From: Thierry Reding <thierry.reding@gmail.com>
To: Stephen Boyd <sboyd@kernel.org>
Cc: Dmitry Osipenko <digetx@gmail.com>,
Sowjanya Komatineni <skomatineni@nvidia.com>,
jason@lakedaemon.net, jonathanh@nvidia.com,
linus.walleij@linaro.org, marc.zyngier@arm.com,
mark.rutland@arm.com, stefan@agner.ch, tglx@linutronix.de,
pdeschrijver@nvidia.com, pgaikwad@nvidia.com,
linux-clk@vger.kernel.org, linux-gpio@vger.kernel.org,
jckuo@nvidia.com, josephl@nvidia.com, talho@nvidia.com,
linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org,
mperttunen@nvidia.com, spatra@nvidia.com, robh+dt@kernel.org,
devicetree@vger.kernel.org, rjw@rjwysocki.net,
viresh.kumar@linaro.org, linux-pm@vger.kernel.org
Subject: Re: [PATCH v9 07/22] clk: Add API to get index of the clock parent
Date: Fri, 8 Nov 2019 19:55:03 +0100 [thread overview]
Message-ID: <20191108185503.GB3384779@ulmo> (raw)
In-Reply-To: <20191108181249.E284E214DB@mail.kernel.org>
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On Fri, Nov 08, 2019 at 10:12:49AM -0800, Stephen Boyd wrote:
> Quoting Thierry Reding (2019-11-08 02:11:16)
> > On Thu, Nov 07, 2019 at 11:19:32AM -0800, Stephen Boyd wrote:
> > > Quoting Thierry Reding (2019-11-07 07:21:15)
> > > > On Thu, Nov 07, 2019 at 03:54:03AM +0300, Dmitry Osipenko wrote:
> > > > > 07.11.2019 02:10, Stephen Boyd пишет:
> > > > > > Quoting Sowjanya Komatineni (2019-08-16 12:41:52)
> > > > > >> This patch adds an API clk_hw_get_parent_index to get index of the
> > > > > >> clock parent to use during the clock restore operations on system
> > > > > >> resume.
> > > > > >
> > > > > > Is there a reason we can't save the clk hw index at suspend time by
> > > > > > reading the hardware to understand the current parent? The parent index
> > > > > > typically doesn't matter unless we're trying to communicate something
> > > > > > from the framework to the provider driver. Put another way, I would
> > > > > > think the provider driver can figure out the index itself without having
> > > > > > to go through the framework to do so.
> > > > >
> > > > > Isn't it a bit wasteful to duplicate information about the parent within
> > > > > a provider if framework already has that info? The whole point of this
> > > > > new API is to allow providers to avoid that unnecessary duplication.
> > > > >
> > > > > Please note that clk_hw_get_parent_index is getting used only at the
> > > > > resume time and not at suspend.
> > > >
> > > > I agree with this. All of the information that we need is already cached
> > > > in the framework. Doing this in the driver would mean essentially adding
> > > > a "saved parent" field along with code to read the value at suspend time
> > > > to the three types of clocks that currently use this core helper.
> > >
> > > Don't we already have a "saved parent" field by storing the pointer to
> > > the clk_hw?
> > >
> > > >
> > > > That's certainly something that we *can* do, but it doesn't sound like a
> > > > better option than simply querying the framework for the value that we
> > > > need.
> > > >
> > >
> > > Let me say this another way. Why does this driver want to know the index
> > > that the framework uses for some clk_hw pointer? Perhaps it happens to
> > > align with the same value that hardware uses, but I still don't
> > > understand why the driver wants to know what the framework has decided
> > > is the index for some clk_hw pointer.
> > >
> > > Or is this something like "give me the index for the parent that the
> > > framework thinks I currently have but in reality don't have anymore
> > > because the register contents were wiped and we need to reparent it"?
