Re: V4L-DVB drivers and BKL

[Mauro Carvalho Chehab <mchehab@redhat.com>]

Hans Verkuil wrote:
> On Thursday 01 April 2010 20:29:52 Devin Heitmueller wrote:
>> On Thu, Apr 1, 2010 at 1:36 PM, Mauro Carvalho Chehab
>> <mchehab@redhat.com> wrote:
>>> If you take a look at em28xx-dvb, it is not lock-protected. If the bug is due
>>> to the async load, we'll need to add the same locking at *alsa and *dvb
>>> parts of em28xx.
>> Yes, that is correct.  The problem effects both dvb and alsa, although
>> empirically it is more visible with the dvb case.
>>
>>> Yet, in this specific case, as the errors are due to the reception of
>>> wrong data from tvp5150, maybe the problem is due to the lack of a
>>> proper lock at the i2c access.
>> The problem is because hald sees the new device and is making v4l2
>> calls against the tvp5150 even though the gpio has been toggled over
>> to digital mode.  Hence an i2c lock won't help.  We would need to
>> implement proper locking of analog versus digital mode, which
>> unfortunately would either result in hald getting back -EBUSY on open
>> of the V4L device or the DVB module loading being deferred while the
>> v4l side of the board is in use (neither of which is a very good
>> solution).
>>
>> This is what got me thinking a few weeks ago that perhaps the
>> submodules should not be loaded asynchronously.  In that case, at
>> least the main em28xx module could continue to hold the lock while the
>> submodules are still being loaded.
> 
> What was the reason behind the asynchronous loading? In general it simplifies
> things a lot if you load modules up front.

The reason is to avoid a dead lock: driver A depends on symbols on B (the
other driver init code) that depends on symbols at A (core stuff, locks, etc). 

There are other approaches to avoid this trouble, like the attach method used
by the DVB modules, but an asynchronous (and parallel) load offers another
advantage: it speeds up boot time, as other processors can take care of the
load of the additonal modules.

-- 

Cheers,
Mauro