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

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Date:   Tue, 04 Dec 2018 14:34:00 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Alex Elder <elder@...aro.org>,
        David Dai <daidavid1@...eaurora.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

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?

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.