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Date: Mon, 17 Jul 2017 08:58:53 -0700
From: Stephen Hemminger <stephen@networkplumber.org>
To: Andrew Rybchenko <arybchenko@solarflare.com>
Cc: <dev@dpdk.org>, Stephen Hemminger <sthemmin@microsoft.com>
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Subject: Re: [dpdk-dev] [RFC 01/14] ethdev: add link status read/write
functions
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On Sun, 16 Jul 2017 16:26:06 +0300
Andrew Rybchenko <arybchenko@solarflare.com> wrote:
> On 07/14/2017 09:30 PM, Stephen Hemminger wrote:
> > Many drivers are all doing copy/paste of the same code to atomicly
> > update the link status. Reduce duplication, and allow for future
> > changes by having common function for this.
> >
> > Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
> > ---
> > lib/librte_ether/rte_ethdev.c | 36 ++++++++++++++++++++++++++++++++++++
> > lib/librte_ether/rte_ethdev.h | 28 ++++++++++++++++++++++++++++
> > 2 files changed, 64 insertions(+)
> >
> > diff --git a/lib/librte_ether/rte_ethdev.c b/lib/librte_ether/rte_ethdev.c
> > index a1b744704f3a..7532fc6b65f0 100644
> > --- a/lib/librte_ether/rte_ethdev.c
> > +++ b/lib/librte_ether/rte_ethdev.c
> > @@ -1332,6 +1332,42 @@ rte_eth_link_get_nowait(uint8_t port_id, struct rte_eth_link *eth_link)
> > }
> >
> > int
> > +_rte_eth_link_update(struct rte_eth_dev *dev,
> > + const struct rte_eth_link *link)
> > +{
> > + volatile struct rte_eth_link *dev_link = &(dev->data->dev_link);
> > + struct rte_eth_link old;
> > +
> > + RTE_BUILD_BUG_ON(sizeof(*link) != sizeof(uint64_t));
> > +
> > + old = *dev_link;
> > +
> > + /* Only reason we use cmpset rather than set is
> > + * that on some architecture may use sign bit as a flag value.
>
> May I ask to provide more details here.
rte_atomic64_set() takes an int64 argument.
This code (taken from ixgbe, virtio and other drivers) uses cmpset
to allow using uint64_t.
My assumption is that some architecture in the past was using the
sign bit a a lock value or something. On 64 bit no special support
for 64bit atomic assignment is necessary. Not sure how this code
got inherited that way.
>
> > + */
> > + while (rte_atomic64_cmpset((volatile uint64_t *)dev_link,
> > + *(volatile uint64_t *)dev_link,
> > + *(const uint64_t *)link) == 0)
>
> Shouldn't it be:
> do {
> old = *dev_link;
> } while (rte_atomic64_cmpset((volatile uint64_t *)dev_link,
> *(uint64_t *)&old, *(const uint64_t *)link) == 0);
>
> At least it has some sense to guarantee transition from old to new
> talking below comparison into account.
Since dev_link is volatile, the compiler is required to refetch
the pointer every time it evaluates the expression. Maybe clearer
to alias devlink to a volatile uint64_t ptr.