Minimal Linux kernel options to enable hid input

Question

I need to handle a HID device (a barcode reader) with a custom Linux (v5.8).

I have a modulerized kernel which works as expected with other USB peripherals (storage and serial are known to work), but I seem unable to scan this device.

Plugging it into a desktop (Linux Mint "Ulyana", if it matters) I get a normal enumeration:

[525428.367216] usb 1-11: new full-speed USB device number 9 using xhci_hcd
[525428.517071] usb 1-11: New USB device found, idVendor=05e0, idProduct=1200, bcdDevice= 1.00
[525428.517077] usb 1-11: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[525428.517080] usb 1-11: Product: Symbol Bar Code Scanner
[525428.517083] usb 1-11: Manufacturer: Symbol Technologies, Inc, 2008
[525428.517086] usb 1-11: SerialNumber: S/N:0641F625A3A943949AF00BAB171ABFE2 Rev:PAACFS00-001-R023
[525428.519932] input: Symbol Technologies, Inc, 2008 Symbol Bar Code Scanner as /devices/pci0000:00/0000:00:14.0/usb1/1-11/1-11:1.0/0003:05E0:1200.0005/input/input32
[525428.579736] hid-generic 0003:05E0:1200.0005: input,hidraw4: USB HID v1.10 Keyboard [Symbol Technologies, Inc, 2008 Symbol Bar Code Scanner] on usb-0000:00:14.0-11/input0

While on my target I see no sign of the new peripheral; lsusb displays just the root HUB even when I manually modprobe (what I think are) the relevant modules:

# lsusb
Bus 001 Device 001: ID 1d6b:0002
# lsmod
Module                  Size  Used by    Not tainted
usbhid                 23296  0 
usbmon                 19424  0 
mt7603e                38048  0 
mt76                   31648  1 mt7603e
mac80211              380000  2 mt7603e,mt76
sha256_generic          2240  0 
libsha256               8480  1 sha256_generic
ehci_platform           4704  0 
cfg80211              236736  3 mt7603e,mt76,mac80211
ehci_hcd               37936  1 ehci_platform
rfkill                  8544  1 cfg80211
usbcore               143456  4 usbhid,usbmon,ehci_platform,ehci_hcd
libarc4                  832  1 mac80211
mtk_eth                30208  0 
usb_common              2768  3 usbmon,ehci_platform,usbcore

I am obviously missing something, but I seem unable to understand what. What should I cross-check?

Note: please feel free to ask for relevant details, I did not put too many things here just to avoid cluttering but I'm fully prepared to give all info deemed useful.

USB bar code scanners generally look like keyboards, which don't usually create device nodes. — stark
@stark Confirmed. This scanner works in "keyboard emulation mode". It's unclear to me what You are trying to say; could You elaborate, please? with desktop I see: "Bus 001 Device 009: ID 05e0:1200 Symbol Technologies Bar Code Scanner" while on target I see nothing and scanner does not work, not even the startup beep, as if not powered on at all. — ZioByte
@stark: Yes. This same hardware works with an ancient kernel version (3.18+tons of vendor patches). I am trying to move to a modern build (for unrelated issues) and this is one of the last things remaining to "port". Unfortunately kernel config changed enough to make it difficult to compare. — ZioByte
Can you try plugging something else in, or is it hard-wired? — Ian Abbott

ZioByte ZioByte · Accepted Answer · 2020-11-03T18:29:57

It turns out OHCI driver is needed to handle low speed peripherals even if they're connected to USB2 (EHCI) controller.

Actually MT7628 has a "secondary" OHCI controller not even advertised in Data Sheet.

This means that, on top of selecting CONFIG_USB_OHCI_HCD=m and CONFIG_USB_OHCI_HCD_PLATFORM=m also the following patch is needed:

diff --git a/arch/mips/boot/dts/ralink/mt7628a.dtsi b/arch/mips/boot/dts/ralink/mt7628a.dtsi
index bf6b6a459bd6..b4ac008fdfdf 100644
--- a/arch/mips/boot/dts/ralink/mt7628a.dtsi
+++ b/arch/mips/boot/dts/ralink/mt7628a.dtsi
@@ -323,6 +323,17 @@ ehci@101c0000 {
        interrupts = <18>;
    };
 
+   ohci@101c1000 {
+       compatible = "generic-ohci";
+       reg = <0x101c1000 0x1000>;
+
+       phys = <&usb_phy>;
+       phy-names = "usb";
+
+       interrupt-parent = <&intc>;
+       interrupts = <18>;
+   };
+
    ethernet: ethernet@10100000 {
        compatible = "ralink,rt5350-eth";
        reg = <0x10100000 0x10000>;

Note address 101c1000 is nowhere mentioned in MT7628 programming manual and it's "one cell away" (0x1000) from standard (and documented) EHCI controller cell (at 101c0000).

I assume similar situation is also true for other SoCs because USB standard mandates to "hand over" low speed devices to a legacy USB1 controller.

I hope this will spare a lot of headache to whoever will stumble in the same problem.

Minimal Linux kernel options to enable hid input

1 Answers