linux-kernel - Re: RFC: android logger feedback request

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Date:	Thu, 22 Dec 2011 12:20:26 +1100
From:	NeilBrown <neilb@...e.de>
To:	Tim Bird <tim.bird@...sony.com>
Cc:	Greg KH <gregkh@...e.de>,
	linux-embedded <linux-embedded@...r.kernel.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	john stultz <johnstul@...ibm.com>,
	Brian Swetland <swetland@...gle.com>,
	Kay Sievers <kay.sievers@...y.org>,
	Lennart Poettering <lennart@...ttering.net>
Subject: Re: RFC: android logger feedback request

On Wed, 21 Dec 2011 16:36:21 -0800 Tim Bird <tim.bird@...sony.com> wrote:

> On 12/21/2011 03:19 PM, Greg KH wrote:
> > That all describes the current code, but you haven't described what's
> > wrong with the existing syslog interface that requires this new driver
> > to be written.  And why can't the existing interface be fixed to address
> > these (potential) shortcomings?
> 
> 
> >> One specific question I have is where is the most appropriate
> >> place for this code to live, in the kernel source tree?
> >> Other embedded systems might want to use this system (it
> >> is simpler than syslog, and superior in some ways), so I don't
> >> think it should remain in an android-specific directory.
> > 
> > What way is it superior?
> 
> Here are some ways that this code is superior to syslog:

It is certainly nice and simple.  It really looks more like a filesystem than
a char device though...  though they aren't really files so much as lossy
pipes.  I don't think that's a problem though, lots of things in filesystems
don't behave exactly like files.

If you created a 'logbuf' filesystem that used libfs to provide a single
directory in which privileged processes could create files then you wouldn't
need the kernel to "know" the allowed logs: radio, events, main, system.
The size could be set by ftruncate() (by privileged used again) rather than
being hardcoded.

You would defined 'read' and 'write' much like you currently do to create a list of
datagrams in a circular buffer and replace the ioctls by more standard
interfaces:

LOGGER_GET_LOG_BUG_SIZE would use 'stat' and the st_blocks field
LOGGER_GET_LOG_LEN would use 'stat' and the st_size field
LOGGER_GET_NEXT_ENTRY_LEN could use the FIONREAD ioctl
LOGGER_FLUSH_LOG could use ftruncate

The result would be much the same amount of code, but an interface which has
fewer details hard-coded and is generally more versatile and accessible.

NeilBrown


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