From mboxrd@z Thu Jan  1 00:00:00 1970
From: Bart.VanAssche@sandisk.com (Bart Van Assche)
Date: Wed, 3 May 2017 16:37:37 +0000
Subject: [PATCH v4, under testing] nvme-rdma: support devices with queue
 size < 32
In-Reply-To: <20170503162708.GO22833@mtr-leonro.local>
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 <20170503155316.GA14334@obsidianresearch.com>
 <1493827096.3901.4.camel@sandisk.com>
 <77ada146-67eb-9dce-d252-6ac630c9a1e8@grimberg.me>
 <20170503162708.GO22833@mtr-leonro.local>
Message-ID: <1493829456.3901.13.camel@sandisk.com>

On Wed, 2017-05-03@19:27 +0300, Leon Romanovsky wrote:
> On Wed, May 03, 2017@07:01:14PM +0300, Sagi Grimberg wrote:
> > > As far as I know the compiler only does that for compile-time constants. In
> > > this case the divisor (max(queue_size / 2, 1)) is not a compile-time constant.
> > 
> > We could theoretically do a (sig_count & max(queue_size / 2, 1)) but
> > that would only do well with power of 2 sized queues...
> 
> IMHO, It is not-so-big-deal limitation.

Hello Marta, Sagi and Leon,

How about changing nvme_rdma_init_sig_count() such that it always returns
a power of two? The "+ 1" in the code below makes sure that the argument of
ilog2() is larger than zero even if queue_size == 1. I'm not sure whether
the time needed to compute ilog2() would make it necessary to cache the
nvme_rdma_init_sig_count() return value.

static inline int nvme_rdma_init_sig_count(int queue_size)
{
        /* Return the largest power of two that is not above half of (queue size + 1) */
 ???????return 1 << ilog2((queue_size + 1) / 2);
}

Bart.