Re: Linux kernel 2.6.x Raw device support

[Bill Davidsen <davidsen@tmr.com>]

Joerg Schilling wrote:

> James Finnall <James@Finnall.net> wrote:
>
>  
>
> > When configuring a Linux kernel 2.6.x series to enable the raw device support, 
> > it states this as being obsolete and deprecated.  The suggestion is to use 
> > the O_DIRECT flag when opening the device name.  I can see how this could be 
> > possible from inside an application where the system calls are directly 
> > available to the programmer.  
> >    
>
> O_DIRECT is a nonstandard extension only found on Linux.
> UNIX supports raw device access via "character special" files.
>
>
>  
>
> > But how can such an option be passed through from the CLI to a program though?
> >    
>
> This is a problem. Why should a portable program introduce an option for 
> something that is Linux specific and done completely different on other OS?
>
>
>  
>
> > It was also stated that in the next kernel development of 2.7 that raw device 
> > support could be removed.  Now I suppose that might be changed by popular 
> > demand as well.
> >    
>
> I believe that the reason for not implementing raw devices on Linux was the fact
> that Linux was designed for x86 and the way Linux did implement DMA to devices 
> would have made a raw device slower than a buffered device. 
>  

But there WERE /dev/raw devices on Linux. You are correct that they 
don't work the way UNIX raw devices do, but the capability was there in 
2.4, and I think in 2.2. Quite fast in 2.4!

As for "designed for x86" could we say "developed on x86?" SunOS was 
based on BSD, developed on a VAX (from memory), while Solaris is sort-of 
based on SysVR3, while I think was avtually developed on either a VAX or 
that funky CPU AT&T used in PBX. I used one, but have happily forgotten.

--
bill davidsen <davidsen@tmr.com>
 CTO TMR Associates, Inc
 Doing interesting things with small computers since 1979