| Message ID | 20210307170742.70949-2-paul@crapouillou.net (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | mmc: jz4740: Support PLL frequency changes | expand |
Quoting Paul Cercueil (2021-03-07 09:07:41) > The purpose of this function is to be used along with the notifier > mechanism. > > When a parent clock can see its rate externally changed at any moment, > and a driver needs a specific clock rate to function, it can register a > notifier on the parent clock, and call clk_set_rate() on the base clock > to adjust its frequency according to the new parent clock. Can the driver use the rate locking mechanism to get a certain rate instead of registering for notifiers and trying to react to changes? > > This works fine, until the base clock has the CLK_SET_RATE_PARENT flag > set. In that case, calling clk_set_rate() on the base clock will call > clk_set_rate() on the parent clock, which will trigger the notifier > again, and we're in a loop. > > For that reason, we need to register the notifier on the parent clock of > the first ancestor of the base clock that will effectively modify its > rate when clk_set_rate() is called, which we can now obtain with > clk_get_first_to_set_rate(). > > Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Hi Stephen, Le sam. 13 mars 2021 à 14:28, Stephen Boyd <sboyd@kernel.org> a écrit : > Quoting Paul Cercueil (2021-03-07 09:07:41) >> The purpose of this function is to be used along with the notifier >> mechanism. >> >> When a parent clock can see its rate externally changed at any >> moment, >> and a driver needs a specific clock rate to function, it can >> register a >> notifier on the parent clock, and call clk_set_rate() on the base >> clock >> to adjust its frequency according to the new parent clock. > > Can the driver use the rate locking mechanism to get a certain rate > instead of registering for notifiers and trying to react to changes? You mean with clk_rate_exclusive_get()? That sounds like a good idea, but what would happen when a different driver calls the non-exclusive clk_set_rate() on this clock (or the parent), would it return -EBUSY, lock on a mutex? ... Cheers, -Paul > >> >> This works fine, until the base clock has the CLK_SET_RATE_PARENT >> flag >> set. In that case, calling clk_set_rate() on the base clock will >> call >> clk_set_rate() on the parent clock, which will trigger the notifier >> again, and we're in a loop. >> >> For that reason, we need to register the notifier on the parent >> clock of >> the first ancestor of the base clock that will effectively modify >> its >> rate when clk_set_rate() is called, which we can now obtain with >> clk_get_first_to_set_rate(). >> >> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c index 5052541a0986..3fd75e283482 100644 --- a/drivers/clk/clk.c +++ b/drivers/clk/clk.c @@ -2450,6 +2450,15 @@ struct clk *clk_get_parent(struct clk *clk) } EXPORT_SYMBOL_GPL(clk_get_parent); +struct clk *clk_get_first_to_set_rate(struct clk *clk) +{ + while (clk && (clk->core->flags & CLK_SET_RATE_PARENT)) + clk = clk_get_parent(clk); + + return clk; +} + + static struct clk_core *__clk_init_parent(struct clk_core *core) { u8 index = 0; diff --git a/include/linux/clk.h b/include/linux/clk.h index 266e8de3cb51..f0ea6ac6aa39 100644 --- a/include/linux/clk.h +++ b/include/linux/clk.h @@ -766,6 +766,17 @@ struct clk *clk_get_parent(struct clk *clk); */ struct clk *clk_get_sys(const char *dev_id, const char *con_id); +/** + * clk_get_first_to_set_rate - get a pointer to the clock that will + * effectively modify its rate when clk_set_rate(clk) is called + * (might be clk itself, or any ancestor) + * @clk: clock source + * + * Returns struct clk corresponding to the matched clock source, or + * NULL on error. + */ +struct clk *clk_get_first_to_set_rate(struct clk *clk); + /** * clk_save_context - save clock context for poweroff * @@ -928,6 +939,11 @@ static inline struct clk *clk_get_parent(struct clk *clk) return NULL; } +static inline struct clk *clk_get_first_to_set_rate(struct clk *clk) +{ + return NULL; +} + static inline struct clk *clk_get_sys(const char *dev_id, const char *con_id) { return NULL;
The purpose of this function is to be used along with the notifier mechanism. When a parent clock can see its rate externally changed at any moment, and a driver needs a specific clock rate to function, it can register a notifier on the parent clock, and call clk_set_rate() on the base clock to adjust its frequency according to the new parent clock. This works fine, until the base clock has the CLK_SET_RATE_PARENT flag set. In that case, calling clk_set_rate() on the base clock will call clk_set_rate() on the parent clock, which will trigger the notifier again, and we're in a loop. For that reason, we need to register the notifier on the parent clock of the first ancestor of the base clock that will effectively modify its rate when clk_set_rate() is called, which we can now obtain with clk_get_first_to_set_rate(). Signed-off-by: Paul Cercueil <paul@crapouillou.net> --- drivers/clk/clk.c | 9 +++++++++ include/linux/clk.h | 16 ++++++++++++++++ 2 files changed, 25 insertions(+)