Re: [Qemu-devel] [PATCH] e1000: document ICS read behaviour

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jan 10, 2013 at 02:03:39PM +0200, Michael S. Tsirkin wrote:
> 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 <mst@redhat.com>

This is v2 of the patch.

> ---
>  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);
>  }
>  
> -- 
> MST