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Subject: Re: [PATCH 1/6] clk: Remove recursion in clk_core_{prepare,enable}()
From:   Jerome Brunet <jbrunet@baylibre.com>
To:     "dbasehore ." <dbasehore@chromium.org>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        linux-clk@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-rockchip@lists.infradead.org, linux-doc@vger.kernel.org,
        sboyd@kernel.org, Michael Turquette <mturquette@baylibre.com>,
        Heiko =?ISO-8859-1?Q?St=FCbner?= <heiko@sntech.de>,
        aisheng.dong@nxp.com, mchehab+samsung@kernel.org,
        Jonathan Corbet <corbet@lwn.net>,
        Stephen Boyd <sboyd@codeaurora.org>
Date:   Thu, 25 Oct 2018 10:57:52 +0200
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         <CAGAzgsrTPoBaP_wZ1fUryTgbu6RhwOtGPDV_njamOoiiH9Or9g@mail.gmail.com>
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On Wed, 2018-10-24 at 13:50 -0700, dbasehore . wrote:
> On Wed, Oct 24, 2018 at 1:15 PM dbasehore . <dbasehore@chromium.org> wrote:
> > 
> > On Wed, Oct 24, 2018 at 2:51 AM Jerome Brunet <jbrunet@baylibre.com> wrote:
> > > 
> > > 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.
> > 
> > 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.
> 
> Nevermind, this is incorrect. The clk_pm_runtime_put is within this if
> statement too, so there isn't an issue here.
> 
> > 
> > > 
> > > 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.
> 
> Nevermind, that's not the case. We add the first core that has a
> non-zero prepare_count to the first (or we go all the way to root).
> That core can't encounter an error since those only happen in the
> prepare_count == 0 case. If it's NULL, clk_core_unprepare just
> returns.

Indeed, the diff is bit hard to follow and I got confused. With th patch
applied, things are more clear. Sorry about that

While correct, this code could simplified a bit

* unless the prepare_list is used anywhere without starting with a list_add(),
reseting the list pointer is not necessary. It should be possible to remove the
list_del_init(). the 'err' label becomes just 'clk_core_unprepare(core->parent)'

* rolling back change under 'if' or 'goto label' are both fine IMO. It would be
easier to follow if only one method was used inside a single function, 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 */
> > > 
> > >