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SubjectRe: [PATCH 1/6] clk: Remove recursion in clk_core_{prepare,enable}()
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On Tue, 2018-10-23 at 18:31 -0700, Derek Basehore wrote:
> From: Stephen Boyd <sboyd@codeaurora.org>
>
> Enabling and preparing clocks can be written quite naturally with
> recursion. We start at some point in the tree and recurse up the
> tree to find the oldest parent clk that needs to be enabled or
> prepared. Then we enable/prepare and return to the caller, going
> back to the clk we started at and enabling/preparing along the
> way.
>
> The problem is recursion isn't great for kernel code where we
> have a limited stack size. Furthermore, we may be calling this
> code inside clk_set_rate() which also has recursion in it, so
> we're really not looking good if we encounter a tall clk tree.
>
> Let's create a stack instead by looping over the parent chain and
> collecting clks of interest. Then the enable/prepare becomes as
> simple as iterating over that list and calling enable.

Hi Derek,

What about unprepare() and disable() ?

This patch removes the recursion from the enable path but keeps it for the
disable path ... this is very odd. Assuming doing so works, It certainly makes
CCF a lot harder to understand.

What about clock protection which essentially works on the same model as prepare
and enable ?

Overall, this change does not look like something that should be merged as it
is. If you were just seeking comments, you should add the "RFC" tag to your
series.

Jerome.

>
> Cc: Jerome Brunet <jbrunet@baylibre.com>

If you don't mind, I would prefer to get the whole series next time. It helps to
get the context.

> Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
> Signed-off-by: Derek Basehore <dbasehore@chromium.org>
> ---
> drivers/clk/clk.c | 113 ++++++++++++++++++++++++++--------------------
> 1 file changed, 64 insertions(+), 49 deletions(-)
>
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index af011974d4ec..95d818f5edb2 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -71,6 +71,8 @@ struct clk_core {
> struct hlist_head children;
> struct hlist_node child_node;
> struct hlist_head clks;
> + struct list_head prepare_list;
> + struct list_head enable_list;
> unsigned int notifier_count;
> #ifdef CONFIG_DEBUG_FS
> struct dentry *dentry;
> @@ -740,49 +742,48 @@ EXPORT_SYMBOL_GPL(clk_unprepare);
> static int clk_core_prepare(struct clk_core *core)
> {
> int ret = 0;
> + struct clk_core *tmp, *parent;
> + LIST_HEAD(head);
>
> lockdep_assert_held(&prepare_lock);
>
> - if (!core)
> - return 0;
> + while (core) {
> + list_add(&core->prepare_list, &head);
> + /* Stop once we see a clk that is already prepared */
> + if (core->prepare_count)
> + break;
> + core = core->parent;
> + }
>
> - if (core->prepare_count == 0) {
> - ret = clk_pm_runtime_get(core);
> - if (ret)
> - return ret;
> + list_for_each_entry_safe(core, tmp, &head, prepare_list) {
> + list_del_init(&core->prepare_list);

Is there any point in removing it from the list ?
Maybe I missed it but it does not seems useful.

Without this, we could use list_for_each_entry()

>
> - ret = clk_core_prepare(core->parent);
> - if (ret)
> - goto runtime_put;
> + if (core->prepare_count == 0) {

Should we really check the count here ? You are not checking the count when the
put() counterpart is called below.

Since PM runtime has ref counting as well, either way would work I guess ... but
we shall be consistent

> + ret = clk_pm_runtime_get(core);
> + if (ret)
> + goto err;
>
> - trace_clk_prepare(core);
> + trace_clk_prepare(core);
>
> - if (core->ops->prepare)
> - ret = core->ops->prepare(core->hw);
> + if (core->ops->prepare)
> + ret = core->ops->prepare(core->hw);
>
> - trace_clk_prepare_complete(core);
> + trace_clk_prepare_complete(core);
>
> - if (ret)
> - goto unprepare;
> + if (ret) {
> + clk_pm_runtime_put(core);
> + goto err;
> + }
> + }
> + core->prepare_count++;
> }
>
> - core->prepare_count++;
> -
> - /*
> - * CLK_SET_RATE_GATE is a special case of clock protection
> - * Instead of a consumer claiming exclusive rate control, it is
> - * actually the provider which prevents any consumer from making any
> - * operation which could result in a rate change or rate glitch while
> - * the clock is prepared.
> - */
> - if (core->flags & CLK_SET_RATE_GATE)
> - clk_core_rate_protect(core);

This gets removed without anything replacing it.

is CLK_SET_RATE_GATE and clock protection support dropped after this change ?

> -
> return 0;
> -unprepare:
> - clk_core_unprepare(core->parent);
> -runtime_put:
> - clk_pm_runtime_put(core);
> +err:
> + parent = core->parent;
> + list_for_each_entry_safe_continue(core, tmp, &head, prepare_list)
> + list_del_init(&core->prepare_list);
> + clk_core_unprepare(parent);

If you get here because of failure clk_pm_runtime_get(), you will unprepare a
clock which may have not been prepared first

Overall the rework of error exit path does not seem right (or necessary)

> return ret;
> }
>
> @@ -878,37 +879,49 @@ EXPORT_SYMBOL_GPL(clk_disable);
> static int clk_core_enable(struct clk_core *core)
> {
> int ret = 0;
> + struct clk_core *tmp, *parent;
> + LIST_HEAD(head);
>
> lockdep_assert_held(&enable_lock);
>
> - if (!core)
> - return 0;
> -
> - if (WARN(core->prepare_count == 0,
> - "Enabling unprepared %s\n", core->name))
> - return -ESHUTDOWN;
> + while (core) {
> + list_add(&core->enable_list, &head);
> + /* Stop once we see a clk that is already enabled */
> + if (core->enable_count)
> + break;
> + core = core->parent;
> + }
>
> - if (core->enable_count == 0) {
> - ret = clk_core_enable(core->parent);
> + list_for_each_entry_safe(core, tmp, &head, enable_list) {
> + list_del_init(&core->enable_list);
>
> - if (ret)
> - return ret;
> + if (WARN_ON(core->prepare_count == 0)) {
> + ret = -ESHUTDOWN;
> + goto err;
> + }
>
> - trace_clk_enable_rcuidle(core);
> + if (core->enable_count == 0) {
> + trace_clk_enable_rcuidle(core);
>
> - if (core->ops->enable)
> - ret = core->ops->enable(core->hw);
> + if (core->ops->enable)
> + ret = core->ops->enable(core->hw);
>
> - trace_clk_enable_complete_rcuidle(core);
> + trace_clk_enable_complete_rcuidle(core);
>
> - if (ret) {
> - clk_core_disable(core->parent);
> - return ret;
> + if (ret)
> + goto err;
> }
> +
> + core->enable_count++;
> }
>
> - core->enable_count++;
> return 0;
> +err:
> + parent = core->parent;
> + list_for_each_entry_safe_continue(core, tmp, &head, enable_list)
> + list_del_init(&core->enable_list);
> + clk_core_disable(parent);
> + return ret;
> }
>
> static int clk_core_enable_lock(struct clk_core *core)
> @@ -3281,6 +3294,8 @@ struct clk *clk_register(struct device *dev, struct clk_hw *hw)
> core->num_parents = hw->init->num_parents;
> core->min_rate = 0;
> core->max_rate = ULONG_MAX;
> + INIT_LIST_HEAD(&core->prepare_list);
> + INIT_LIST_HEAD(&core->enable_list);
> hw->core = core;
>
> /* allocate local copy in case parent_names is __initdata */


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