maybe race condition & alsa

maybe race condition & alsa

[Francisco Lobo]

SALVE

After a couple of days... Success! I now have linux-2.6 + udev, and 
finally got rid of the devfs names. Thanks to everyone!

I just have ONE device node on the physical filesystem: /dev/console 
(i.e. mknod /dev/console c 5 1), because init must have it at boot. 
Then, rc.sysinit has this before any real work (fs checks):

/bin/mount -n /dev
/bin/mount -n /proc
/bin/mount -n /sys
mkdir /dev/pts
mkdir /dev/shm
/sbin/udevstart
ln -snf /proc/self/fd /dev/fd
ln -snf /proc/self/fd/0 /dev/stdin
ln -snf /proc/self/fd/1 /dev/stdout
ln -snf /proc/self/fd/2 /dev/stderr
ln -snf /proc/kcore /dev/core

PRONTO!

Maybe the documentation for udev should hint that /dev/console is the 
only device needed to boot (or maybe I didn't read it well).

Getting alsa to work was a very different story! Although this problem 
might just be alsa related (alsactl in particular), I though the 
following info should help people migrate to udev, so:

1. with udev the following modprobe.conf lines are not working for me

install snd-card-0 /sbin/modprobe --ignore-install snd-card-0 && \
	{ /usr/sbin/alsactl restore >/dev/null 2>&1 || :; }
remove snd-card-0 { /usr/sbin/alsactl store >/dev/null 2>&1 || :; } ; \
	/sbin/modprobe -r --ignore-remove snd-card-0

2. although there are no error messages, the fact is the alsactl cannot
find the sound card (/dev entries), so when one tries to play something 
the sound is mute

The solution I found is:

1. replace hotplug init script with:

echo "/sbin/udev" > /proc/sys/kernel/hotplug

2. immediately after that load the snd-card-0 and other module(s) in a 
modules init script

3. then in a later init script do the alsactl part, and by "later" I 
mean after a few other ones, or else same problem as above - you might 
use some adequate sleep value before alsactl command (but?)

What I mean by a possible race condition is that alsactl will not find 
the /dev entries if this procedure is not exactly followed. Also, this 
will ONLY work with udev as the hotplug program! The hotplug scripts are 
simply too slow to respond (I've tried sleep 2 before alsactl and 
nothing, at least with hotplug-2004_01_05 - NOTE: I don't have 
pcimodules, yet hotplug should use sysfs shouldn't it?)

Of course, alsactl could/should do more to find the sound card... I did 
not check its sources to see how this is done... But it seems plausible 
that it is only doing a stat on /dev entries, and maybe it could ask the 
kernel for sound devices, which might then make the modprobe / udev / 
hotplug processes to finish whatever they have to do. (I don't know much 
about this, it just feels that something like it could sort things out.)

The obvious problem with this solution is that one always has the sound 
modules, even if we're not going to jam!

Salu!

Francisco.


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Re: maybe race condition & alsa

[Greg KH]

On Wed, Apr 07, 2004 at 09:40:19PM +0100, Francisco Lobo wrote:
> 
> Maybe the documentation for udev should hint that /dev/console is the 
> only device needed to boot (or maybe I didn't read it well).

It's not needed for me. It all depends on your distro's needs.

> 1. replace hotplug init script with:
> 
> echo "/sbin/udev" > /proc/sys/kernel/hotplug

Ick, ick, ick.  NO.  do not do this.  Lots of other stuff will not work
properly in your system if you do this.

The problem is that you need to wait a bit of time before running
alsactl for the device nodes to show up.  So call out to a shell script
instead to do this wait, instead of trying to do it all in a
modprobe.conf line.

That's what works for me.

thanks,

greg k-h


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Re: maybe race condition & alsa

[Marco d'Itri]

On Apr 08, Greg KH <greg@kroah.com> wrote:

> The problem is that you need to wait a bit of time before running
> alsactl for the device nodes to show up.  So call out to a shell script
> instead to do this wait, instead of trying to do it all in a
> modprobe.conf line.
> 
> That's what works for me.
I'm using this:

/etc/dev.d/snd/controlC0/alsa.dev:

#!/bin/sh -e
exec /usr/sbin/alsactl restore 0

-- 
ciao, |
Marco | [5624 caqcc7nk9K4Dg]


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Re: maybe race condition & alsa

[Greg KH]

On Thu, Apr 08, 2004 at 10:44:15PM +0200, Marco d'Itri wrote:
> On Apr 08, Greg KH <greg@kroah.com> wrote:
> 
> > The problem is that you need to wait a bit of time before running
> > alsactl for the device nodes to show up.  So call out to a shell script
> > instead to do this wait, instead of trying to do it all in a
> > modprobe.conf line.
> > 
> > That's what works for me.
> I'm using this:
> 
> /etc/dev.d/snd/controlC0/alsa.dev:
> 
> #!/bin/sh -e
> exec /usr/sbin/alsactl restore 0

That's a great example.  I think I'll go add it to the scripts...

But wait, that really works?  Heh, I guess when the DEVNAME includes a
directory, that's what is needed, pretty funny.

thanks,

greg k-h


> 
> -- 
> ciao, |
> Marco | [5624 caqcc7nk9K4Dg]


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Re: maybe race condition & alsa

[Francisco Lobo]

Marco d'Itri wrote:
 >
 > I'm using this:
 >
 > /etc/dev.d/snd/controlC0/alsa.dev:
 >
 > #!/bin/sh -e
 > exec /usr/sbin/alsactl restore 0
 >

Brilliant !!!
I must start thinking in udev ways from now on...

Greg KH wrote: (about /dev/console)
 >
 > It's not needed for me. It all depends on your distro's needs.
 >

I don't really understand: at present I am running Arch Linux, but I 
can't see anything special with their boot setup, it's a plain BSD style 
init; and their init compilation is very much LFS style, no special hacks.

Do you mean using a /linuxrc to start udev, before init? (Maybe I should 
try that someday...) The init manpage says it defaults the system 
console to /dev/console or inherits the CONSOLE var from the kernel 
command-line; it needs it to report its messages I guess.

Well... I would really like to know how you did it.

Greg KH wrote: (about hotplug)
 >
 > Ick, ick, ick.  NO.  do not do this.  Lots of other stuff will not work
 > properly in your system if you do this.
 >

Hum... I see everything (I use) working... Although I have to confess 
that I don't use usb devices, or plug and unplug any hardware regularly.

What will not work?
Is there any specific problem with using /sbin/udev as the hotplug program?

As far as I understand (and for my simple needs), I only require the 
correct device nodes to be created after I insert a kernel module. I 
shouldn't need a script that calls a script that calls a script..., 
every time I load a module. But this is my personal point of view, and I 
understand that a general all-scenarios solution cannot be done in 
another way.

Thank you both!
Francisco.


PS:
I did try the latest hotplug in the meantime, which now does a good job 
of discovering hardware via sysfs!

Yet I have some reservations with respect to its init script. I don't 
want to configure all my hardware at boot but only as I need it, so I am 
force to blacklist a lot of modules. Also, it is curious that my alsa 
sound card module does not get loaded, because the oss one comes first 
in modules.pcimap and I put it on the blacklist (from what I could 
understand from the scripts), so I cannot have several modules for the 
same device(?).

Of course, this is not related to using hotplug as the "hotplug 
program"; these are simply examples, which I think begin to support my 
notion that what the hotplug scripts try to automate is in fact 
something that is very specific to system/user preference (i.e. like 
init scripts). Without having the time to get familiar with all the 
scripts at the moment, I would rather do without it.


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