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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Ira Weiny <ira.weiny@intel.com>,
        Vaibhav Jain <vaibhav@linux.ibm.com>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-nvdimm@lists.01.org,
        linux-kernel@vger.kernel.org,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [RESEND PATCH v7 4/5] ndctl/papr_scm,uapi: Add support for PAPR nvdimm specific methods
In-Reply-To: <20200520153209.GC3660833@iweiny-DESK2.sc.intel.com>
References: <20200519190058.257981-1-vaibhav@linux.ibm.com> <20200519190058.257981-5-vaibhav@linux.ibm.com> <20200520153209.GC3660833@iweiny-DESK2.sc.intel.com>
Date:   Thu, 21 May 2020 16:48:40 +1000
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Ira Weiny <ira.weiny@intel.com> writes:
> On Wed, May 20, 2020 at 12:30:57AM +0530, Vaibhav Jain wrote:
>> Introduce support for Papr nvDimm Specific Methods (PDSM) in papr_scm
>> modules and add the command family to the white list of NVDIMM command
>> sets. Also advertise support for ND_CMD_CALL for the dimm
>> command mask and implement necessary scaffolding in the module to
>> handle ND_CMD_CALL ioctl and PDSM requests that we receive.
...
>> + *
>> + * Payload Version:
>> + *
>> + * A 'payload_version' field is present in PDSM header that indicates a specific
>> + * version of the structure present in PDSM Payload for a given PDSM command.
>> + * This provides backward compatibility in case the PDSM Payload structure
>> + * evolves and different structures are supported by 'papr_scm' and 'libndctl'.
>> + *
>> + * When sending a PDSM Payload to 'papr_scm', 'libndctl' should send the version
>> + * of the payload struct it supports via 'payload_version' field. The 'papr_scm'
>> + * module when servicing the PDSM envelope checks the 'payload_version' and then
>> + * uses 'payload struct version' == MIN('payload_version field',
>> + * 'max payload-struct-version supported by papr_scm') to service the PDSM.
>> + * After servicing the PDSM, 'papr_scm' put the negotiated version of payload
>> + * struct in returned 'payload_version' field.
>
> FWIW many people believe using a size rather than version is more sustainable.
> It is expected that new payload structures are larger (more features) than the
> previous payload structure.
>
> I can't find references at the moment through.

I think clone_args is a good modern example:

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/sched.h#n88

cheers