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Subjectkernel/printk.c: increasing the buffer size to capture devfsd debug messages.

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Subject: kernel/printk.c: increase the buffer size to capture devfsd
debug messages.

Short Question: Can I use (128*1024) for the buffer size in
kernel/printk.c
and still boot the kernel using loadlin?

Long question and background:

I have been trying to track down a subtle
anomaly (not a fatal bug, but an annoying feature)
of devfs device name registration.

I am using scsi devices, and the problem is
(quoting from my earlier post):

>Namely the name "generic" or "disc" seem to
>exist from the start (after bootup), but
>the entry "cd" doesn't exist until I do something
>about accessing the CD somehow.
>(It seems that I fail the initial
>attempt to mount due to the missing name.)

Strange thing is that "generic" is registered
right from the start by the time I try to list
the device entries under a particular LUN of a
multi-CD changer device. (Doug Gilbert was kind
enough to look over my config files, and he could not
find anomaly in my setup.)

Anyway, by enabling the devfs debug message generation
in the kernel, I can follow what devfs (and devfsd)
are doing during the bootup and subsequent multi-user
transition phase, etc..

By tracing the messages, I am hoping I learn how the devfs
and devfsd interact in a real world setting.
There may be a thing or two that may have
been overlooked during the transition to the final 2.4.x kernel.

However, during this effort, I noticed one problem.
printk() uses rather smallish buffer (16K by default. Actually large
enough
for ordinary use, I think.)
Since devfs generates large amount of messages (depending
on the boot command line option.),
the initial part of messages is gone from dmesg output.

I have learned to used "-s" option of
dmesg to use a non-default value of the kernel buffer that
stores the messages that dmesg prints out.

16K is too small for devfs debugging.
I increased the kernel message size in kernel/printk.c
to 32KB. Still no luck.
Then to 64K. I still don't see the initial part of
boot up message in dmesg.
(However, I am now seeing the interesting part where
somehow "generic" is registered where "sr" names aren't.
lookup of "sr" names are done just prior to SCSI
listing in the dmesg whereas "generic" registration
happens after the listing. I am trying to figure out why.)

Now I used the following loadlin command line.
c:\loadlin\loadlin c:\loadlin\241-64k.dbg -noheap \
devfs=mount,dmod,dreg,dilookup root=/dev/sda6 ro vga=3 \
scsihosts=sym53c8xx:tmscsim
(No linebreaks in the real bat file.)

Now I have a big question about the dmesg message buffer, or
rather its size.

Can I use an arbitrary value for buffer size in kernel/printk.c? That
is, does the size need to be in the form of 2**N (two to the power of
some integer number)? I vaguely recall having read something
about this. I checked the code, and the code in printk.c itself doesn't
seem to have such a restrction. But you never know.

I am worried because
- if such restrction is there, the next value I can try is 128K, which
is rather large for a static kernel data structure, and

- at 128KB, I am wondering if that large buffer size
may have issues with loader program loadlin.exe invoked
from DOS.
(I am not sure if the big area used by such a large array
may interfere with the initial memory allocation done
by loadlin.exe. The bzipped kernel image is already slight
ly over 700Kb. I have heard some problems hitting
memory size limit during loading, etc.)

Has anyone tried 128K buffer size in kernel/printk.c
and still have the kernel boot (without
hard to notice memory corruption problems and other subtle bugs)?
Any hints and tips will be appreciated.

Happy Hacking,

Chiaki Ishikawa

PS: FYI, I am quoting my initial question to see
if anyone has experienced something similar.
Maybe the strange beast (multi-LUN CD changer)
is to blame.

---
(Below is a partially edited quote of an e-mail I sent
to linux-scsi mailing list.)
I have begun using devfs for about a couple of weeks now and
thank you for the great addition to linux.
Now I am happy to see the device names on the
scsi chain which won't be changed just because
I add/delete a device.

However, I noticed that there seems to be a subtle interaction of
devfs (+devfsd) and
the device names that appear under luns for
a scsi chain.

Namely the name "generic" or "disc" seem to
exist from the start (after bootup), but
the entry "cd" doesn't exist until I do something
about accessing the CD somehow.
(It seems that I fail the initial
attempt to mount due to the missing name.)


