[dpdk-dev] [RFC 01/14] ethdev: add link status read/write functions
Andrew Rybchenko
arybchenko at solarflare.com
Mon Jul 17 18:31:30 CEST 2017
On 07/17/2017 07:21 PM, Stephen Hemminger wrote:
> On Mon, 17 Jul 2017 19:12:01 +0300
> Andrew Rybchenko <arybchenko at 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 at 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 at 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.
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