linux-kernel - Re: [PATCH] RFC: AMBA bus discardable probe() function

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Date:	Wed, 4 Aug 2010 23:24:10 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Linus Walleij <linus.walleij@...ricsson.com>
Cc:	linux-kernel@...r.kernel.org, Greg Kroah-Hartman <gregkh@...e.de>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>
Subject: Re: [PATCH] RFC: AMBA bus discardable probe() function

On Wed, Aug 04, 2010 at 02:39:03PM +0200, Linus Walleij wrote:
> Fighting a compilation warning when using __init on probe():s in
> the AMBA (PrimeCell) bus abstraction, the intended effect of
> discarding AMBA probe():s is not achieveable without first adding
> the amba_driver_probe() function akin to platform_driver_probe().

Well, I've decided to investigate what kind of savings we're looking
at.  Going by my most recently built Realview kernel, I see in
System.map:

c01754c4 t pl061_probe
c01757b0 t pl061_irq_handler
	=> 748 bytes

c01842dc t clcdfb_probe
c0184658 t clcdfb_check_var
	=> 892 bytes

c01a3860 t pl011_probe
c01a39e4 T dev_driver_string
	=> 388 bytes

c01ee0c4 t pl030_probe
c01ee1dc t pl031_alarm_irq_enable
	=> 280 bytes

c01ee7e8 t pl031_probe
c01ee950 t pl031_interrupt
	=> 360 bytes

c02b43dc t amba_kmi_probe
c02b453c t ds1307_probe
	=> 352 bytes

c02b4b84 t mmci_probe
c02b4f74 t aaci_probe
c02b53fc t smc_drv_remove
	=> 2168 bytes

which gives a total of 5188 bytes for all the above probe functions.
However, in order to free this, we're adding 184 bytes of code and
literal pool to achieve this:

c01847b8 t amba_driver_probe_fail
c01847cc t resource_show
	=> 20 bytes

c0184e40 T amba_driver_probe
c0184ee4 W unxlate_dev_mem_ptr
	=> 164 bytes

This reduces the saving to 5004 bytes.

The overall kernel size is 3877020 bytes, which means we're potentially
allowing for 0.13% of the kernel to be freed at run time - which may
equate to one or at most two additional pages.
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