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

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Message-ID: <154411934006.88331.2149751521264448532@swboyd.mtv.corp.google.com>
Date:   Thu, 06 Dec 2018 10:02:20 -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 David Dai (2018-12-05 17:24:18)
> 
> 
> On 12/4/2018 11:15 PM, Stephen Boyd wrote:
> > Quoting David Dai (2018-12-04 17:14:10)
> >> On 12/4/2018 2:34 PM, Stephen Boyd wrote:
> >>> Quoting Alex Elder (2018-12-04 13:41:47)
> >>>>
> >>> 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.
> > How is this different from bus bandwidth requests? In those cases the
> > bandwidth is calculated in bits per second or something like that, and
> > written to the hardware so it can convert that bandwidth into kHz and
> > set a bus clk frequency in the clock controller? So in the IPA case
> > we've skipped the bps to kHz conversion step and gone straight to the
> > clk frequency setting part? Is a BCM able to aggregate units of
> > bandwidth or kHz depending on how it's configured and this BCM is
> > configured for kHz?
> 
> The data written to the hardware is just a 14bit scalar value that it 
> takes to select a performance/frequency from a preset table. It's not 
> really doing any sort of conversion in hardware in this case, instead 
> the value is computed by software based on the aux data given. Think of 
> it as a generic aggregator as opposed to being strictly bandwidth and 
> the aggregator itself does not care what type of value it is(be it Khz 
> or BW/s).
> 

Got it. Thanks!