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

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Date:   Mon, 2 Dec 2019 10:25:37 +0100
From:   Marc Gonzalez <marc.w.gonzalez@...e.fr>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     linux-clk <linux-clk@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH v1] clk: Convert managed get functions to devm_add_action
 API

On 02/12/2019 02:42, Dmitry Torokhov wrote:

> On Thu, Nov 28, 2019 at 10:56:30AM -0800, Bjorn Andersson wrote:
> 
>> On Tue 26 Nov 08:13 PST 2019, Marc Gonzalez wrote:
>>
>>> Date: Tue, 26 Nov 2019 13:56:53 +0100
>>>
>>> Using devm_add_action_or_reset() produces simpler code and smaller
>>> object size:
>>>
>>> 1 file changed, 16 insertions(+), 46 deletions(-)
>>>
>>>     text	   data	    bss	    dec	    hex	filename
>>> -   1797	     80	      0	   1877	    755	drivers/clk/clk-devres.o
>>> +   1499	     56	      0	   1555	    613	drivers/clk/clk-devres.o
>>>
>>> Signed-off-by: Marc Gonzalez <marc.w.gonzalez@...e.fr>
>>
>> Looks neat
>>
>> Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> 
> This however increases the runtime costs as each custom action cost us
> an extra pointer. Given that in a system we likely have many clocks
> managed by devres, I am not sure that this code savings is actually
> gives us overall win. It might still, I just want to understand how we
> are allocating/packing devres structures.

I'm not 100% sure what you are saying.

Are you arguing that the proposed patch increases the run-time cost of
devm_clk_put() so much that the listed improvements (simpler source code,
smaller object size) are not worth it?

AFAIU, the release action is only called
- explicitly, when devm_clk_put() is called
- implicitly, when the device is removed

How often are clocks removed?

In hot code-path (called hundreds of times per second) it makes sense to
write more complex code, to shave a few cycles every iteration. But in
cold code-path, I think it's better to write short/simple code.

Regards.