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

[Laurent Hardy <laurent.hardy@6wind.com>]

On 1/8/20 2:06 PM, Thomas Monjalon wrote:
> 08/01/2020 13:59, Ferruh Yigit:
>> 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.
> I agree capabilities in ethdev are a bit of a mess.
> I would appreciate someone makes a complete audit of it
> so we can discuss how to improve the situation.
>
>
>> Perhaps we should have explicit capabilities and status fields, even in the
>> rte_device level which inherited by net/crypto devices etc..
> No, ethdev capabilities should stay in ethdev.
>
>
>> 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.
> It means the dev_ops is resetted at init if a device does not support the feature.
> It is against having const dev_ops.
>
>
>> 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.

I can provide a V2 which will remove the useless dev_led_ctrl_capable ops.

About the 'is_supported()' versions of APIs, in the current patch I 
factorize
the check on dev ops on and off availability in a same function named
"led_ctrl_capable" but I can rename it if required.

Just in this specific case I don't dissociate on and off capability, as 
being
able to set the led off without a way to set it on again sounds a bit 
unusual :)

> The other alternatives are in rte_eth_dev_info and dev_flags.
>
>
>