linux-kernel - Re: [PATCH] IB/mad: Use ID allocator routines to allocate agent number

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Message-ID: <20180529164032.GB18457@ziepe.ca>
Date:   Tue, 29 May 2018 10:40:32 -0600
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Håkon Bugge <haakon.bugge@...cle.com>
Cc:     Hans Westgaard Ry <hans.westgaard.ry@...cle.com>,
        Doug Ledford <dledford@...hat.com>,
        Jack Morgenstein <jackm@....mellanox.co.il>,
        Daniel Jurgens <danielj@...lanox.com>,
        Parav Pandit <parav@...lanox.com>,
        Pravin Shedge <pravin.shedge4linux@...il.com>,
        OFED mailing list <linux-rdma@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] IB/mad: Use ID allocator routines to allocate agent
 number

On Tue, May 29, 2018 at 06:16:14PM +0200, Håkon Bugge wrote:
>
> > On 29 May 2018, at 17:49, Jason Gunthorpe <jgg@...pe.ca> wrote:
> > 
> > On Tue, May 29, 2018 at 09:38:08AM +0200, Hans Westgaard Ry wrote:
> >> The agent TID is a 64 bit value split in two dwords.  The least
> >> significant dword is the TID running counter. The most significant
> >> dword is the agent number. In the CX-3 shared port model, the mlx4
> >> driver uses the most significant byte of the agent number to store the
> >> slave number, making agent numbers greater and equal to 2^24 (3 bytes)
> >> unusable.
> > 
> > There is no reason for this to be an ida, just do something like
> > 
> > mad_agent_priv->agent.hi_tid = atomic_inc_return(&ib_mad_client_id) & mad_agent_priv->ib_dev->tid_mask;
> > 
> > And have the driver set tid_mask to 3 bytes of 0xFF
> 
> The issue is that some of the mad agents are long-lived, so you will
> wrap and use the same TID twice.

We already have that problem, and using ida is problematic because we
need to maximize the time between TID re-use, which ida isn't doing.

Preventing re-use seems like a seperate issue from limiting the range
to be compatible with mlx4.

Jason