From owner-freebsd-hackers Sat Nov 29 14:25:04 1997 Return-Path: Received: (from root@localhost) by hub.freebsd.org (8.8.7/8.8.7) id OAA24003 for hackers-outgoing; Sat, 29 Nov 1997 14:25:04 -0800 (PST) (envelope-from owner-freebsd-hackers) Received: from ns2.cetlink.net (root@ns2.cetlink.net [209.54.54.20]) by hub.freebsd.org (8.8.7/8.8.7) with ESMTP id OAA23994 for ; Sat, 29 Nov 1997 14:24:59 -0800 (PST) (envelope-from jak@cetlink.net) Received: from hot1.auctionfever.com (ts1-cltnc-46.cetlink.net [209.54.58.46]) by ns2.cetlink.net (8.8.5/8.8.5) with SMTP id RAA17738; Sat, 29 Nov 1997 17:24:33 -0500 (EST) From: jak@cetlink.net (John Kelly) To: Bruce Evans Cc: hackers@freebsd.org Subject: Re: 650 UART, SIO driver, 8259 PIC Date: Sat, 29 Nov 1997 23:25:37 GMT Message-ID: <3481a093.3224591@mail.cetlink.net> References: <199711292017.HAA16179@godzilla.zeta.org.au> In-Reply-To: <199711292017.HAA16179@godzilla.zeta.org.au> X-Mailer: Forte Agent 1.01/16.397 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by hub.freebsd.org id OAA23998 Sender: owner-freebsd-hackers@freebsd.org X-Loop: FreeBSD.org Precedence: bulk On Sun, 30 Nov 1997 07:17:23 +1100, Bruce Evans wrote: >BTW, I remember why an EOI is sent on return and not at the start >for the non-auto-EOI case. That was my next question. :-) >It is to reduce interrupt latency. I guess that depends on how you measure latency. But it seems that the requesting device must wait the same length of time to get service either way, at least in the case where the ISR leaves CPU interrupts disabled until exiting with IRET automatically reenables them. Perhaps EOI at the start (like auto-EOI) would be useful if the ISR reenabled CPU interrupts to allow other devices to pre-empt the current ISR. John