Re: [dpdk-dev] [PATCH] librte_ethdev: extend dpdk api led control to query capability

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 1/8/2020 10:31 AM, Laurent Hardy wrote:
> Hi all,
> 
> On 1/8/20 10:55 AM, David Marchand wrote:
>> On Wed, Jan 8, 2020 at 10:09 AM Ferruh Yigit <ferruh.yigit@intel.com> wrote:
>>> On 1/8/2020 8:56 AM, David Marchand wrote:
>>>> Hello Laurent,
>>>>
>>>> Bonne année.
>>>>
>>>> Cc: maintainers.
>>>>
>>>> On Tue, Jan 7, 2020 at 3:57 PM Laurent Hardy <laurent.hardy@6wind.com> wrote:
>>>>> In current led control API we have no way to know if a device is able
>>>>> to handle on/off requests coming from the application.
>>>>> Knowing if the device is led control capable could be useful to avoid
>>>>> exchanges between application and kernel.
>>>>> Using the on/off requests to flag if the device is led control capable
>>>>> (based on the ENOSUP returned error) is not convenient as such request
>>>>> can change the led state on device.
>>>>>
>>>>> This patch adds a new function rte_eth_led_ctrl_capable() that will look
>>>>> for led_off/on dev ops availability on the related pmd, to know if the
>>>>> device is able to handle such led control requests (on/off).
>>>> This patch breaks the ABI, which is BAD :-).
>>> Why it is an ABI break, dev_ops should be between library and drivers, so it
>>> should be out of the ABI concern, isn't it.
>> You are right.
>> So in our context, this is not an ABI breakage.
>> But abidiff still reports it, so maybe some filtering is required to
>> avoid this false positive.
>>
>> Note that if we insert an ops before rx_queue_count, we would have a
>> real ABI breakage, as this ops is accessed via an inline wrapper by
>> applications.
>>
>>
>>>> This new api only needs to look at the existing ops, so you can remove
>>>> the (unused in your patch) dev_led_ctrl_capable ops.
>>>>
>>>> OTOH, would it make sense to expose this capability in dev_flags?
>>>>
>>> 'rte_eth_led_on()' & 'rte_eth_led_off()' APIs returns '-ENOTSUP' when the not
>>> supported, can that help application to understand?
>> You might want to know it is supported without changing the state.
>> Laurent?
> 
> First, happy new year :)
> 
> Yes exactly, the purpose of this patch is to query if the device is led 
> control capable or not without changing the led state.
> 
> About exposing the capability through a dev_flags, means to make some 
> modification in each pmds. It looks more easy in term of pmds 
> maintenance to relying on the rte_eth_led_off()/on() dev ops 
> availability at rte_ethdev level, right ?
> 

'dev_flag' definition is not clear, right now it holds the combination of status
and capability. And we have 'rte_eth_dev_info' struct, which is again
combination of device capability and status.
Perhaps we should have explicit capabilities and status fields, even in the
rte_device level which inherited by net/crypto devices etc..

But for dev_ops, instead of having another capabilities indicator, which
requires PMDs to keep this synchronized, I think it is better if we can self
contain this information within dev_ops, like not implementing dev_ops would
mean it is not supported, this way it is easier to maintain and less error prone.

Only we should have it without side effect,

1- adding an additional 'dry-run' parameter can work, but this means breaking
ABI and updating majority of the ethdev APIs :)
2- Adding 'is_supported' versions of the APIs as we need can be an option, like
'rte_eth_led_on_is_supported()'
3- Olivier's suggestion to add a new API to get the led status, so that this
information can be used select led API which won't cause side affect and let us
learn if it is supported.

Any other alternatives?

I would prefer the 2) in above ones, which is very similar to the original patch.