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SubjectRe: [PATCH V2] leds: trigger: Introduce an USB port trigger
HI Rob,

I got problems following your objections, so it took me some time to
go back to this.

On 21 July 2016 at 22:42, Rob Herring <robh@kernel.org> wrote:
> On Wed, Jul 20, 2016 at 10:06:23AM +0200, Rafał Miłecki wrote:
>> On 20 July 2016 at 03:02, Rob Herring <robh@kernel.org> wrote:
>> > On Fri, Jul 15, 2016 at 11:10:45PM +0200, Rafał Miłecki wrote:
>> >> This commit adds a new trigger that can turn on LED when USB device gets
>> >> connected to the USB port. This can be useful for various home routers
>> >> that have USB port and a proper LED telling user a device is connected.
>> >>
>> >> Right now this trigger is usable with a proper DT only, there isn't a
>> >> way to specify USB ports from user space. This may change in a future.
>> >>
>> >> Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
>> >> ---
>> >> V2: The first version got support for specifying list of USB ports from
>> >> user space only. There was a (big try &) discussion on adding DT
>> >> support. It led to a pretty simple solution of comparing of_node of
>> >> usb_device to of_node specified in usb-ports property.
>> >> Since it appeared DT support may be simpler and non-DT a bit more
>> >> complex, this version drops previous support for "ports" and
>> >> "new_port" and focuses on DT only. The plan is to see if this
>> >> solution with DT is OK, get it accepted and then work on non-DT.
>> >>
>> >> Felipe: if there won't be any objections I'd like to ask for your Ack.
>> >> ---
>> >> Documentation/devicetree/bindings/leds/common.txt | 11 ++
>> >> Documentation/leds/ledtrig-usbport.txt | 19 ++
>> >> drivers/leds/trigger/Kconfig | 8 +
>> >> drivers/leds/trigger/Makefile | 1 +
>> >> drivers/leds/trigger/ledtrig-usbport.c | 206 ++++++++++++++++++++++
>> >> 5 files changed, 245 insertions(+)
>> >> create mode 100644 Documentation/leds/ledtrig-usbport.txt
>> >> create mode 100644 drivers/leds/trigger/ledtrig-usbport.c
>> >>
>> >> diff --git a/Documentation/devicetree/bindings/leds/common.txt b/Documentation/devicetree/bindings/leds/common.txt
>> >> index af10678..75536f7 100644
>> >> --- a/Documentation/devicetree/bindings/leds/common.txt
>> >> +++ b/Documentation/devicetree/bindings/leds/common.txt
>> >> @@ -50,6 +50,12 @@ property can be omitted.
>> >> For controllers that have no configurable timeout the flash-max-timeout-us
>> >> property can be omitted.
>> >>
>> >> +Trigger specific properties for child nodes:
>> >> +
>> >> +usbport trigger:
>> >> +- usb-ports : List of USB ports that usbport should observed for turning on a
>> >> + given LED.
>> >
>> > I think this should be more generic such that it could work for disk or
>> > network LEDs too. Perhaps 'led-triggers = <nodes>'? trigger is a bit of
>> > a Linux name, but I haven't thought of anything better. Really, I'd
>> > prefer the link in the other direction (e.g. port node have a 'leds" or
>> > '*-leds' property), but that's maybe harder to parse.
>>
>> I was already thinking on this as I plan to add "netdev" trigger at
>> some point in the future. I'd like to use "net-devices" for it (as a
>> equivalent or "usb-ports").
>>
>> However there is a problem with your idea of sharing "led-triggers"
>> between triggers.. Consider device with generic purpose LED that
>> should be controllable by a user. I believe we should let user switch
>> it's state, e.g. with:
>> echo usbport > trigger
>> echo netdev > trigger
>> with a shared property I don't see how we couldn't handle it properly.
>
> Well, the userspace interface and DT bindings are independent things.
> You can't argue for what the DT binding should look like based on the
> userspace interface.

