Re: [dpdk-dev] [RFC 01/14] ethdev: add link status read/write functions

[Stephen Hemminger <stephen@networkplumber.org>]

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.