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

[Olivier Matz <olivier.matz@6wind.com>]

On Wed, Jan 08, 2020 at 09:09:29AM +0000, Ferruh Yigit 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.
> 
> > 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?

In our case, it is not possible to use rte_eth_led_on/off() to check if
the feature is supported: on success, it would change the value of the
led, and it seems it is not recoverable on some drivers.

Today it is not possible to know if the feature is available without
side effect.