From owner-freebsd-hackers  Wed Jul 30 10:30:20 1997
Return-Path: <owner-freebsd-hackers>
Received: (from root@localhost)
          by hub.freebsd.org (8.8.5/8.8.5) id KAA18239
          for hackers-outgoing; Wed, 30 Jul 1997 10:30:20 -0700 (PDT)
Received: from phaeton.artisoft.com (phaeton.Artisoft.COM [198.17.250.50])
          by hub.freebsd.org (8.8.5/8.8.5) with SMTP id KAA18223
          for <freebsd-hackers@FreeBSD.ORG>; Wed, 30 Jul 1997 10:30:14 -0700 (PDT)
Received: (from terry@localhost) by phaeton.artisoft.com (8.6.11/8.6.9) id KAA05314; Wed, 30 Jul 1997 10:27:15 -0700
From: Terry Lambert <terry@lambert.org>
Message-Id: <199707301727.KAA05314@phaeton.artisoft.com>
Subject: Re: Location of copyin() and copyout()..
To: msmith@atrad.adelaide.edu.au (Michael Smith)
Date: Wed, 30 Jul 1997 10:27:15 -0700 (MST)
Cc: vinay@agni.nuko.com, dg@root.com, terry@lambert.org,
        freebsd-hackers@FreeBSD.ORG
In-Reply-To: <199707300311.MAA17635@genesis.atrad.adelaide.edu.au> from "Michael Smith" at Jul 30, 97 12:41:26 pm
X-Mailer: ELM [version 2.4 PL24]
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: owner-freebsd-hackers@FreeBSD.ORG
X-Loop: FreeBSD.org
Precedence: bulk

> Vinay Bannai stands accused of saying:
> > 
> > So in a situation where I pin the user pages down (for DMA), it is okay to
> > use copyin()/copyout().
> 
> You don't have to pin them down for copyin/copyout, but it will work
> fine, yes.

Note: you can get a bigger win, depending on how you pin it.

If you pin it in the process of mmap()ing the device from user space,
then you create a bufer in KVM that you export to user space; this
is what the SVGA stuff does.

Because the memory exists in both kernel and user space, you can
use bcopy() to the kernel space address.  This saves the creation
and deletion (potentially) of mapping the target address of the
user process into and out of the kernel address space.

If you only allow one user space process at a time, you can further
cache the translation factor in the device instance data, and save
the lookup for the user->kernel address conversion when you are in
the kernel processing a user address request.


					Terry Lambert
					terry@lambert.org
---
Any opinions in this posting are my own and not those of my present
or previous employers.