Subject: Re: [RFC PATCH 08/11] IB/Verbs: Use management helper has_iwarp() for, iwarp-check

From: Tom Talpey <[email-masked]>
Date: Tue, 31 Mar 2015 09:56:29 -0400

On 3/31/2015 7:41 AM, Michael Wang wrote:
> Hi, Tom
>
> Thanks for the comments :-)
>
> On 03/31/2015 01:19 PM, Tom Talpey wrote:

[oops - repeating my reply with full cc's]

>> [snip]
>>
>>>
>>> Actually I'm thinking about Doug's idea to use rdma_transport_is_XX()
>>> instead of the current basic helper, thus may be use rdma_transport_is_iwarp()
>>> in here could be better, since it's actually a feature of iwarp tech
>>> that RDMA Read only support one scatter-gather entry.
>>
>> No, you should expose an attribute to surface the maximum length of
>> the remote gather list, which varies by adapter as well as protocol.
>> The fact that iWARP is different from IB is not relevant, and conflates
>> unrelated properties.
>
> To be confirmed, so your point is that the max-read-sges will be different
> even the transport is the same IWRAP, and that depends on the capability
> of adapter, correct?

Yes, in fact the iWARP protocol does not preclude multiple read SGEs,
even though most iWARP implementations have chosen to support just one.

Even for multi-SGE-capable adapters, there is a limit of SGL size, based
on the adapter's work request format and other factors. So my argument
is that upper layers can and should query that, not make a blanket
decision based on protocol type.

>
> I currently only find this one place where infer max-read-sges from
> transport type, it looks more like a special case to me rather than a generic
> method we could exposed... and also  not very related with IB management
> helper concept IMHO.

It is most certainly not a special case, but you could decide to
introduce it in many ways. I'm not commenting on that.

My main concern is that you do not introduce a new and clumsy "is iWARP"
rule as an adapter-specific API requirement to expose the RDMA Read SGE
behavior. That's what your initial message seemed to imply?

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