'd feel differently.

Rob