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

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Message-ID: <20140515215646.GA16326@cloud>
Date:	Thu, 15 May 2014 14:56:46 -0700
From:	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 Tue, May 13, 2014 at 03:10:59PM -0700, H. Peter Anvin wrote:
> On 05/09/2014 03:38 PM, Josh Triplett wrote:
> > 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.
> 
> I really have a problem with the logic "our current interface is wrong,
> so let's introduce another wrong interface which solves a narrow use
> case".  In some ways it would actually be *better* to use an ioctl
> interface on /dev/port in that case...

ioport{8,16,32} seems preferable to an ioctl on /dev/port, but in any
case, I'd be happy to adapt this patch to whatever interface seems
preferable.  I just don't want to let the perfect be the enemy of the
good here; 16-bit and 32-bit port operations are currently completely
impossible via /dev/port, and I'm primarily interested in fixing that,
not necessarily in creating a completely generalized interface for doing
high-performance repeated I/O operations that ought to be in the kernel
anyway.

- Josh Triplett
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