linux-kernel - Re: [RFC PATCH] clk: qcom: clk-rpmh: Add IPA clock support

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Date:   Tue, 4 Dec 2018 17:14:10 -0800
From:   David Dai <daidavid1@...eaurora.org>
To:     Stephen Boyd <sboyd@...nel.org>, Alex Elder <elder@...aro.org>,
        linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     georgi.djakov@...aro.org, bjorn.andersson@...aro.org,
        evgreen@...gle.com, tdas@...eaurora.org
Subject: Re: [RFC PATCH] clk: qcom: clk-rpmh: Add IPA clock support


On 12/4/2018 2:34 PM, Stephen Boyd wrote:
> Quoting Alex Elder (2018-12-04 13:41:47)
>> On 12/4/18 1:24 PM, Stephen Boyd wrote:
>>> Quoting David Dai (2018-12-03 19:50:13)
>>>> Add IPA clock support by extending the current clk rpmh driver to support
>>>> clocks that are managed by a different type of RPMh resource known as
>>>> Bus Clock Manager(BCM).
>>> Yes, but why? Does the IPA driver need to set clk rates and that somehow
>>> doesn't work as a bandwidth request?
>> The IPA core clock is a *clock*, not a bus.  Representing it as if
>> it were a bus, abusing the interconnect interface--pretending a bandwidth
>> request is really a clock rate request--is kind of kludgy.  I think Bjorn
>> and David (and maybe Georgi? I don't know) decided a long time ago that
>> exposing this as a clock is the right way to do it.  I agree with that.
>>
> But then we translate that clock rate into a bandwidth request to the
> BCM hardware? Seems really weird because it's doing the opposite of what
> you say is abusive. What does the IPA driver plan to do with this clk?
> Calculate a frequency by knowing that it really boils down to some
> bandwidth that then gets converted back into some clock frequency? Do we
> have the user somewhere that can be pointed to?
The clock rate is translated into a unitless threshold value sent as 
part of the rpmh msg
that BCM takes to select a performance. In this case, the unit 
conversion is based on
the unit value read from the aux data which is in Khz. I understand that 
this wasn't
explicitly mentioned anywhere and I'll improve on that next patch. 
Here's a link to
the IPA driver implementation: https://lkml.org/lkml/2018/11/7/220

>
> Of course, none of these details are in the commit text so it's really
> hard for me as a bystander to figure this all out.  So again, please add
> these sorts of details to the commit text so we can be "sold" on the
> idea of the patch instead of stating what the patch does.
Understood, I'll be as detailed and as explicit as I can in the future.

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