netdev - Re: [PATCH] phylib: add mdio-gpio bus driver (v2)

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Message-ID: <fa686aa40810280830i1a5a7b84u1dfa8cd97c6b1420@mail.gmail.com>
Date:	Tue, 28 Oct 2008 09:30:10 -0600
From:	"Grant Likely" <grant.likely@...retlab.ca>
To:	"Russell King - ARM Linux" <linux@....linux.org.uk>
Cc:	"Paulius Zaleckas" <paulius.zaleckas@...tonika.lt>,
	netdev@...r.kernel.org, linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-embedded@...r.kernel.org
Subject: Re: [PATCH] phylib: add mdio-gpio bus driver (v2)

On Tue, Oct 28, 2008 at 9:17 AM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Tue, Oct 28, 2008 at 07:08:06AM -0600, Grant Likely wrote:
>> On Tue, Oct 28, 2008 at 1:46 AM, Paulius Zaleckas
>> <paulius.zaleckas@...tonika.lt> wrote:
>> > Grant Likely wrote:
>> >> The IRQ array is fixed size.  You can add it to the mdio_gpio_info
>> >> structure and then just set the pointer here so that only one kzalloc
>> >> is needed.
>> >
>> > It can be put in mdio_gpio_info, but please note that mdio_gpio_info is
>> > allocated with kzalloc() and irq with kmalloc(), because there is no need
>> > to fill this array with zeros(see below).
>>
>> Adding an additional 32 words to be zeroed in the mdio_gpio_info
>> kzalloc is considerably cheaper than doing an additional kmalloc.
>> Plus, once the array is zeroed it is then in the cache and so the
>> filling it with -1 also becomes cheaper.
>
> Actually no, it doesn't become cheaper.  You're making the assumption
> that cache lines are allocated when memory is written to.  This isn't
> the case with the vast majority of ARM CPUs.

Okay, I wasn't aware of that on ARM.

However it is still true that increasing the size of the kzalloc is
cheaper than doing 2 allocs.

g.


-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.
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