Re: [PATCH 1/6] clk: Remove recursion in clk_core_{prepare,enable}()

From: dbasehore .
Date: Wed Oct 24 2018 - 16:23:57 EST

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On Wed, Oct 24, 2018 at 1:15 PM dbasehore . <dbasehore@xxxxxxxxxxxx> wrote:
>
> On Wed, Oct 24, 2018 at 2:51 AM Jerome Brunet <jbrunet@xxxxxxxxxxxx> wrote:
> >
> > On Tue, 2018-10-23 at 18:31 -0700, Derek Basehore wrote:
> > > From: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxx>
> > > Signed-off-by: Derek Basehore <dbasehore@xxxxxxxxxxxx>
> > > ---
> > > 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.
>
> I think I accidentally messed that up when I picked up the patch.
> There were some merge conflicts with the addition of the
> clk_pm_runtime code.
>
> >
> > 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 ?
>
> No, I think I just accidentally removed this when resolving conflicts.
>
> >
> > > -
> > > 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)
> >
>
> Yeah, all of this seems to just be a poor resolution of patch
> conflicts on my part. Will fix.
>

Actually, I think that the rework of the error exit path makes sense
since the prior exit path was for the recursive case, not the list
iteration case. I will fix the issue where we call clk_core_unprepare
when it shouldn't, though.

> > > 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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