linux-kernel - Re: [PATCH v5 00/10] clk: implement clock rate protection mechanism

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Date:   Fri, 02 Feb 2018 13:50:28 +0100
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Stephen Boyd <sboyd@...eaurora.org>
Cc:     Michael Turquette <mturquette@...libre.com>,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        Russell King <linux@...linux.org.uk>,
        Linus Walleij <linus.walleij@...aro.org>,
        Quentin Schulz <quentin.schulz@...e-electrons.com>,
        Kevin Hilman <khilman@...libre.com>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>
Subject: Re: [PATCH v5 00/10] clk: implement clock rate protection mechanism

On Thu, 2018-02-01 at 09:43 -0800, Stephen Boyd wrote:
> > > > Applied to clk-protect-rate, with the exception that I did not apply
> > > > "clk: fix CLK_SET_RATE_GATE with clock rate protection" as it breaks
> > > > qcom clk code.
> > > > 
> > > > Stephen, do you plan to fix up the qcom clock code so that the
> > > > SET_RATE_GATE improvement can go in?
> > > > 
> > > 
> > > I started working on it a while back. Let's see if I can finish
> > > it off this weekend.
> > > 
> > 
> > Hi Stephen,
> > 
> > Have you been able find something to fix the qcom code regarding this issue ?
> > 
> 
> This is what I have. I'm unhappy with a few things. First, I made
> a spinlock for each clk, which is overkill. Most likely, just a
> single spinlock is needed per clk-controller device. Second, I
> haven't finished off the branch/gate part, so gating/ungating of
> branches needs to be locked as well to prevent branches from
> turning on while rates change. And finally, the 'branches' list is
> duplicating a bunch of information about the child clks of an
> RCG, so it feels like we need a core framework API to enable and
> disable clks forcibly while remembering what is enabled/disabled
> or at least to walk the clk tree and call some function.

Looks similar to Mike's CCR idea ;)

> 
> The spinlock per clk-controller is duplicating the regmap lock we
> already have, so we may want a regmap API to grab the lock, and
> then another regmap API to do reads/writes without grabbing the
> lock, and then finally release the lock with a regmap unlock API.

There is 'regsequence' for multiple write in a burst, but that's only if you do
write only ... I suppose you are more in read/update/writeback mode, so it
probably does not help much.

Maybe we could extend regmap's regsequence, to do a sequence of
regmap_update_bits() ?

Another possibility could be to provide your own lock/unlock ops when
registering the regmap. With this, you might be able to supply your own spinlock
to regmap. This is already supported by regmap, would it help ?

> This part is mostly an optimization, but it would be nice to have
> so that multiple writes could be done in sequence. This way, the
> RCG code could do the special locking sequence and the branch
> code could do the fire and forget single bit update.