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

[Andrew Rybchenko <arybchenko@solarflare.com>]

On 07/17/2017 07:21 PM, Stephen Hemminger wrote:
> On Mon, 17 Jul 2017 19:12:01 +0300
> Andrew Rybchenko <arybchenko@solarflare.com> wrote:
>
>> On 07/17/2017 06:58 PM, Stephen Hemminger wrote:
>>> 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.
>> Many thanks. May be it would be useful in the comment as well.
> Maybe one of the original developers could clarify.
> It would be cleaner just to do rte_atomcic64_set(), it might just
> be a leftover semantic from Linux/BSD/??? where the original developer
> was looking.

Agree.

>>>>> +	 */
>>>>> +	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.
>> I meant that dev_link value may change after old value saved in original
>> patch,
>> but before cmpset which actually replaces dev_link value here. As the result
>> two _rte_eth_link_update() run in parallel changing to the same value
>> may return
>> "changes done", but actually only one did the job.
>> I'm not sure if it is really important here, since requirements are not
>> clear.
> Since there is no locking here. There can not be a guarantee of ordering possible.
> The only guarantee is that the set of values (duplex, speed, flags) is consistent.
> I.e one caller wins, the streams don't get crossed.

Results of the update operation is used by some driver to log link up/down
change. So, it could result in duplicate up/down logs. Not a big deal, but
could be confusing.

I guess many are very busy right now with 17.08 release. So, I hope
we'll see more feedback when 17.08 release is done.