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Message-Id: <200711051305.13980.david-b@pacbell.net>
Date: Mon, 5 Nov 2007 13:05:13 -0800
From: David Brownell <david-b@...bell.net>
To: Linux Kernel list <linux-kernel@...r.kernel.org>
Cc: Felipe Balbi <felipebalbi@...rs.sourceforge.net>,
Bill Gatliff <bgat@...lgatliff.com>,
Haavard Skinnemoen <hskinnemoen@...el.com>,
Andrew Victor <andrew@...people.com>,
Tony Lindgren <tony@...mide.com>,
Jean Delvare <khali@...ux-fr.org>,
"eric miao" <eric.y.miao@...il.com>,
Kevin Hilman <khilman@...sta.com>,
Paul Mundt <lethal@...ux-sh.org>,
Ben Dooks <ben@...nity.fluff.org>
Subject: Re: [patch/rfc 1/4] GPIO implementation framework
On Monday 29 October 2007, David Brownell wrote:
>
> Provides new implementation infrastructure that platforms may choose to use
> when implementing the GPIO programming interface. Platforms can update their
> GPIO support to use this. The downside is slower access to non-inlined GPIOs;
> rarely a problem except when bitbanging some protocol.
I was asked just what that overhead *is* ... and it surprised me.
A summary of the results is appended to this note.
Fortuntely it turns out those problems all go away if the gpiolib
code uses a *raw* spinlock to guard its table lookups. With a raw
spinlock, any performance impact of gpiolib seems to be well under
a microsecond in this bitbang context (and not objectionable).
Preempt became free; enabling debug options had only a minor cost.
That's as it should be, since the only substantive changes were to
grab and release a lock, do one table lookup a bit differently, and
add one indirection function call ... changes which should not have
any visible performance impact on per-bit codepaths, and one might
expect to cost on the order of one dozen instructions.
So the next version of this code will include a few minor bugfixes,
and will also use a raw spinlock to protect that table. A raw lock
seems appropriate there in any case, since non-sleeping GPIOs should
be accessible from hardirq contexts even on RT kernels.
If anyone has any strong arguments against using a raw spinlock
to protect that table, it'd be nice to know them sooner rather
than later.
- Dave
SUMMARY:
Using the i2c-gpio driver on a preempt kernel with all the usual
kernel debug options enabled, the per-bit times (*) went up in a
bad way: from about 6.4 usec/bit (original GPIO code on this board)
up to about 11.2 usec/bit (just switching to gpiolib), which is
well into "objectionable overhead" territory for bit access.
Just enabling preempt shot the time up to 7.4 usec/bit ... which is
also objectionable (it's all-the-time overhead that is clearly
needless), but much less so.
Converting the table lock to be a raw spinlock essentially removed
all non-debug overheads. It took enabling all those debug options
plus internal gpiolib debugging overhead to get those times up to
the 7.4 usec/bit that previously applied even with just preempt.
(*) Those times being eyeballed medians; I didn't make time to find
a way to export a few thousand measurements from the tool and
do the math. The typical range was +/- one usec.
The numbers include udelay() calls, so the relevant point is
the time *delta* attributable only to increased gpiolib costs,
not the base time (with udelays). The delta probably reflects
on the order of four GPIO calls: set two different bits, clear
one of them, and read it to make sure it cleared.
> The upside is:
>
> * Providing two features which were "want to have (but OK to defer)" when
> GPIO interfaces were first discussed in November 2006:
>
> - A "struct gpio_chip" to plug in GPIOs that aren't directly supported
> by SOC platforms, but come from FPGAs or other multifunction devices
> (like UCB-1x00 GPIOs).
>
> - Full support for message-based GPIO expanders, needing a gpio_chip
> hookup; previous support for this part of the programming interface
> was just stubs. (One example: the widely used pcf8574 I2C chips,
> with 8 GPIOs each.)
>
> * Including a non-stub implementation of the gpio_{request,free}() calls,
> which makes those calls much more useful. The diagnostic labels are
> also recorded given DEBUG_FS, so /sys/kernel/debug/gpio can show a
> snapshot of all GPIOs known to this infrastructure.
>
> The driver programming interfaces introduced in 2.6.21 do not change at all;
> this new infrastructure is entirely below the covers.
-
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