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Wysocki" , "Viresh Kumar" , "Danilo Krummrich" , "Alice Ryhl" , "Maarten Lankhorst" , "Maxime Ripard" , "Thomas Zimmermann" , "David Airlie" , "Simona Vetter" , "Drew Fustini" , "Guo Ren" , "Fu Wei" , =?utf-8?q?Uwe_Kleine-K=C3=B6nig?= , "Michael Turquette" , "Stephen Boyd" , "Miguel Ojeda" , "Gary Guo" , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , "Benno Lossin" , "Andreas Hindborg" , "Trevor Gross" , "Michal Wilczynski" , "Boqun Feng" , , , , , , , , "Boris Brezillon" , =?utf-8?q?Onur_=C3=96zkan?= , "Maurice" Subject: Re: [PATCH v5 2/4] rust: clk: implement Clone for Clk From: "Gary Guo" X-Mailer: aerc 0.21.0 References: <20260706-clk-type-state-v5-0-67c5f326a16c@collabora.com> <20260706-clk-type-state-v5-2-67c5f326a16c@collabora.com> In-Reply-To: X-ClientProxiedBy: LO4P123CA0421.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:18b::12) To LOAP265MB8560.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:4ab::19) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: LOAP265MB8560:EE_|LOAP265MB8981:EE_ X-MS-Office365-Filtering-Correlation-Id: bae2df13-9e10-4a11-02e7-08def0c4708c X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|10070799003|376014|7416014|23010399003|22082099003|18002099003|4143699003|5023799004|3023799007|10067099003|56012099006; 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If you make it generic over `Deref Clk>` you could even make it work with `&Clk` (for > short-lived enables) and `Arc>` (for enable periods longer > than a code block). This seems to be exactly matching my issue with the current API design: https://lore.kernel.org/rust-for-linux/DKEO7DXQ8604.1CB09QFM2EO68@garyguo.n= et/ i.e. the type state is used for two things. Best, Gary > >> >> Implement Clone for Clk: each clone is an independent view of the >> same underlying clock, in the same state, owning its own >> clk_prepare()/clk_enable() counts, e.g.: >> >> let enabled_clk =3D prepared_clk.clone().enable()?; >> >> // Do stuff that requires the clock to be enabled. >> >> // enabled_clk goes out of scope and releases the counts it >> // owns; the clock remains prepared through prepared_clk. > > Looks indeed like something an `EnabledGuard` would cover. > >> >> Since struct clk is not refcounted on the C side, share it between >> clones by wrapping the pointer in an Arc'd RawClk, whose drop >> implementation calls clk_put() exactly once, when the last clone goes >> out of scope. This costs one small allocation per clk_get(), which was >> deemed negligible when compared against the pre-existing indirections >> in the clk framework. >> >> Cloning a prepared (or enabled) Clk calls clk_prepare() (and >> clk_enable()) on the underlying clock. These calls cannot fail here: >> the value being cloned already holds a count of each, so the C side >> only increments the respective counts. This is what makes an infallible >> Clone implementation possible. > > Another point to consider is that `clk_prepare` (and thus `clone()`, as > you mentioned in the doccomment of the patch) can sleep. An > `EnabledGuard`, otoh, would only need to call `clk_enable` and thus > wouldn't sleep, making it usable in atomic context. > >> >> The DISABLE_ON_DROP and UNPREPARE_ON_DROP constants are renamed to >> ENABLED and PREPARED respectively, since they now describe the state > > Let's give them their final name in patch 1 then, since it's also > accurate (and arguably better) even before this patch. > > I am not quite familiar with the client code that will make use of this; > a pointer would be welcome. But I strongly suspect that multiple calls > to `get` and an `EnabledGuard` can cover most (if not all) use-cases. > Introducing an `Arc` here forces a storage decision upon users while > hiding it. > > It sets us up for some dilemmas in the future: for instance, Maurice's > series exposing `clk_rate_exclusive_get` turns a `Clk` into an > `ExclusiveClk`, but its potential clones are still around as regular > `Clk`s. Moreover, `ExclusiveClk` derefs to `Clk`, which can be > cloned as per this patch; so is it really exclusive? > > So I'd like to keep discussing this design a bit more, with more > visibility over user code. Maybe it should be moved to the tail of the > series, or even extracted into its own series so as to not block the > other patches.