linux-kernel - Re: [PATCH] drivers/char/mem.c: Add /dev/ioports, supporting 16-bit and 32-bit ports

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Date:	Fri, 9 May 2014 15:38:39 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH] drivers/char/mem.c: Add /dev/ioports, supporting 16-bit
 and 32-bit ports

On Fri, May 09, 2014 at 02:20:45PM -0700, H. Peter Anvin wrote:
> On 05/09/2014 02:12 PM, Arnd Bergmann wrote:
> > 
> >> However, if we're going to have these devices I'm wondering if having
> >> /dev/portw and /dev/portl (or something like that) might not make sense,
> >> rather than requiring a system call per transaction.
> > 
> > Actually the behavior of /dev/port for >1 byte writes seems questionable
> > already: There are very few devices on which writing to consecutive
> > port numbers makes sense. Normally you just want to write a series
> > of bytes (or 16/32 bit words) into the same port number instead,
> > as the outsb()/outsw()/outsl() functions do.
> > 
> 
> Indeed.  I missed the detail that it increments the port index; it is
> virtually guaranteed to be bogus.

Exactly.  It might make sense to have ioport8/ioport16/ioport32 devices
that accept arbitrary-length reads and writes (divisible by the size)
and do the equivalent of the string I/O instructions outs/ins, but for
the moment I'd like to add the single device that people always seem to
want and can't get from /dev/port.  If someone's doing enough writes
that doing a syscall per in/out instruction seems like too much
overhead, they can write a real device driver or use ioperm/iopl.

- Josh triplett
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