linux-kernel - Re: [PATCH 6/7] clk: scmi: Allocate CLK operations dynamically

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Message-ID: <1d0baf6dbaa1c2ca6594f9a2bcade2c4.sboyd@kernel.org>
Date: Wed, 28 Feb 2024 18:20:34 -0800
From: Stephen Boyd <sboyd@...nel.org>
To: Cristian Marussi <cristian.marussi@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, sudeep.holla@....com, james.quinlan@...adcom.com, f.fainelli@...il.com, vincent.guittot@...aro.org, peng.fan@....nxp.com, michal.simek@....com, quic_sibis@...cinc.com, quic_nkela@...cinc.com, souvik.chakravarty@....com, Michael Turquette <mturquette@...libre.com>, linux-clk@...r.kernel.org
Subject: Re: [PATCH 6/7] clk: scmi: Allocate CLK operations dynamically

Quoting Cristian Marussi (2024-02-22 00:28:41)
> On Wed, Feb 21, 2024 at 09:44:14PM -0800, Stephen Boyd wrote:
> > 
> > It's not great to move these function pointer structs out of RO memory
> > to RW. I'm also not convinced that it's any better to construct them at
> > runtime. Isn't there a constant set of possible clk configurations? Or
> > why can't we simply add some failures to the clk_ops functions instead?
> 
> Well, the real clock devices managed by the SCMI server can be a of

SCMI is a server!? :)

> varying nature and so the minimum set of possible clk configurations
> to cover will amount to all the possible combinations of supported ops
> regarding the specific clock properties (i.e. .set_parent / .set_rate /
> .enable / .get/set_duty_cycle / atomic_capability ... for now)...we
> simply cannot know in advance what the backend SCMI server is handling.
> 
> These seemed to me too much in number (and growing) to be pre-allocated
> in all possible combinations. (and mostly wasted since you dont really
> probably use all combinations all the time)
> 
> Moreover, SCMI latest spec now exposes some clock properties (or not) to
> be able avoid even sending an actual SCMI message that we know will be
> denied all the time; one option is that we return an error,, as you said,
> but what is the point (I thought) to provide at all a clk-callback that
> we know upfront will fail to be executed every time ? (and some consumer
> drivers have been reported by partners not to be happy with these errors)
> 
> What I think could be optimized here instead, and I will try in the next
> respin, it is that now I am allocating one set of custom ops for each clock
> at the end, even if exactly the same ops are provided since the clock
> capabilities are the same; I could instead allocate dynamically and fill only
> one single set of ops for each distinct set of combinations, so as to avoid
> useless duplication and use only the miminum strict amount of RW memory
> needed.
> 

Yes please don't allocate a clk_op per clk. And, please add these
answers to the commit text so that we know why it's not possible to know
all combinations or fail clk_ops calls.