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Path: sserve!newshost.anu.edu.au!munnari.oz.au!metro!sequoia!ultima!kralizec.zeta.org.au!not-for-mail From: bde@kralizec.zeta.org.au (Bruce Evans) Newsgroups: comp.os.386bsd.questions Subject: Re: The reason for stray interrupts Date: 30 Oct 1993 09:21:17 +1000 Organization: Kralizec Dialup Unix Sydney: +61-2-837-1183 V.32bis Lines: 33 Message-ID: <2as8hd$op6@kralizec.zeta.org.au> References: <2ais9gINN2t8@xs4all.hacktic.nl> <2alpnh$1cm@werple.apana.org.au> <vandys.751745453@cisco.com> NNTP-Posting-Host: kralizec.zeta.org.au In <vandys.751745453@cisco.com> vandys@cisco.com (Andrew Valencia) writes: >Sigh. >The problem is the way interrupts are cleared from the two chained >PIC's. If you disable the chained IRQ to the second PIC, or if you >use auto-EIO mode, or if you use selective EOI to finish a pending >interrupt, it doesn't happen. The simplistic code which shotguns >the two PICs in a row unconditionally is at fault. Sigh. I think this doesn't apply to most systems, at least for 0.1 and later versions of 386BSD. 0.1 does the pair of EOI's in the "wrong" order, but this shouldn't matter because stray interrupts are only supposed to happen when an interrupt goes away during the interrupt ACK cycle, and interrupts are disabled so there won't be another interrupt ACK cycle for some time. 0.0 did the EOI's at the end of the interrupt instead of at the start. This might have caused stray interrupts iff interrupts were not disabled. The 0.2.4 patchkit and later versions have ifdefs to enable auto-EOI. I enable them but I tell everyone not to enable them both because auto-EOI is documented not to work for the slave. I think it might cause stray interrupts because of timing problems :-). Perhaps this depends on whether the system uses interrupt gates or trap gates for interrupts. There should be no problems using interrupt gates gates, for the reasons given above. 386BSD uses interrupt gates. I think linux uses trap gates (relying on the 8259's interrupt priorities for locking). 8088 systems use interrupt gates because trap gates are not an option. -- Bruce Evans bde@kralizec.zeta.org.au