Re: [RFC PATCH] libnvdimm: Update the meaning for persistence_domain values

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 1/15/20 10:57 PM, Aneesh Kumar K.V wrote:
> On 1/15/20 10:25 PM, Jeff Moyer wrote:
>> "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:
>>
>>> Currently, kernel shows the below values
>>>     "persistence_domain":"cpu_cache"
>>>     "persistence_domain":"memory_controller"
>>>     "persistence_domain":"unknown"
>>>
>>> This patch updates the meaning of these values such that
>>>
>>> "cpu_cache" indicates no extra instructions is needed to ensure the 
>>> persistence
>>> of data in the pmem media on power failure.
>>>
>>> "memory_controller" indicates platform provided instructions need to 
>>> be issued
>>> as per documented sequence to make sure data flushed is guaranteed to 
>>> be on pmem
>>> media in case of system power loss.
>>>
>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>>> ---
>>>   arch/powerpc/platforms/pseries/papr_scm.c | 7 ++++++-
>>>   include/linux/libnvdimm.h                 | 6 +++---
>>>   2 files changed, 9 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/powerpc/platforms/pseries/papr_scm.c 
>>> b/arch/powerpc/platforms/pseries/papr_scm.c
>>> index c2ef320ba1bf..26a5ef263758 100644
>>> --- a/arch/powerpc/platforms/pseries/papr_scm.c
>>> +++ b/arch/powerpc/platforms/pseries/papr_scm.c
>>> @@ -360,8 +360,13 @@ static int papr_scm_nvdimm_init(struct 
>>> papr_scm_priv *p)
>>>       if (p->is_volatile)
>>>           p->region = nvdimm_volatile_region_create(p->bus, &ndr_desc);
>>> -    else
>>> +    else {
>>> +        /*
>>> +         * We need to flush things correctly to guarantee persistance
>>> +         */
>>> +        set_bit(ND_REGION_PERSIST_MEMCTRL, &ndr_desc.flags);
>>>           p->region = nvdimm_pmem_region_create(p->bus, &ndr_desc);
>>> +    }
>>>       if (!p->region) {
>>>           dev_err(dev, "Error registering region %pR from %pOF\n",
>>>                   ndr_desc.res, p->dn);
>>
>> Would you also update of_pmem to indicate the persistence domain,
>> please?
>>
> 
> sure.
> 
> 
>>> diff --git a/include/linux/libnvdimm.h b/include/linux/libnvdimm.h
>>> index f2a33f2e3ba8..9126737377e1 100644
>>> --- a/include/linux/libnvdimm.h
>>> +++ b/include/linux/libnvdimm.h
>>> @@ -52,9 +52,9 @@ enum {
>>>        */
>>>       ND_REGION_PERSIST_CACHE = 1,
>>>       /*
>>> -     * Platform provides mechanisms to automatically flush outstanding
>>> -     * write data from memory controler to pmem on system power loss.
>>> -     * (ADR)
>>> +     * Platform provides instructions to flush data such that on 
>>> completion
>>> +     * of the instructions, data flushed is guaranteed to be on pmem 
>>> even
>>> +     * in case of a system power loss.
>>
>> I find the prior description easier to understand.
> 
> 
> I was trying to avoid the term 'automatically, 'memory controler' and 
> ADR. Can I update the above as
> 
> /*
>   * Platform provides mechanisms to flush outstanding write data
>   * to pmem on system power loss.
>   */
> 

Wanted to add more details. So with the above interpretation, if the 
persistence_domain is found to be 'cpu_cache', application can expect a 
store instruction to guarantee persistence. If it is 'none' there is no 
persistence ( I am not sure how that is the difference from 'volatile' 
pmem region). If it is  'memory_controller' ( I am not sure whether that 
is the right term), application needs to follow the recommended 
mechanism to flush write data to pmem.

-aneesh
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