linux-kernel - Re: [PATCH V6 02/15] cpufreq: Add cpufreq_table

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Message-ID: <2025010855-cargo-quickness-a08f@gregkh>
Date: Wed, 8 Jan 2025 12:50:31 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Danilo Krummrich <dakr@...hat.com>, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	linux-pm@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Stephen Boyd <sboyd@...nel.org>, Nishanth Menon <nm@...com>,
	rust-for-linux@...r.kernel.org,
	Manos Pitsidianakis <manos.pitsidianakis@...aro.org>,
	Erik Schilling <erik.schilling@...aro.org>,
	Alex Bennée <alex.bennee@...aro.org>,
	Joakim Bech <joakim.bech@...aro.org>, Rob Herring <robh@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V6 02/15] cpufreq: Add cpufreq_table_len()

On Wed, Jan 08, 2025 at 04:42:53PM +0530, Viresh Kumar wrote:
> On 07-01-25, 12:59, Greg KH wrote:
> > On Tue, Jan 07, 2025 at 04:51:35PM +0530, Viresh Kumar wrote:
> > > Add a function to calculate number of entries in the cpufreq table. This
> > > will be used by the Rust implementation.
> > 
> > Again, why is Rust unique here?  Why wouldn't the C code also need this?
> 
> How about this:
> 
>     cpufreq: Add cpufreq_table_len()
> 
>     The last entry of a cpufreq table is marked by setting the frequency
>     field to a special value: CPUFREQ_TABLE_END. The C code manages to
>     traverse the table by checking the frequency field, until it reaches
>     CPUFREQ_TABLE_END.
> 
>     The Rust cpufreq bindings though will need to know the length of the
>     cpufreq table in advance, for example to check against an invalid index
>     value.
> 
>     Provide a helper to calculate number of entries in the cpufreq table.
>     will be used by the Rust implementation.

Odd, why can't Rust also know where CPUFREQ_TABLE_END is?  Why do you
have to do extra work here?  That feels wrong.

thanks,

greg k-h