I don't understand that. It sounds like you don't want user to have
control over a LED that was specified in DT. If I got it right, it
sounds like a bad idea to me. It's a known case (in marketing, usage
model) to allow user disable all LEDs (e.g. to don't disturb him
during the night). That sounds like a valid usage for me, so I want
user to be able to switch between triggers. And this is of course what
is already supported by triggers and user space interface. With
*current* triggers implementation you can do:
cd /sys/class/leds/foo/
echo none > trigger
echo timer > trigger

So I'm trying to have trigger specific entry in order to support more
than a single trigger.


> Perhaps we need a "device trigger" where you echo the device to
> be the trigger (similar to how bind works). If you have something to id
> the device, then you can lookup its struct device and then its of_node
> ptr.

Are you describing mixing user space interface with DT interface at
this point? Because echoing device sounds like doing some "echo foo >
bar" in user space. If so, I didn't design setting trigger details
from user space yet. I'm aware it'll require more discussion, so I
left it fo later.


> The other problem with your userspace interface is it only works if
> the trigger is defined in DT. It doesn't allow for specifying which usb
> port or which netdev. Any trigger source should be assignable to any
> LED? I can assign the disk activity trigger to the numlock LED if I
> want to...

Well, it's not a surprise, I pretty straightly described it in the
commit message:
> Right now this trigger is usable with a proper DT only, there isn't a
> way to specify USB ports from user space. This may change in a future.
I mean to discuss and add a way of specifying sources to the trigger
in the future.

To answer your question: you can't assign trigger source to the LED.
First you need to assign trigger to the LED. Then some triggers may
allow setting extra parameters.

There are not limitations on assigning triggers. So you can e.g.
assign disk activity trigger to the USB LED. LEDs subsystem doesn't
store info about LED type, only its name.


>> I don't think we can use anything like:
>> led-triggers = <&ohci_port1>, <&ehci_port1>, <&ethmac0>;
>> I'd get even more complicated if we ever need cells for any trigger.
>> AFAIK it's not allowed to mix handles with different cells like:
>> led-triggers = <&ohci_port1>, <&foo 5>, <&ethmac0>;
>
> It is allowed, but you would have to have a '#led-trigger-cells'
> property in each phandle target.
>
>> These problems made me use trigger-specific properies for specifying
>> LED triggers. Do you have any other idea for sharing a property &
>> avoiding above problems at the same time?
>>
>>
>> >> +
>> >> Examples:
>> >>
>> >> system-status {
>> >> @@ -58,6 +64,11 @@ system-status {
>> >> ...
>> >> };
>> >>
>> >> +usb {
>> >> + label = "USB";
>> >
>> > It's not really clear in the example this is an LED node as it is
>> > incomplete.
>>
>> What do you mean by incomplete? Should I include e.g. ohci_port1 in
>> this example?
>
> label and usb-ports alone in this node does not make an LED node.

I just followed the current form of
Documentation/devicetree/bindings/leds/common.txt . Should we improve
this while in general? It already has a similar example like this:
system-status {
label = "Status";
linux,default-trigger = "heartbeat";
...
};


>> >> + usb-ports = <&ohci_port1>, <&ehci_port1>;
>> >> +};
>> >> +
>> >> camera-flash {
>> >> label = "Flash";
>> >> led-sources = <0>, <1>;
>> >> diff --git a/Documentation/leds/ledtrig-usbport.txt b/Documentation/leds/ledtrig-usbport.txt
>> >> new file mode 100644
>> >> index 0000000..642c4cd
>> >> --- /dev/null
>> >> +++ b/Documentation/leds/ledtrig-usbport.txt
>> >> @@ -0,0 +1,19 @@
>> >> +USB port LED trigger
>> >> +====================
>> >> +
>> >> +This LED trigger can be used for signaling user a presence of USB device in a
>> >> +given port. It simply turns on LED when device appears and turns it off when it
>> >> +disappears.
>> >> +
>> >> +It requires specifying a list of USB ports that should be observed. This can be
>> >> +done in DT by setting a proper property with list of a phandles. If more than
>> >> +one port is specified, LED will be turned on as along as there is at least one
>> >> +device connected to any of ports.
>> >> +
>> >> +This trigger can be activated from user space on led class devices as shown
>> >> +below:
>> >> +
>> >> + echo usbport > trigger
>> > Why do I have to do this (by default)? I already specified in the DT
>> > what the connection is. It should come up working OOTB, or don't put it
>> > in DT.
>>
>> Just specifying connection with "usb-ports" (or "led-triggers") won't
>> enable a given trigger and it shouldn't. There is already a way to
>> specify default trigger using property "linux,default-trigger". So you
>> can specify:
>> 1) Default one with:
>> linux,default-trigger = "usbport";
>> 2) On runtime:
>> echo usbport > trigger
>
> IMO, these should be mutually exclusive. Either you specify a DT node as
> the trigger or you specify a linux specific trigger.

