linux-kernel - Re: [PATCH v1] clk: Convert managed get functions to devm_add

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Date:   Thu, 12 Dec 2019 11:10:02 -0800
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Marc Gonzalez <marc.w.gonzalez@...e.fr>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Guenter Roeck <linux@...ck-us.net>,
        linux-clk <linux-clk@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        x86 <x86@...nel.org>
Subject: Re: [PATCH v1] clk: Convert managed get functions to devm_add_action
 API

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