> Below is the more detailed problem about devfs
> name recognition thing.
> Has anyone seen something like this before?
>
> I am not entirely sure where to report this.
> (It might as well be the scsi system problem...)
> Help will be appreciated where to send the bug reports, etc..
> (Forwarding will be fine.)
>
> Happy Hacking,
>
> Chiaki
>
> ---
> I use Debian GNU/Linux, kernel 2.4.1.
> ishikawa@duron$ uname -a
> Linux duron 2.4.1 #18 Fri Feb 9 02:18:50 JST 2001 i686 unknown
>
> I have enabled devfs and installed devfsd.
>
> devfs is mounted at the boot time.
> Kernel command line: devfs=mount root=/dev/sda6 ro
> scsihosts=sym53c8xx:tmscsim BOOT_IMAGE=241.ey2
>
> devfs related message lines from dmesg:
> dmesg | grep -i devfs
> Kernel command line: devfs=mount root=/dev/sda6 ro
> scsihosts=sym53c8xx:tmscsim BOOT_IMAGE=241.ey2
> devfs: v0.102 (20000622) Richard Gooch (rgooch@atnf.csiro.au)
> devfs: boot_options: 0x0
> Mounted devfs on /dev
>
> I have observed an anomaly in device listing
> when done by "ls".
>
> I have a couple of CD devices on
> a SCSI chain.
> In the following listing,
> the device at host1/bus0/target6 is a 7 lun
> CD changer device.
> The device at target5 is a two lun CD/PD (PD
> seems to be recognized as optical device of some sort, i.e. an optical

> disc.
> Lun 0 for this combo is used for PD (disc).
> Lun 1 for this compo is used for CD.
>
> Please note that on the first try, ls didn't show any "cd" entry for
> the said devices. For comparison purposes,
> look specifically at target6/lun0.
>
> However, after playing with legacy names and
> cd into one of the device directories,
> the "cd" entries show up.
> See the ls listing under target6/lun0.
>
> Is this normal?
> I thought the entry for "cd" should be there from the beginning.
>
> NOTE: Come to think of it since "disc" is present for
> (id 5, lun 0) PD (disc) device from the start,
> this problem may be present only for CD devices.
>
> >From typescript :
>

(Please be advised that the mailer may
mungle long lines.)

