[linux-dvb] Re: More than 4 DVB cards - again

[Johannes Stezenbach <js@convergence.de>]

Michael Hunold wrote:
> On 06/22/04 10:27, Peter Maersk-Moller wrote:
> >Seriously, using "extended minor number". How do I do that ?
> >Are you talking about changing something in a config file
> >or are you talking about rewriting the device driver ?
> 
> This must be changed in the dvb-core device handling.

dev_t has now 32 bit (12 bit for the major and 20 bit for the minor
number). See http://lwn.net/Articles/49684/

BTW, I now remember that the CONFIG_DVB_DEVFS_ONLY stuff was
dropped because of changes in devfs. It just wouldn't work
that way anymore in 2.6.

> >What is the limitation regarding more DVB devices than 4 ?
> 
> There can be 9 different types of devices per adapter 
> (video,audio,sec,frontend,demux,dvr,ca, net) and each device can be 
> present up to 4 times (ie. demux0, demux1, demux2, demux3)
> 
> Because everything needs to be a power of two, one adapter consumes 
> 16*4=64 minor numbers. Because of the fact you only have 256 minor 
> numbers you're stuck with 4 adapters.
> 
> For 2.4 alone, the devfs implementation wasn't tied to major/minor 
> numbers, so if you used devfs alone it was possible to support more that 
> 4 adapters.

The "power of two" thing is not a requirement, it's usually
just convenient because you can simply mask the adapter number
or device type from the minor number.

However, if you look at dvbdev.c, dvbdev_find_device() does not
make use of this encoding. Wierd...

In theory:
It would be possible to use 3 bits for the adapter number and
make a table which maps the remaining 5 bits to device type
and number, and adapt MAKEDEV-DVB.sh to the new numbering scheme.
However, as this would create some headaches for many users
during driver upgrade, I don't want to do this. Either use
the new dev_t bits or use a second major number.

> In 2.6 this has changed -- devfs is now more tightly coupled and is 
> limited to major/minor numbers again.
> 
> But: major/minor numbers have been extended from 8 to 16 bit types, so 

It's 12/20 bit, see kdev_t.h.

> it should be possible to support more devices, but this also affects the 
> surrounding system (ie. glibc must support this)

Specifically mknod must allow you to create device nodes with major or minor > 255.


Johannes