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Message-ID: <7291dd1b2d16fd9bbd90988ac5bcc3a46d17e3f4.camel@linux.ibm.com>
Subject: Re: [RFC PATCH net-next v5 04/11] net/smc: implement some
 unsupported operations of loopback-ism
From: Niklas Schnelle <schnelle@linux.ibm.com>
To: Wen Gu <guwen@linux.alibaba.com>, Gerd Bayer <gbayer@linux.ibm.com>,
        wintera@linux.ibm.com, twinkler@linux.ibm.com, hca@linux.ibm.com,
        gor@linux.ibm.com, agordeev@linux.ibm.com, davem@davemloft.net,
        edumazet@google.com, kuba@kernel.org, pabeni@redhat.com,
        wenjia@linux.ibm.com, jaka@linux.ibm.com
Cc: borntraeger@linux.ibm.com, svens@linux.ibm.com, alibuda@linux.alibaba.com,
        tonylu@linux.alibaba.com, linux-kernel@vger.kernel.org,
        linux-s390@vger.kernel.org, netdev@vger.kernel.org
Date: Thu, 04 Apr 2024 17:15:17 +0200
In-Reply-To: <7a0fc481-658e-4c99-add7-ccbd5f9dce1e@linux.alibaba.com>
References: <20240324135522.108564-1-guwen@linux.alibaba.com>
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	 <3b3ff37643e9030ec1246e67720683a2cf5660e5.camel@linux.ibm.com>
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On Thu, 2024-04-04 at 21:12 +0800, Wen Gu wrote:
>=20
> On 2024/4/4 19:42, Niklas Schnelle wrote:
> > On Thu, 2024-04-04 at 17:32 +0800, Wen Gu wrote:
> > >=20
> > > On 2024/4/4 00:25, Gerd Bayer wrote:
> > > > On Sun, 2024-03-24 at 21:55 +0800, Wen Gu wrote:
> > > > > This implements some operations that loopback-ism does not suppor=
t
> > > > > currently:
> > > > >    - vlan operations, since there is no strong use-case for it.
> > > > >    - signal_event operations, since there is no event to be proce=
ssed
> > > > > by the loopback-ism device.
> > > >=20
> > > > Hi Wen,
> > > >=20
> > > > I wonder if the these operations that are not supported by loopback=
-ism
> > > > should rather be marked "optional" in the struct smcd_ops, and the
> > > > calling code should call these only when they are implemented.
> > > >=20
> > > > Of course this would mean more changes to net/smc/smc_core.c - but
> > > > loopback-ism could omit these "boiler-plate" functions.
> > > >=20
> > >=20
> > > Hi Gerd.
> > >=20
> > > Thank you for the thoughts! I agree that checks like 'if(smcd->ops->x=
xx)'
> > > can avoid the device driver from implementing unsupported operations.=
 But I
> > > am afraid that which operations need to be defined as 'optional' may =
differ
> > > from different device perspectives (e.g. for loopback-ism they are vl=
an-related
> > > opts and signal_event). So I perfer to simply let the smc protocol as=
sume
> > > that all operations have been implemented, and let drivers to decide =
which
> > > ones are unsupported in implementation. What do you think?
> > >=20
> > > Thanks!
> > >=20
> >=20
> > I agree with Gerd, in my opinion it is better to document ops as
> > optional and then allow their function pointers to be NULL and check
> > for that. Acting like they are supported and then they turn out to be
> > nops to me seems to contradict the principle of least surprises. I also
> > think we can find a subset of mandatory ops without which SMC-D is
> > impossible and then everything else should be optional.
>=20
> I see. If we all agree to classify smcd_ops into mandatory and optional o=
nes,
> I'll add a patch to mark the optional ops and check if they are implement=
ed.

Keep in mind I don't speak for the SMC maintainers but that does sound
reasonable to me.

>=20
> >=20
> > As a first guess I think the following options may be mandatory:
> >=20
> > * query_remote_gid()
> > * register_dmb()/unregister_dmb()
> > * move_data()
> >    For this one could argue that either move_data() or
> >    attach_dmb()/detach_dmb() is required though personally I would
> >    prefer to always have move_data() as a fallback and simple API
> > * supports_v2()
> > * get_local_gid()
> > * get_chid()
> > * get_dev()
> I agree with this classification. Just one point, maybe we can take
> supports_v2() as an optional ops, like support_dmb_nocopy()? e.g. if
> it is not implemented, we treat it as an ISMv1.
>=20
> Thanks!

Interpreting a NULL supports_v2() as not supporting v2 sounds
reasonable to me.