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To: current@freebsd.org
Subject: device driver open semantics...
Reply-to: phk@freebsd.org
Date: Sat, 01 Feb 1997 13:31:40 +0100
Message-ID: <3050.854800300@critter.dk.tfs.com>
From: Poul-Henning Kamp <phk@critter.dk.tfs.com>
Sender: owner-current@freebsd.org
X-Loop: FreeBSD.org
Precedence: bulk


The way things work now, this scenario exists:


	Proc A          Proc B          Device foo
	------------------------------------------

	open(foo,RD)
		    \__________________
				       \
					->open(foo, RD,A)

			open(foo,WR)
				    \__
				       \
					->open(foo, WR,B)

			close(foo)

					>>nothing<<

	close(foo)
		  \____________________
				       \
					->close(foo,A)


I'm dissatisfied with the >>nothing<< step.  At this time
there is nobody who has the device open for writing but
we fail to communicate this to the driver.

Many drivers don't really care who opened them, and it would
generally be sufficient to be informed of the current agregate
open mode:

        Proc A          Proc B          Device foo
        ------------------------------------------

        open(foo,RD)
                    \__________________
                                       \
                                        ->isopen(foo, RD)
                                
                        open(foo,WR)
                                    \__
                                       \
                                        ->isopen(foo, RD|WR)
  
                        close(foo)
                                  \____
                                       \
                                        ->isopen(foo, RD)

        close(foo)
                  \____________________
                                       \
                                        ->isopen(foo,0)

(we would still have to pass a proc * argument in case we
need to sleep, but it does not necessarily correspond to 
the process that effected the change in status)

Comments ?

I realize that changing this behaviour in general would
probably surprise most if not all of our device-drivers,
So we're probably talking about a per-driver flag or possibly
a different open/close entry point in the [cb]devsw structure.

Preferences ? 

Objections ?

--
Poul-Henning Kamp           | phk@FreeBSD.ORG       FreeBSD Core-team.
http://www.freebsd.org/~phk | phk@login.dknet.dk    Private mailbox.
whois: [PHK]                | phk@tfs.com           TRW Financial Systems, Inc.
Future will arrive by its own means, progress not so.