From: Jan Klemkow <jan@openbsd.org>
Subject: ahci: fix interrupt issues
To: tech@openbsd.org
Date: Thu, 21 Nov 2024 18:20:44 +0100

Hi,

Attaching ahci(4) with newer AMD 600 Series AHCI controler hangs.  This is
caused via atascsi_attach() which creates residual bits in the interrupt status
register.  atascsi_attach() runs in polling mode, but sets the interrupt status
bits anyway.  When these bits are already set, we never get an interrupt and
run in a 60 second timeout for every ATA command send from sd(4) layer.  Which
causes a kernel boot to last over an hour.

The SATA AHCI 1.3.1 specification also saids:

	10.1.2 System Software Specific Initialization

	[...] system software must always ensure that the PxIS (clear
	this first) and IS.IPS (clear this second) registers are cleared to ‘0’
	before programming the PxIE and GHC.IE registers.  This will prevent any
	residual bits set in these registers from causing an interrupt to be
	asserted.

The diff below simply clears the interupt status register, which fixes this
issue.

Tested with:
ahci0 at pci12 dev 0 function 0 "AMD 600 Series AHCI" rev 0x01: msi, AHCI 1.3.1

ok?

bye,
Jan

Index: dev/ic/ahci.c
===================================================================
RCS file: /cvs/src/sys/dev/ic/ahci.c,v
diff -u -p -r1.42 ahci.c
--- dev/ic/ahci.c	4 Sep 2024 07:54:52 -0000	1.42
+++ dev/ic/ahci.c	21 Nov 2024 16:39:16 -0000
@@ -321,6 +321,9 @@ noccc:
 
 	sc->sc_atascsi = atascsi_attach(&sc->sc_dev, &aaa);
 
+	/* Flush all residual bits of the interrupt status register */
+	ahci_write(sc, AHCI_REG_IS, ahci_read(sc, AHCI_REG_IS));
+
 	/* Enable interrupts */
 	ahci_write(sc, AHCI_REG_GHC, AHCI_REG_GHC_AE | AHCI_REG_GHC_IE);