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From: "Tod E. Kurt" <>
To: Heiner Kallweit <>
Cc: "open list:HID CORE LAYER" <>,
	Linux USB Mailing List <>
Subject: Re: Linux "hid_led" vs "hidraw" USB HID question 
Date: Sun, 10 Nov 2019 15:07:22 -0800
Message-ID: <> (raw)
In-Reply-To: <>

Hi Heiner,

Yes, that was what I was starting from. I was hoping there was now an in-situ way of replacing kernel modules without a full kernel recompile.

I have tried this patch on Raspian 4.19.81 and Ubuntu 19 kernel 5.0.0-32 with no config changes.  I believe it solves the main issue.  However, I'm seeing another problem that I can't explain.

- On reboot, device shows up in /sys/class/hidraw/
- Opening with hidapi and (mis)using hidapi functions works, but,
- Device is then removed from /sys/class/hidraw/

Under what circumstances can a device be removed from /sys/class/hidraw? 

If you'd like to trigger this yourself, run "blink1-tool --list" and see the blink(1) disappear. And yet "blink1-tool" can subsequently still communicate with the blink(1). Which I don't quite understand yet either.


> On Nov 10, 2019, at 9:25 a, Heiner Kallweit <> wrote:
> On 10.11.2019 17:59, Tod E. Kurt wrote:
>> Hi Heiner, (and others on the list)
>> Thanks for your quick and detailed response. 
>> I am endeavoring to try the patch out now.  But I've not built a desktop Linux kernel or kernel modules in about 15 years.  Do you have a preferred recipe for applying this patch to an existing distro?  Since 'hid-quirks' isn't a module, this means recompiling the entire HID driver, correct?  I mostly test against various modern Ubuntu or Raspian flavors of Debian, if that matters.
> See e.g. here:
>> Also, is there a mitigation for users running production distros that doesn't involve a recompile? 
> I don't think so (at least from kernel perspective). What you could do is adding an integration with
> the /sys/class/led interface in your library (if it should be suitable for your use cases) and
> auto-detect which interface to use.
>> Thanks,
>> -=Tod
> Heiner
>>> On Nov 10, 2019, at 2:44 a, Heiner Kallweit <> wrote:
>>> On 10.11.2019 04:26, Tod E. Kurt wrote:
>>>> HiI
>>> Hi Tod,
>>>> Since you are listed as author of "hid-led.c": 
>>>> I was wondering if you could offer some insight on an issue I'm seeing.
>>>> I'm a maintainer of "hidapi" and "node-hid", cross-platform C and NodeJs libraries for accessing HID devices, and I'm the creator of the ThingM blink(1) USB LED that "hid-led" controls.
>>>> On the distros I've seen (Ubuntu, Raspian) where "hid-led" is enabled, when inserting a blink(1) device, the "hid-led" driver appears to grab the device and remove it from available "hidraw" devices.  This makes generic hidraw-based systems (like "hidapi" and the upcoming Chrome WebHID) unable to see the blink(1)
>>>> I have tried blacklisting the "hid-led" module but the problem persist. Ffrom dmesg and lsmod the module does appear to not be loaded on reboot.
>>>> Any insights on what's going on?  Any tips on how to debug this or how to prevent this from occurring?
>>> When hid-led was written it was needed to add the LED devices to hid_have_special_driver list.
>>> Else the driver can't take control over the device. Side effect is that even if hid-led
>>> isn't loaded hid-generic can't take control.
>>> I think since e04a0442d33b ("HID: core: remove the absolute need of hid_have_special_driver[]")
>>> it's no longer needed to have the LED devices in hid_have_special_driver.
>>> Could you please test the following patch? If hid-led is loaded is should control the
>>> device, if it's blacklisted hid-generic should have the control.
>>> diff --git a/drivers/hid/hid-quirks.c b/drivers/hid/hid-quirks.c
>>> index c50bcd967..bdaab79f7 100644
>>> --- a/drivers/hid/hid-quirks.c
>>> +++ b/drivers/hid/hid-quirks.c
>>> @@ -419,13 +419,6 @@ static const struct hid_device_id hid_have_special_driver[] = {
>>> #endif
>>> -#endif
>>> -- 
>>> 2.24.0
>>>> I can send you blink(1) devices and "hidapi" test programs if you'd like to try to replicate this.
>>> Thanks for the offer, I've got a blink(1) already. Just tested the patch with this device
>>> and it worked properly like described above.
>>>> Thanks!
>>>> -=Tod
>>> Heiner

  reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
2019-11-10 10:44 ` Heiner Kallweit
2019-11-10 16:59   ` Tod E. Kurt
2019-11-10 17:25     ` Heiner Kallweit
2019-11-10 23:07       ` Tod E. Kurt [this message]
2019-11-11 21:31         ` Heiner Kallweit

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