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

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Date:   Wed, 5 Dec 2018 23:33:04 -0800
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Stephen Boyd <sboyd@...nel.org>
Cc:     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, georgi.djakov@...aro.org,
        evgreen@...gle.com, tdas@...eaurora.org
Subject: Re: [RFC PATCH] clk: qcom: clk-rpmh: Add IPA clock support

On Tue 04 Dec 23:15 PST 2018, 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)
> > >> 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. 
> 
> 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?
> 

My objection to the use of msm_bus vs clock framework is not related to
how the actual interface of configuring the hardware looks like. It's
simply a matter of how this is represented in software, between some
provider and the IPA driver.

The IPA driver wants the IPA block to tick at 75MHz and I do not think
it's appropriate to achieve that by requesting a path between IPA Core
and IPA Core of 75000000 Kbytes/s.

Regards,
Bjorn