From patchwork Thu Jan 10 12:03:39 2013 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: e1000: document ICS read behaviour Date: Thu, 10 Jan 2013 02:03:39 -0000 From: Michael S. Tsirkin X-Patchwork-Id: 210983 Message-Id: <20130110120339.GA21794@redhat.com> To: qemu-devel@nongnu.org, Bill Paul Cc: Paolo Bonzini , Anthony Liguori , Jason Wang , Stefan Hajnoczi , "Michael S. Tsirkin" Add code comment to clarify the reason we set ICS with ICR: the reason was previously undocumented and git log (commit b1332393cdd7d023de8f1f8aa136ee7866a18968) confused rather than clarified the comments. Digging in the mailing list archives gives the real reason https://lists.gnu.org/archive/html/qemu-devel/2009-06/msg00401.html Add code comment with an explanation supplied by Bill Paul. 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 92fb00a..d10119c 100644 --- a/hw/e1000.c +++ b/hw/e1000.c @@ -230,7 +230,17 @@ 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); }