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[46.91.226.206]) by smtp.gmail.com with ESMTPSA id y15sm4712432wrh.94.2019.11.08.02.11.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 08 Nov 2019 02:11:17 -0800 (PST) Date: Fri, 8 Nov 2019 11:11:16 +0100 From: Thierry Reding To: Stephen Boyd Cc: Dmitry Osipenko , Sowjanya Komatineni , 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 Message-ID: <20191108101116.GA2583136@ulmo> References: <1565984527-5272-1-git-send-email-skomatineni@nvidia.com> <1565984527-5272-8-git-send-email-skomatineni@nvidia.com> <20191106231005.F2CD820869@mail.kernel.org> <20191107152115.GA2580600@ulmo> <20191107191933.0B18021D6C@mail.kernel.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="h31gzZEtNLTqOjlF" Content-Disposition: inline In-Reply-To: <20191107191933.0B18021D6C@mail.kernel.org> User-Agent: Mutt/1.12.2 (2019-09-21) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --h31gzZEtNLTqOjlF Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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 =D0=BF=D0=B8=D1=88=D0=B5=D1=82: > > > > 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. > > > > =20 > > > > 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 i= ndex > > > > typically doesn't matter unless we're trying to communicate somethi= ng > > > > from the framework to the provider driver. Put another way, I would > > > > think the provider driver can figure out the index itself without h= aving > > > > to go through the framework to do so. > > >=20 > > > Isn't it a bit wasteful to duplicate information about the parent wit= hin > > > a provider if framework already has that info? The whole point of this > > > new API is to allow providers to avoid that unnecessary duplication. > > >=20 > > > Please note that clk_hw_get_parent_index is getting used only at the > > > resume time and not at suspend. > >=20 > > 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. >=20 > Don't we already have a "saved parent" field by storing the pointer to > the clk_hw? >=20 > >=20 > > 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. > >=20 >=20 > 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. >=20 > 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. > 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 g= iven * clock does not have a current parent. */ int clk_hw_get_parent_index(struct clk_hw *hw) { struct clk_hw *parent =3D 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. 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. Any thoughts? 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