Re: [PATCH V3 4/5] clk: core: add CLK_OPS_PARENT_ON flags to support clocks require parent on

[Dong Aisheng]

On Mon, Aug 17, 2015 at 08:45:18PM +0800, Dong Aisheng wrote:
> On Thu, Aug 13, 2015 at 06:01:09PM -0700, Stephen Boyd wrote:
> > On 07/28, Dong Aisheng wrote:
> > > On Freescale i.MX7D platform, all clocks operations, including
> > > enable/disable, rate change and re-parent, requires its parent
> > > clock on. Current clock core can not support it well.
> > > This patch introduce a new flag CLK_OPS_PARENT_ON to handle this
> > > special case in clock core that enable its parent clock firstly for
> > > each operation and disable it later after operation complete.
> > > 
> > > This patch fixes disaling clocks while its parent is off.
> > > This is a special case that is caused by a state mis-align between
> > > HW and SW in clock tree during booting.
> > > Usually in uboot, we may enable all clocks in HW by default.
> > > And during kernel booting time, the parent clock could be disabled in its
> > > driver probe due to calling clk_prepare_enable and clk_disable_unprepare.
> > > Because it's child clock is only enabled in HW while its SW usecount
> > > in clock tree is still 0, so clk_disable of parent clock will gate
> > > the parent clock in both HW and SW usecount ultimately.
> > > Then there will be a clock is on in HW while its parent is disabled.
> > > 
> > > Later when clock core does clk_disable_unused, this clock disable
> > > will cause system hang due to the limitation of operation requiring
> > > its parent clock on.
> > > 
> > > Cc: Mike Turquette <mturquette@xxxxxxxxxx>
> > > Cc: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
> > > Signed-off-by: Dong Aisheng <aisheng.dong@xxxxxxxxxxxxx>
> > > ---
> > 
> > Sorry, I still don't agree with this patch. There's no reason we
> > should be turning on clocks during late init so that we can turn
> > off clocks.
> 
> Can't the reason be that it's fairly possible one clock is enabled
> in HW while it's parent is disabled in initial clock tree state
> and to enable parent clocks to disable itself is required by its special
> HW characteristic?
> 
> Dosen't it quite clear from HW point of view?
> 
> > If there's some sort of problem in doing that we
> > should figure it out and make it so that during late init we turn
> > off clocks from the leaves of the tree to the root.
> > 
> 
> Turning off clocks from the leaves to root probably may require clock core
> to provide a way to keep the parent clock enabled once finding its child
> is still on in HW (clk_core_is_enabled() returns true) but enable_count
> is zero before late init.
> 
> One possible solution may be leaving parent clocks on in HW during disable
> once finding its child is on in HW and only decrease the parent's refcount,
> and then replying on the later clk_disable_unused() to disable both child
> and parent from leave to root.
> e.g. clock A: parent, clock B: child of A
> Initial state: clock B is enabled in HW while refcount is zero
> Step1: Driver A enable clock A during probe
>  A: refcount becomes 1 HW state: enabled
> Step2: Driver A disable clock A after probe
>  A: refcount becomes 0 HW state: enabled (only decrease refcount)
> 
> Then Clock A will be the same state as B, HW enabled while refcount is zero(
> means no users), the later clk_disable_unusersd() will disable them all
> from leave to root.
> 
> This is a workable solution but seems much more complicated than the exist
> one in this patch which is only 5 lines of code changes.
> 
> And the question is:
> since we already have the support of CLK_OPS_PARENT_ON (required by
> clock set_rate/re-parent), why we still need invent another complicated
> mechanism to support avoiding enable parent clock only for clk_disable_unused()?
> Is that really worthy?
> And it's also less power efficiency than the one in the patch.
> 
> > I agree that there's a problem here where we don't properly
> > handle keeping children clocks on if a driver comes in and turns
> > off a clock in the middle of the tree before late init. That's a
> > real bug, and we should fix it.
> 
> Sorry, i still can't understand it's a bug.
> Can you help explain more?
> 
> It looks to me like reasonable.
> Enable/disable clock in driver is just one case, the initial clock tree may
> also have such cases.
> (Here i took the 'children clocks' you said as the one who's child clock is on
> in HW while refcount is zero, fix me if wrong)
> 
> And it seems not so quite make sense to not physically disable the clock
> when there's already no child users(refcount becomes zero) and i don't
> think the child clock's default enablement state in HW means a valid user
> since it's just caused by misalignment between HW and SW clock tree during
> kernel booting phase which is meaningless.
> And that seems is why the clk_disable_unused() function exist for fixing
> this state misalignment issue.
> 
> > Mike Turquette has been doing
> > some work to have a way to indicate that certain clocks should be
> > kept on until client drivers grab them. 
> 
> Sorry i can't see how this help on my issue.
> 
> > I think it will also make
> > sure to up refcounts on parent clocks in the middle of the tree
> > when it figures out that a child clock is enabled. Would that be
> > all that we need to do to fix this problem?
> > 
> 
> Then when will we down the refcounts on parent clocks and when to disable it?
> The current clk_disable_unused() only handle HW clk enable/disable, no
> refcount operations.
> Not sure how this is going to fix my issues.
> 
> And again, as i said above, i don't think it makes much sense to not disable
> parent only if child is enabled in HW, unless there's more strong reasons.
> 
> > Also, the subject of this patch and patch 5 are the same. Why?
> > 
> 
> Sorry, mainly because the full feature of CLK_OPS_PARENT_ON is divided into
> two patches for better review, their commit message is different.
> patch 4 is adding support for clk_disable_unused() while patch 5 is for
> clk_setrate/clk_reparent.
> I could reform the subject if needed.
> 
> Regards
> Dong Aisheng
> 

Hi Mike & Stephen,

Any comments about this?

Regards
Dong Aisheng

> > -- 
> > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> > a Linux Foundation Collaborative Project
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