Re: [PATCH] pl031: Expose RTCICR as proper WC register

[Peter Maydell]

On Mon, 4 Nov 2019 at 11:52, Alexander Graf <address@hidden> wrote:
>
> The current pl031 RTCICR register implementation always clears the IRQ
> pending status on a register write, regardless of the value it writes.
>
> To justify that behavior, it references the arm926e documentation
> (DDI0287B) and indicates that said document states that any write clears
> the internal IRQ state. I could however not find any text in that document
> backing the statement. In fact, it explicitly says:
>
>   "Writing 1 to bit 0 of RTCICR clears the RTCINTR flag."
>
> which describes it as much as a write-to-clear register as the PL031 spec
> (DDI0224) does:
>
>   "Writing 1 to bit position 0 clears the corresponding interrupt.
>    Writing 0 has no effect."

DDI0287B page 11-2 section 11.1 says
"The interrupt is cleared by writing any data value to the
interrupt clear register RTCICR". As you note, this contradicts
what it says later on in section 11.2.2.

(Interestingly, the PL030 does have a "write any value to
clear the interrupt" register, RTCEOI.)

I'm fairly sure this patch is right and the DDI0287B document
has an error, since it isn't internally consistent and doesn't
match the proper PL031 TRM.

Did you find this because you had a guest that assumed the
other behaviour? This bug has been in QEMU for a very long time,
and it seems odd for a guest to deliberately perform an action
(writing 0) which is documented to have no effect on the device...

> Let's remove the bogus comment and instead follow both specs to what they
> say.
>
> Reported-by: Hendrik Borghorst <address@hidden>
> Signed-off-by: Alexander Graf <address@hidden>
> ---
>  hw/rtc/pl031.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/hw/rtc/pl031.c b/hw/rtc/pl031.c
> index 3a982752a2..c57cf83165 100644
> --- a/hw/rtc/pl031.c
> +++ b/hw/rtc/pl031.c
> @@ -149,11 +149,7 @@ static void pl031_write(void * opaque, hwaddr offset,
>          pl031_update(s);
>          break;
>      case RTC_ICR:
> -        /* The PL031 documentation (DDI0224B) states that the interrupt is
> -           cleared when bit 0 of the written value is set.  However the
> -           arm926e documentation (DDI0287B) states that the interrupt is
> -           cleared when any value is written.  */
> -        s->is = 0;
> +        s->is &= ~value;
>          pl031_update(s);
>          break;
>      case RTC_CR:
> --
> 2.17.1

thanks
-- PMM