linux-kernel - Re: [PATCH v2 1/2] firmware: arm_scmi: get only min/max clock rates

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Message-ID: <80ea462fbe044781bb4b1e7a80ab8b80@foss.st.com>
Date: Tue, 10 Dec 2024 10:58:05 +0000
From: Etienne CARRIERE - foss <etienne.carriere@...s.st.com>
To: Cristian Marussi <cristian.marussi@....com>
CC: Sudeep Holla <sudeep.holla@....com>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        "arm-scmi@...r.kernel.org"
	<arm-scmi@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>,
        "linux-clk@...r.kernel.org"
	<linux-clk@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] firmware: arm_scmi: get only min/max clock rates

On Monday, December 9, 2024, Cristian Marussi wrote:
> On Mon, Dec 09, 2024 at 01:48:48PM +0000, Etienne CARRIERE - foss wrote:
> > On Monday, December 9, 2024, Sudeep Holla wrote:
> > > On Tue, Dec 03, 2024 at 06:39:07PM +0100, Etienne Carriere wrote:
> > > > Remove limitation of 16 clock rates max for discrete clock rates
> > > > description when the SCMI firmware supports SCMI Clock protocol v2.0
> > > > or later.
> > > >
> > > > Driver clk-scmi.c is only interested in the min and max clock rates.
> > > > Get these by querying the first and last discrete rates with SCMI
> > > > clock protocol message ID CLOCK_DESCRIBE_RATES since the SCMI
> > > > specification v2.0 and later states that rates enumerated by this
> > > > command are to be enumerated in "numeric ascending order" [1].
> > > >
> > > > Preserve the implementation that queries all discrete rates (16 rates
> > > > max) to support SCMI firmware built on SCMI specification v1.0 [2]
> > > > where SCMI Clock protocol v1.0 does not explicitly require rates
> > > > described with CLOCK_DESCRIBE_RATES to be in ascending order.
> > > >
> > > > Link: https://developer.arm.com/documentation/den0056 [1]
> > > > Link: https://developer.arm.com/documentation/den0056/a [2]
> > > > Signed-off-by: Etienne Carriere <etienne.carriere@...s.st.com>
> > > > ---
> 
> Hi,
> 
> > >
> > > [...]
> > >
> > > > +
> > > > +static int scmi_clock_get_rates_bound(const struct scmi_protocol_handle *ph,
> > > > +                                   u32 clk_id, struct scmi_clock_info *clk)
> > > > +{
> > >
> > > This new function seem to have unwraped the scmi_iterator_ops(namely
> > > prepare_message, update_state and process_response instead of reusing them.
> > > Can you please explain why it wasn't possible to reuse them ?
> >
> > Since we're interested here only in min and max rates, let's query the
> > first and last rates only. This can save a bit of useless transactions between
> > agent and firmware in case there are many clocks with somewhat large
> > the discrete rate lists.
> >
> > I though using the iterator for this specific case would add a bit more
> > complexity: it's expected to iterate (st->desc_index incremented from the
> > common scmi_iterator_run() function) whereas here I propose to send
> > only 2 messages.
> 
> Yes, indeed the core iterator helpers are meant to issue a 'full scan'
> retrievieng all the resources that are returned while handling in a
> common way the underlying machinery common to all messages that, like
> DESCRIBE_RATES, could possibly return their results in chunks as a
> multi-part reply...
> 
> ...having said that I can certainly extend the iterators to be configurable
> enough to fit this new usecase and retrieve only the desired part of the
> 'scan' so that can be used for this kind of max/min query or for the
> bisection case.
> 
> I would avoid to re-introduce ad-hoc code to handle these new usecases
> that do not fit into the existing iterator logic, since iterators
> were introduced to remove duplication and unify under common
> methods...and this new iterator scenario seems to me that has already 2
> usecases and certainly more protocol could want to perform similar 'lazy
> partial queries' in the future, so I'd prefer to address this in a more
> general way upfront if possible...I will think about it and post something
> next week in the form of some new iterator extensions, if it's fine for you.
> 

Hi Cristian,
Thanks for looking at this. Any help here is very welcome.

BR,
Etienne


> Thanks,
> Cristian
>