> >
> > Yeah, that's exactly what this is being used for. It's used to restore
> > the parent/child relationship during resume after the registers have
> > been wiped during supend.
>
> Ok cool. Our whole suspend/resume and save/restore story hasn't really
> been well thought out so we may want to pull all this logic into the
> core one day. For now it's OK to do the heavy lifting from provider
> drivers until someone gets a better grasp on how this should all work.
Ah, that would explain why I was scratching my head trying to understand
how exactly this was supposed to work. It did feel like there was some
infrastructure there, but looking around there wasn't a very consistent
usage pattern that I could find.
I think suspend/resume is always a little tricky. For example the clocks
may required a slightly different logical sequences between SoCs. Maybe
even different types of clocks have different needs. We seem to have a
bit of that on Tegra alone already. Without having delved into this too
much, it seems to me like the core can't do a whole lot without stepping
(potentially) on drivers' toes.
The current save_context/restore_context seems to be mostly that,
though, so I think it's a good starting point. You're right that we may
eventually see clearer patterns appear.
> > > A generic API to get any index for this question is overkill and we should
> > > consider adding some sort of API like clk_hw_get_current_parent_index(),
> > > or a framework flag that tells the framework this parent is incorrect
> > > and we need to call the .set_parent() op again to reconfigure it.
> >
> > Okay, I think I see what you're saying. The current implementation does
> > carry a bit of a risk because users could be calling this function with
> > any arbitrary pair of struct clk_hw *, even completely unrelated ones.
> >
> > How about we turn it into this instead:
> >
> > /**
> > * clk_hw_get_parent_index - return the index of the parent clock
> > * @hw: clk_hw associated with the clk being consumed
> > *
> > * Fetches and returns the index of parent clock. Returns -EINVAL if the given
> > * clock does not have a current parent.
> > */
> > int clk_hw_get_parent_index(struct clk_hw *hw)
> > {
> > struct clk_hw *parent = clk_hw_get_parent(hw);
> >
> > if (!parent)
> > return -EINVAL;
> >
> > return clk_fetch_parent_index(hw->core, parent->core);
> > }
> > EXPORT_SYMBOL_GPL(clk_hw_get_parent_index);
> >
> > I think that has the advantage that we can't pass it a parent that's not
> > really a parent. There's still the slightly weird case where the clock
> > doesn't have a current parent, but hopefully that's something we are not
> > going to encounter much. After all this only makes sense to be called on
> > mux clocks and they always do have a parent by definition.
>
> Right.
>
> >
> > Perhaps we should be more explicit and wrap that !parent conditional in
> > a WARN_ON()? In my local patches I do that at the call sites because
> > they are all functions returning void, so we'd be silently ignoring the
> > cases, but I think it may make sense to have it in the core.
> >
>
> Sure a WARN_ON() sounds fair. That will not take the whole task down
> and makes sure that drivers aren't doing something incorrect. Otherwise,
> this looks good and we can optimize by caching the parent index later if
> we really need to.
Okay, great. I'll go replace the above patch in the branch that I have.
I'm not sure if you saw it, but I had sent this in a pull request for
v5.5-rc1 about a week ago because I've got Tegra clock driver patches
that depend on this. I can replace this patch with the above proposal
and update the Tegra clock driver branch and then resend the two pull
requests.
Does that sound like a plan?
