Re: [PATCH v1] clk: Convert managed get functions to devm_add_action API

From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Marc Gonzalez <marc.w.gonzalez@free.fr>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>,
	Stephen Boyd <sboyd@kernel.org>,
	Michael Turquette <mturquette@baylibre.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
	Russell King <rmk+kernel@armlinux.org.uk>,
	Guenter Roeck <linux@roeck-us.net>,
	linux-clk <linux-clk@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	x86 <x86@kernel.org>
Subject: Re: [PATCH v1] clk: Convert managed get functions to devm_add_action API
Date: Thu, 12 Dec 2019 11:10:02 -0800	[thread overview]
Message-ID: <20191212191002.GA101194@dtor-ws> (raw)
In-Reply-To: <6ce49a67-8065-277b-5f80-ed47011e50d6@arm.com>

On Thu, Dec 12, 2019 at 06:15:16PM +0000, Robin Murphy wrote:
> On 12/12/2019 4:59 pm, Marc Gonzalez wrote:
> > On 12/12/2019 15:47, Robin Murphy wrote:
> > 
> > > On 12/12/2019 1:53 pm, Marc Gonzalez wrote:
> > > 
> > > > On 11/12/2019 23:28, Dmitry Torokhov wrote:
> > > > 
> > > > > On Wed, Dec 11, 2019 at 05:17:28PM +0100, Marc Gonzalez wrote:
> > > > > 
> > > > > > What is the rationale for the devm_add_action API?
> > > > > 
> > > > > For one-off and maybe complex unwind actions in drivers that wish to use
> > > > > devm API (as mixing devm and manual release is verboten). Also is often
> > > > > used when some core subsystem does not provide enough devm APIs.
> > > > 
> > > > Thanks for the insight, Dmitry. Thanks to Robin too.
> > > > 
> > > > This is what I understand so far:
> > > > 
> > > > devm_add_action() is nice because it hides/factorizes the complexity
> > > > of the devres API, but it incurs a small storage overhead of one
> > > > pointer per call, which makes it unfit for frequently used actions,
> > > > such as clk_get.
> > > > 
> > > > Is that correct?
> > > > 
> > > > My question is: why not design the API without the small overhead?
> > > 
> > > Probably because on most architectures, ARCH_KMALLOC_MINALIGN is at
> > > least as big as two pointers anyway, so this "overhead" should mostly be
> > > free in practice. Plus the devres API is almost entirely about being
> > > able to write simple robust code, rather than absolute efficiency - I
> > > mean, struct devres itself is already 5 pointers large at the absolute
> > > minimum ;)
> > 
> > (3 pointers: 1 list_head + 1 function pointer)
> 
> Ah yes, I failed to mentally preprocess the debug config :)
> 
> > I'm confused. The first patch was criticized for potentially adding
> > an extra pointer for every devm_clk_get (e.g. 800 bytes on a 64-bit
> > platform with 100 clocks).
> 
> I'm not sure it was a criticism so much as an observation of an aspect that
> deserved consideration (certainly it was on my part, and I read Dmitry's "It
> might still, ..." as implying the same). I'd say by this point it has been
> thoroughly considered, and personally I'm now happy with the conclusion that
> the kind of embedded platforms that will have many dozens of clocks are also
> the kind that will tend to have enough padding to make it moot, and thus the
> code simplification probably is worthwhile overall.

I wonder if we could actually avoid allocating the data with
ARCH_KMALLOC_MINALIGN in all the cases. It is definitely needed for the
devm_k*alloc() group of functions as they are direct replacement for
k*alloc() APIs that give users aligned memory, but for other data
structures (clocks, regulators, etc, etc) it is not required.

Thanks.

-- 
Dmitry