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

[Thomas Monjalon <thomas@monjalon.net>]

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.

The other alternatives are in rte_eth_dev_info and dev_flags.