Re: [RFC PATCH v2 1/5] leds: trigger: add API for HW offloading of triggers

[Ansuel Smith <ansuelsmth@gmail.com>]

On Mon, Nov 08, 2021 at 07:41:42PM +0100, Marek Behún wrote:
> On Mon, 8 Nov 2021 18:58:38 +0100
> Ansuel Smith <ansuelsmth@gmail.com> wrote:
> 
> > Are you aware of any device that can have some trigger offloaded and
> > still have the led triggered manually?
> 
> I don't understand why we would need such a thing.
> 
> Look, just to make it clear via an example: I have a device with a
> Marvell PHY chip inside. There is a LED connected to one of the PHY LED
> pins.
> 
> Marvell PHY has LED[0] control register, which supports the following
> modes:
>   LED is OFF
>   LED is ON
>   LED is ON when Link is up
>   LED blinks on RX activity
>   LED blinks on TX activity
>   LED blinks on RX/TX activity
>   LED is ON and blinks on RX/TX activity
>   ...
> 
> I have code that exports this LED as a LED classdev
> 
> When I activate netdev trigger on this LED, the netdev trigger currently
> just blinks the LED in software, by calling the .brightness_set()
> method, which configures LED[0] control register to one of the first
> two modes above (LED is OFF, LED is ON).
> 
> But I have also another patch that adds support to offloading netdev
> trigger upon offloadable settings. The netdev trigger code calls the
> .trigger_offload() method, which is implemented in PHY driver. This
> method checks whether it is a netdev trigger that is to be offloaded,
> and whether device_name is the name of the device attached to the PHY,
> and then chooses one of the modes above, according to netdev trigger
> settings.
> 
> So when I request netdev trigger for eth0, to indicate link and blink
> on activity, the netdev trigger doesn't do anything in software. It
> just calls the offload method ONCE (at the moment I am changing netdev
> trigger settings). The blinking is then done by the PHY chip. Netdev
> trigger doesn't do anything, at least not until I change the settings
> again.
> 
> > Talking about mixed mode, so HW and SW.
> 
> What exactly do you mean by mixed mode? There is no mixed mode.
>

Ok.

> > Asking to understand as currently the only way to impement all
> > of this in netdev trigger is that:
> > IF any hw offload trigger is supported (and enabled) then the entire
> > netdev trigger can't work as it won't be able to simulate missing
> > trigger in SW. And that would leave some flexibility.
> 
> What do you mean by missing trigger here? I think we need to clarify
> what we mean by the word "trigger". Are you talking about the various
> blinking modes that the PHY supports? If so, please let's call them HW
> control modes, and not triggers. By "triggers" I understand triggers
> that can be enabled on a LED via /sys/class/leds/<LED>/trigger.
> 

offload triggers = blinking modes supported

> > We need to understand how to operate in this condition. Should netdev
> > detect that and ""hide"" the sysfs triggers? Should we report error?
> 
> So if I understand you correctly, you are asking about what should we
> do if user asked for netdev trigger settings (currently only link, rx,
> tx, interval) that can't be offloaded to the PHY chip.
> 
> Well, if the PHY allows to manipulate the LEDs ON/OFF state (in other
> words "full control by SW", or ability to implement brightness_set()
> method), then netdev trigger should blink the LED in SW via this
> mechanism (which is something it would do now). A new sysfs file,
> "offloaded", can indicate whether the trigger is offloaded to HW or not.
> 

Are all these sysfs entry OK? I mean if we want to add support for he
main blinking modes, the number will increase to at least 10 additional
entry. 

> If, on the other hand, the LED cannot be controlled by SW, and it only
> support some HW control modes, then there are multiple ways how to
> implement what should be done, and we need to discuss this.
> 
> For example suppose that the PHY LED pin supports indicating LINK,
> blinking on activity, or both, but it doesn't support blinking on rx
> only, or tx only.
> 
> Since the LED is always indicating something about one network device,
> the netdev trigger should be always activated for this LED and it
> should be impossible to deactivate it. Also, it should be impossible to
> change device_name.
> 
>   $ cd /sys/class/leds/<LED>
>   $ cat device_name
>   eth0
>   $ echo eth1 >device_name
>   Operation not supported.
>   $ echo none >trigger
>   Operation not supported.
> 
> Now suppose that the driver by default enabled link indication, so we
> have:
>   $ cat link
>   1
>   $ cat rx
>   0
>   $ cat tx
>   0
> 
> We want to enable blink on activity, but the LED supports only blinking
> on both rx/tx activity, rx only or tx only is not supported.
> 
> Currently the only way to enable this is to do
>   $ echo 1 >rx
>   $ echo 1 >tx
> but the first call asks for (link=1, rx=1, tx=0), which is impossible.
> 
> There are multiple things which can be done:
> - "echo 1 >rx" indicates error, but remembers the setting
> - "echo 1 >rx" quietly fails, without error indication. Something can
>   be written to dmesg about nonsupported mode
> - "echo 1 >rx" succeeds, but also sets tx=1
> - rx and tx are non-writable, writing always fails. Another sysfs file
>   is created, which lists modes that are actually supported, and allows
>   to select between them. When a mode is selected, link,rx,tx are
>   filled automatically, so that user may read them to know what the LED
>   is actually doing
> - something different?
> 

Expose only the supported blinking modes? (in conjunciong with a generic
traffic blinking mode)

The initial question was Should we support a mixed mode offloaed
blinking modes and blinking modes simulated by sw? I assume no as i
don't think a device that supports that exist.

> Marek

-- 
	Ansuel