From a52a8841038638afe54ffb00e0aca48de0b1539a Mon Sep 17 00:00:00 2001 From: "Michael S. Tsirkin" Date: Wed, 9 Jan 2013 23:50:00 +0200 Subject: [PATCH] e1000: document ICS read behaviour Add code comment to clarify the reason we set ICS with ICR: the reason was previously undocumented and git log confused rather than clarified the comments. Signed-off-by: Michael S. Tsirkin --- hw/e1000.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/hw/e1000.c b/hw/e1000.c index ef06ca1894..ee85c53d38 100644 --- a/hw/e1000.c +++ b/hw/e1000.c @@ -237,7 +237,17 @@ set_interrupt_cause(E1000State *s, int index, uint32_t val) val |= E1000_ICR_INT_ASSERTED; } s->mac_reg[ICR] = val; + + /* + * Make sure ICR and ICS registers have the same value. + * The spec says that the ICS register is write-only. However in practice, + * on real hardware ICS is readable, and for reads it has the same value as + * ICR (except that ICS does not have the clear on read behaviour of ICR). + * + * The VxWorks PRO/1000 driver uses this behaviour. + */ s->mac_reg[ICS] = val; + qemu_set_irq(s->dev.irq[0], (s->mac_reg[IMS] & s->mac_reg[ICR]) != 0); } -- 2.11.0