Thierry
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next prev parent reply other threads:[~2019-11-08 18:55 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-16 19:41 [PATCH v9 00/22] SC7 entry and exit support for Tegra210 Sowjanya Komatineni
2019-08-16 19:41 ` [PATCH v9 01/22] pinctrl: tegra: Fix write barrier placement in pmx_writel Sowjanya Komatineni
2019-08-18 22:20 ` Linus Walleij
2019-08-16 19:41 ` [PATCH v9 02/22] pinctrl: tegra: Flush pinctrl writes during resume Sowjanya Komatineni
2019-08-18 22:20 ` Linus Walleij
2019-08-16 19:41 ` [PATCH v9 03/22] clk: tegra: divider: Save and restore divider rate Sowjanya Komatineni
2019-08-16 19:41 ` [PATCH v9 04/22] clk: tegra: pllout: Save and restore pllout context Sowjanya Komatineni
2019-08-16 19:41 ` [PATCH v9 05/22] clk: tegra: pll: Save and restore pll context Sowjanya Komatineni
2019-08-16 19:41 ` [PATCH v9 06/22] clk: tegra: Support for OSC context save and restore Sowjanya Komatineni
2019-08-16 19:41 ` [PATCH v9 07/22] clk: Add API to get index of the clock parent Sowjanya Komatineni
2019-11-06 23:10 ` Stephen Boyd
2019-11-07 0:54 ` Dmitry Osipenko
2019-11-07 15:21 ` Thierry Reding
2019-11-07 19:19 ` Stephen Boyd
2019-11-08 10:11 ` Thierry Reding
2019-11-08 18:12 ` Stephen Boyd
2019-11-08 18:55 ` Thierry Reding [this message]
2019-11-08 21:15 ` Stephen Boyd
2019-08-16 19:41 ` [PATCH v9 08/22] clk: tegra: periph: Add restore_context support Sowjanya Komatineni
2019-08-16 19:41 ` [PATCH v9 09/22] clk: tegra: clk-super: Fix to enable PLLP branches to CPU Sowjanya Komatineni
2019-08-16 19:41 ` [PATCH v9 10/22] clk: tegra: clk-super: Add restore-context support Sowjanya Komatineni
2019-08-16 19:41 ` [PATCH v9 11/22] clk: tegra: clk-dfll: Add suspend and resume support Sowjanya Komatineni
2019-11-08 21:20 ` Stephen Boyd
2019-11-08 23:38 ` Dmitry Osipenko
2019-08-16 19:41 ` [PATCH v9 12/22] cpufreq: tegra124: " Sowjanya Komatineni
2019-11-02 14:42 ` Thierry Reding
2019-08-16 19:41 ` [PATCH v9 13/22] clk: tegra210: Use fence_udelay during PLLU init Sowjanya Komatineni
2019-08-16 19:41 ` [PATCH v9 14/22] clk: tegra: Share clk and rst register defines with Tegra clock driver Sowjanya Komatineni
2019-08-16 19:42 ` [PATCH v9 15/22] clk: tegra210: Add suspend and resume support Sowjanya Komatineni
2019-08-19 16:47 ` Dmitry Osipenko
2019-08-16 19:42 ` [PATCH v9 16/22] soc/tegra: pmc: Allow to support more tegras wake Sowjanya Komatineni
2019-08-16 19:42 ` [PATCH v9 17/22] soc/tegra: pmc: Add pmc wake support for tegra210 Sowjanya Komatineni
2019-08-16 19:42 ` [PATCH v9 18/22] arm64: tegra: Enable wake from deep sleep on RTC alarm Sowjanya Komatineni
2019-08-16 19:42 ` [PATCH v9 19/22] soc/tegra: pmc: Configure core power request polarity Sowjanya Komatineni
2019-08-16 19:42 ` [PATCH v9 20/22] soc/tegra: pmc: Configure deep sleep control settings Sowjanya Komatineni
2019-08-19 16:48 ` Dmitry Osipenko
2019-08-19 18:20 ` Sowjanya Komatineni
2019-08-19 19:07 ` Sowjanya Komatineni
2019-08-19 19:33 ` Dmitry Osipenko
2019-08-16 19:42 ` [PATCH v9 21/22] arm64: dts: tegra210-p2180: Jetson TX1 SC7 timings Sowjanya Komatineni
2019-08-16 19:42 ` [PATCH v9 22/22] arm64: dts: tegra210-p3450: Jetson Nano " Sowjanya Komatineni
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