linux-kernel - Re: [RFC] clk: Remove cached cores in parent map during unregister

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Date:   Mon, 26 Aug 2019 14:24:14 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Michael Turquette <mturquette@...libre.com>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Cc:     linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, Rob Clark <robclark@...il.com>,
        Sean Paul <seanpaul@...omium.org>
Subject: Re: [RFC] clk: Remove cached cores in parent map during unregister

Quoting Stephen Boyd (2019-08-21 11:10:08)
> Quoting Stephen Boyd (2019-07-29 15:46:51)
> > Quoting Bjorn Andersson (2019-07-22 22:14:46)
> > > As clocks are registered their parents are resolved and the parent_map
> > > is updated to cache the clk_core objects of each existing parent.
> > > But in the event of a clock being unregistered this cache will carry
> > > dangling pointers if not invalidated, so do this for all children of the
> > > clock being unregistered.
> > > 
> > > Signed-off-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> > > ---
> > > 
> > > This resolves the issue seen where the DSI PLL (and it's provided clocks) is
> > > being registered and unregistered multiple times due to probe deferral.
> > > 
> > > Marking it RFC because I don't fully understand the life of the clock yet.
> > 
> > The concept sounds sane but the implementation is going to be not much
> > fun. The problem is that a clk can be in many different parent caches,
> > even ones for clks that aren't currently parented to it. We would need
> > to walk the entire tree(s) and find anywhere that we've cached the
> > clk_core pointer and invalidate it. Maybe we can speed that up a little
> > bit by keeping a reference to the entry of each parent cache that is for
> > the parent we're removing, essentially holding an inverse cache, but I'm
> > not sure it will provide any benefit besides wasting space for this one
> > operation that we shouldn't be doing very often if at all.
> > 
> > It certainly sounds easier to iterate through the whole tree and just
> > invalidate entries in all the caches under the prepare lock. We can
> > optimize it later.
> 
> Here's an attempt at the simple approach. There's another problem where
> the cached 'hw' member of the parent data is held around when we don't
> know when the caller has destroyed it. Not much else we can do for that
> though.
> 
> ---8<---
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index c0990703ce54..f42a803fb11a 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -3737,6 +3737,37 @@ static const struct clk_ops clk_nodrv_ops = {
>         .set_parent     = clk_nodrv_set_parent,
>  };
>  
> +static void clk_core_evict_parent_cache_subtree(struct clk_core *root,
> +                                               struct clk_core *target)
> +{
> +       int i;
> +       struct clk_core *child;
> +
> +       if (!root)
> +               return;

I don't think we need this part. Child is always a valid pointer.