Re: [Qemu-devel] [PATCH v2]booke: timer: Deactivate timer for target_bit above 61

[Alexander Graf <agraf@suse.de>]

On 06/10/2013 02:47 PM, Bhushan Bharat-R65777 wrote:
>
>> -----Original Message-----
>> From: Andreas Färber [mailto:afaerber@suse.de]
>> Sent: Monday, June 10, 2013 5:43 PM
>> To: Bhushan Bharat-R65777
>> Cc: qemu-ppc@nongnu.org; qemu-devel@nongnu.org; agraf@suse.de; Wood Scott-
>> B07421; Bhushan Bharat-R65777
>> Subject: Re: [Qemu-devel] [PATCH v2]booke: timer: Deactivate timer for
>> target_bit above 61
>>
>> Am 10.06.2013 09:55, schrieb Bharat Bhushan:
>>> QEMU timer supports a maximum timer of INT64_MAX. So starting timer
>>> only for time which is calculated using target_bit<  62 and
>>> deactivate/stop timer if the target bit is above 61.
>>>
>>> This patch also fix the time calculation from target_bit.
>>> The code was doing (1<<  (target_bit + 1)) while this should be (1ULL
>>> <<  (target_bit + 1)).
>>>
>>> Signed-off-by: Bharat Bhushan<bharat.bhushan@freescale.com>
>>> ---
>>> v1->v2
>>>   - Added "booke: timer:" in patch subject
>>>
>>>   hw/ppc/ppc_booke.c |    8 +++++++-
>>>   1 files changed, 7 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/hw/ppc/ppc_booke.c b/hw/ppc/ppc_booke.c index
>>> e41b036..f4eda15 100644
>>> --- a/hw/ppc/ppc_booke.c
>>> +++ b/hw/ppc/ppc_booke.c
>>> @@ -133,9 +133,15 @@ static void booke_update_fixed_timer(CPUPPCState
>> *env,
>>>       ppc_tb_t *tb_env = env->tb_env;
>>>       uint64_t lapse;
>>>       uint64_t tb;
>>> -    uint64_t period = 1<<  (target_bit + 1);
>>> +    uint64_t period;
>>>       uint64_t now;
>>>
>>> +    /* Deactivate timer for target_bit>  61 */
>>> +    if (target_bit>  61)
>>> +        return;
>> Braces missing and trailing whitespace after return.
> Ok, will correct
>
>> So IIUC we can only allow 63 bits due to signedness, thus a maximum of
>> (1<<  62), thus target_bit<= 61.
>>
>> Any chance at least the comment can be worded to explain that any better? Maybe
>> also use (target-bit + 1>= 63) or period>  INT64_MAX as condition?
> How about this:
>      /* QEMU timer supports a maximum timer of INT64_MAX (0x7fffffff_ffffffff).
>       * Run booke fit/wdog timer when
>       * ((1ULL<<  target_bit + 1)<  0x40000000_00000000), i.e target_bit = 61.
>       * Also the time with this maximum target_bit (with current range of
>       * CPU frequency PowerPC supports) will be many many years. So it is
>       * pretty safe to stop the timer above this threshold. */

How about

   /* This timeout will take years to trigger. Treat the timer as 
disabled. */


Alex