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

From: David Dai
Date: Tue Dec 04 2018 - 20:14:15 EST


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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