>
> ishikawa@duron$ ls /dev/scsi/host1
> ./ ../ bus0/
> ishikawa@duron$ ls /dev/scsi/host1/bus0
> ./ ../ target5/ target6/
> ishikawa@duron$ ls /dev/scsi/host1/bus0/target5
> ./ ../ lun0/ lun1/
> ishikawa@duron$ ls /dev/scsi/host1/bus0/target5/lun0 <== "disc" is
there.
>
> ./ ../ disc generic
> ishikawa@duron$ ls /dev/scsi/host1/bus0/target5/lun1 <== CD device.
No
> "cd".
> ./ ../ generic
> ishikawa@duron$ ls -l /dev/scsi/host1/bus0/target6
> 0
> drwxr-xr-x 1 root root 0 Jan 1 1970 ./
> drwxr-xr-x 1 root root 0 Jan 1 1970 ../
> drwxr-xr-x 1 root root 0 Jan 1 1970 lun0/
> drwxr-xr-x 1 root root 0 Jan 1 1970 lun1/
> drwxr-xr-x 1 root root 0 Jan 1 1970 lun2/
> drwxr-xr-x 1 root root 0 Jan 1 1970 lun3/
> drwxr-xr-x 1 root root 0 Jan 1 1970 lun4/
> drwxr-xr-x 1 root root 0 Jan 1 1970 lun5/
> drwxr-xr-x 1 root root 0 Jan 1 1970 lun6/
> ishikawa@duron$ ls -l /dev/scsi/host1/bus0/target6/lun0
> 0
> drwxr-xr-x 1 root root 0 Jan 1 1970 ./
> drwxr-xr-x 1 root root 0 Jan 1 1970 ../
> crw------- 1 root root 21, 3 Jan 1 1970 generic
> ishikawa@duron$ ls -l /dev/scsi/host1/bus0/target6/lun1
> 0
> drwxr-xr-x 1 root root 0 Jan 1 1970 ./
> drwxr-xr-x 1 root root 0 Jan 1 1970 ../
> crw------- 1 root root 21, 4 Jan 1 1970 generic
> ishikawa@duron$
> ishikawa@duron$ echo "Have you noticed that no CD entries are found
above?"
>
> Have you noticed that no CD entries are found above?
> ishikawa@duron$
> ishikawa@duron$ ls -l /dev/scsi/host1/bus0/target6/lun6
> 0
> drwxr-xr-x 1 root root 0 Jan 1 1970 ./
> drwxr-xr-x 1 root root 0 Jan 1 1970 ../
> crw------- 1 root root 21, 9 Jan 1 1970 generic
> ishikawa@duron$
> ishikawa@duron$ echo "However, now I do something using the
copatibility
> device names."
> However, now I do something using the copatibility device names.
> ishikawa@duron$
> ishikawa@duron$
> ishikawa@duron$
> ishikawa@duron$ ls -l /dev/scd*
> lr-xr-xr-x 1 root root 3 Feb 9 2001 /dev/scd0 ->
sr0
> lr-xr-xr-x 1 root root 3 Feb 9 2001 /dev/scd1 ->
sr1
> lr-xr-xr-x 1 root root 3 Feb 9 2001 /dev/scd2 ->
sr2
> lr-xr-xr-x 1 root root 3 Feb 9 2001 /dev/scd3 ->
sr3
> lr-xr-xr-x 1 root root 3 Feb 9 2001 /dev/scd4 ->
sr4
> lr-xr-xr-x 1 root root 3 Feb 9 2001 /dev/scd5 ->
sr5
> lr-xr-xr-x 1 root root 3 Feb 9 2001 /dev/scd6 ->
sr6
> lr-xr-xr-x 1 root root 3 Feb 9 2001 /dev/scd7 ->
sr7
> ishikawa@duron$ ls -l /dev/sr*
> lr-xr-xr-x 1 root root 31 Feb 9 13:09 /dev/sr0 ->
> scsi/host1/bus0/target5/lun1/cd
> lr-xr-xr-x 1 root root 31 Feb 9 13:09 /dev/sr1 ->
> scsi/host1/bus0/target6/lun0/cd
> lr-xr-xr-x 1 root root 31 Feb 9 13:09 /dev/sr2 ->
> scsi/host1/bus0/target6/lun1/cd
> lr-xr-xr-x 1 root root 31 Feb 9 13:09 /dev/sr3 ->
> scsi/host1/bus0/target6/lun2/cd
> lr-xr-xr-x 1 root root 31 Feb 9 13:09 /dev/sr4 ->
> scsi/host1/bus0/target6/lun3/cd
> lr-xr-xr-x 1 root root 31 Feb 9 13:09 /dev/sr5 ->
> scsi/host1/bus0/target6/lun4/cd
> lr-xr-xr-x 1 root root 31 Feb 9 13:09 /dev/sr6 ->
> scsi/host1/bus0/target6/lun5/cd
> lr-xr-xr-x 1 root root 31 Feb 9 13:09 /dev/sr7 ->
> scsi/host1/bus0/target6/lun6/cd
>
> ishikawa@duron$ cd /dev/scsi/host1/bus0/target6/lun0
> ishikawa@duron$ ls -l
> 0
> drwxr-xr-x 1 root root 0 Jan 1 1970 ./
> drwxr-xr-x 1 root root 0 Jan 1 1970 ../
> brw-rw---- 1 root cdrom 11, 1 Jan 1 1970 cd <==
CD
> device!
> crw------- 1 root root 21, 3 Jan 1 1970 generic
> ishikawa@duron$
> ishikawa@duron$ echo "Am I missing something?"
> Am I missing something?
> ishikawa@duron$
>
> COMMENT:
> Come to think of accessing the symlinks probably
> had no bearing on the behavior change. Maybe "cd" ing to
> the device directory made a difference.
> Changing directory to the larget lun0 might have triggered
> some sort of device registration.
> But I think this is strange.
> Devfs ought to have done it in advance at the startup time.
>
> I am using Debian GNU/Linux
> Kernel 2.4.1 (+ a patch that floated on the linux-scsi.)
>
> devfsd and related init scripts
> is from the Debian unstable distribution.
>
>

Is it possible that the accessing the CD using the
compatibility device name /dev/sr* forced
the creation of the "cd" device name?

I consider the possibility of module loading. Both SCSI CD and
SCSI generic (sg) are modules now.
I checked /etc/devfs/devfs.conf (experimental Debian package
puts the config file here! ) has the following line:

LOOKUP .* MODLOAD

So the module autoloading ought to work. ("generic" exists
somehow from the start.)

I admit that not many people have scsi CD changers
and so these problems may not show up often enough to be noticed.
I was trying to test some CD access functions and then I realized this
anomaly.

Happy Hacking,

Chiaki




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