I don't think I'm even able to make them exclusive without reworking
triggers subsystem. It's what is already implemented there and trigger
can't really say it refuses to be unassigned from the LED.
Also if we don't allow changing current trigger (by echo foo >
trigger) it won't be possible to disable LED (e.g. for a night) or
change trigger for some multi purpose LEDs.


>> >> +Nevertheless, current there isn't a way to specify list of USB ports from user
>> >> +space.
>> >
>> > s/current/currently/
>> >
>> > This is a problem since if it works by default and you switch to a
>> > different trigger, there's no way to get back to the default (unless
>> > you remember the ports).
>>
>> There is no such problem. You can freely do:
>> echo none > trigger
>> (e.g. to disable LED during night or whatever)
>> and then:
>> echo usbport > trigger
>>
>> This will make "usbport" use triggers specified in DT as expected.
>
> I think the singular "usbport" trigger is problematic. Look at how cpu
> triggers are done. I can specify which cpu is the trigger. You should be
> able specify which port is the trigger, too.

Maybe it would be possible to register separate trigger for every USB
port. I can see some problems like fetching list of possible USB port
(we don't have anything lovely like for_each_possible_cpu). Should
list of ports be read from DT? Or maybe we should create/delete
triggers dynamically as new USB hubs appear? That would require more
logic like detecting how many ports a hub supports.

Anyway, the serious limitation I see is assigning more than a single
port to a LED. One LED can have only 1 trigger assigned at the time.
There are plenty of devices with 1 USB LED and few USB ports. Also
some physical ports are handled by more than 1 controller (up to 3 or
more). We really need a way to assign more than 1 USB port to a single
LED.
This seems to be also a limitation of CPU trigger I didn't notice so
far. What if you have dual CPU device with only 1 CPU activity LED?


> In summary, I guess what I'm saying is don't push the problems of the
> current userspace interface down to DT.

Thanks for your time and looking at this problem with me. I'm afraid
I'll need some more help (or so it seems so far to me).

I'm wondering if we have some misunderstanding on this whole thing
with USB port trigger. Maybe I didn't explain my use case well enough
and it's clear to me only?

I'm coming from OpenWrt/LEDE project which is basically a distribution
for home routers. We deal with a wide range of devices you can find on
a market. We also have plenty of out of tree drivers and solutions I'm
not a big fan of. I'm trying to improve/rewrite them, get them in an
acceptable form and upstream.

One of problems we got are USB LEDs. We have some out of tree driver for them:
http://git.openwrt.org/?p=15.05/openwrt.git;a=blob;f=target/linux/generic/files/drivers/leds/ledtrig-usbdev.c;hb=HEAD
but it doesn't work well. It doesn't have any DT support. It requires
user to know USB ports. He has to write some script that will setup a
trigger on every boot. It works for 1 USB port only.

So I would like to:
1) Have a USB trigger selectable with "linux,default-trigger"
2) Have property next to "linux,default-trigger" specifying USB ports
3) Have trigger follow ports, light LED on if any of them has device connected
4) Allow user to deactivate this trigger (e.g. no LEDs on during night)

Some extra notes:
1) I don't think it's possible for a single trigger to handle various
devices like USB or network ones. Later we may may want to add more
complex stuff like blinking on activity. It's too much to handle in a
single trigger.
2) I'm trying to be thinking future-proof and I think we may need
switching between triggers in the future. I mean allowing user to
choose between some more complex triggers, not just "none" and "foo".
Maybe some status LED (with default-on trigger) that can get network
activity trigger assigned during WPS action? Such a possibility of
changing triggers would go in pair with current implementation (echo
"foo" > trigger).
Does it make my attempts to make any more sense? Can you suggest some
way I should proceed?

--
Rafał

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