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rpi-4.4.y: Add USB gadget support to Pi Zero #1239

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@notro notro commented Dec 26, 2015

Build the dwc2 usb driver as a module to be able to use the USB On-the-Go functionality (supports both host and device/peripheral mode).
Add a specific Device Tree file for Pi Zero with dwc2 enabled in otg mode. Also fix the inverted ACT led.

This requires a change in the bootloader to give the Zero a dtb of it's own. Currently it uses this:

~$ sudo vcdbg log msg
001289.085: Loading 'bcm2708-rpi-b-plus.dtb' from SD card

This PR can be tested by setting the dtb manually in /boot/config.txt:



This will give a serial console on a connected computer:

sudo modprobe g_serial
sudo systemctl start getty@ttyGS0.service

On my Windows 8 computer, the USB serial port shows up as: ELMO GMAS (COM5)

Hotswapping to a keyboard works.

Controller fifo

The fifo assignment isn't optimal. This PR uses only 1024 of the 4080 available dynamic fifo entries:

[    8.779556] dwc2 20980000.usb: EPs: 8, dedicated fifos, 4080 entries in SPRAM

Currently assigned in Device Tree:

&usb {
    g-np-tx-fifo-size = <32>;
    g-rx-fifo-size = <256>;
    g-tx-fifo-size = <256 128 128 64 64 64 32>;

I don't know how to best distribute the remaining entries.


When I disconnect the keyboard, I get a warning:

[   93.639089] g_serial gadget: Gadget Serial v2.4
[   93.639120] g_serial gadget: g_serial ready
[   93.642186] dwc2 20980000.usb: bound driver g_serial
<connect computer>
[  101.877358] dwc2 20980000.usb: new device is high-speed
[  101.893135] dwc2 20980000.usb: new address 5
[  101.908600] g_serial gadget: high-speed config #2: CDC ACM config
<disconnect computer and connect keyboard>
[  158.138483] usb 1-1: new low-speed USB device number 2 using dwc2
[  158.355959] usb 1-1: New USB device found, idVendor=046d, idProduct=c313
[  158.356001] usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[  158.356019] usb 1-1: Product: USB Multimedia Keyboard
[  158.356036] usb 1-1: Manufacturer: LITEON Technology
[  158.378340] input: LITEON Technology USB Multimedia Keyboard as /devices/platform/soc/20980000.usb/usb1/1-1/1-1:1.0/0003:046D:C313.0001/input/input0
[  158.441305] hid-generic 0003:046D:C313.0001: input,hidraw0: USB HID v1.10 Keyboard [LITEON Technology USB Multimedia Keyboard] on usb-20980000.usb-1/input0
[  158.454182] input: LITEON Technology USB Multimedia Keyboard as /devices/platform/soc/20980000.usb/usb1/1-1/1-1:1.1/0003:046D:C313.0002/input/input1
[  158.509498] hid-generic 0003:046D:C313.0002: input,hidraw1: USB HID v1.10 Device [LITEON Technology USB Multimedia Keyboard] on usb-20980000.usb-1/input1
<disconnect keyboard>
[  186.039220] usb 1-1: USB disconnect, device number 2
[  186.219073] ------------[ cut here ]------------
[  186.219241] WARNING: CPU: 0 PID: 67 at drivers/usb/dwc2/gadget.c:176 dwc2_hsotg_init_fifo+0x18c/0x1ac [dwc2]()
[  186.219258] Modules linked in: evdev usb_f_acm u_serial g_serial libcomposite cfg80211 rfkill dwc2 udc_core bcm2835_gpiomem bcm2835_wdt uio_pdrv_genirq uio i2c_dev snd_bcm2835 snd_pcm snd_timer snd fuse
[  186.219359] CPU: 0 PID: 67 Comm: kworker/u2:2 Not tainted 4.4.0-rc6+ #2
[  186.219371] Hardware name: BCM2708
[  186.219435] Workqueue: dwc2 dwc2_conn_id_status_change [dwc2]
[  186.219508] [<c0016ccc>] (unwind_backtrace) from [<c0013b3c>] (show_stack+0x20/0x24)
[  186.219548] [<c0013b3c>] (show_stack) from [<c032682c>] (dump_stack+0x20/0x28)
[  186.219584] [<c032682c>] (dump_stack) from [<c00222c8>] (warn_slowpath_common+0x8c/0xc4)
[  186.219613] [<c00222c8>] (warn_slowpath_common) from [<c00223bc>] (warn_slowpath_null+0x2c/0x34)
[  186.219697] [<c00223bc>] (warn_slowpath_null) from [<bf0ae6ac>] (dwc2_hsotg_init_fifo+0x18c/0x1ac [dwc2])
[  186.219831] [<bf0ae6ac>] (dwc2_hsotg_init_fifo [dwc2]) from [<bf0b04e4>] (dwc2_hsotg_core_init_disconnected+0x60/0x35c [dwc2])
[  186.219937] [<bf0b04e4>] (dwc2_hsotg_core_init_disconnected [dwc2]) from [<bf0a6824>] (dwc2_conn_id_status_change+0xe8/0x21c [dwc2])
[  186.220001] [<bf0a6824>] (dwc2_conn_id_status_change [dwc2]) from [<c0039c70>] (process_one_work+0x13c/0x490)
[  186.220029] [<c0039c70>] (process_one_work) from [<c003a13c>] (worker_thread+0x178/0x538)
[  186.220064] [<c003a13c>] (worker_thread) from [<c003fee8>] (kthread+0xe0/0xfc)
[  186.220097] [<c003fee8>] (kthread) from [<c000f788>] (ret_from_fork+0x14/0x2c)
[  186.220114] ---[ end trace 900ecfdec3d84a77 ]---
<connect computer>
[  189.792998] dwc2 20980000.usb: new device is high-speed
[  189.808759] dwc2 20980000.usb: new address 13
[  189.831151] g_serial gadget: high-speed config #2: CDC ACM config

Failed on 4.1

I also tried to do this PR on rpi-4.1.y but it didn't work and I didn't try to find out why. Got this error:

[    7.463269] dwc2 20980000.usb: no platform data or transceiver defined

This PR is based on work done by several people in #1212

@notro notro changed the title Add USB gadget support to Pi Zero rpi-4.4.y: Add USB gadget support to Pi Zero Dec 26, 2015
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notro commented Dec 26, 2015

I only get the warning on keyboard disconnect if I have g_serial loaded.

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popcornmix commented Dec 26, 2015

Thanks. No objections. @pelwell ?

I did mention the inverted ACT led shortly before launch, but I think Mike liked the inverted, mostly on behaviour due to the absence of a PWR led, so it wasn't fixed then. I still think fixing it is probably the right thing to do, but we'll see what @pelwell and others think.

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MrEngman commented Dec 27, 2015

The ACT led is great how it works at the moment. Ideal as there is no PWR led. Please don't change it.

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notro commented Dec 27, 2015

The ACT led is great how it works at the moment. Ideal as there is no PWR led.

In what way ideal, that it works excatly as a PWR led? The trigger can be changed to default-on to mimick a power led.

The problem with the inverted behaviour is that trigger none turns the led on and default-on turns it off. heartbeat gives a strange pulsing, and mmc0 is steady on apart from small "inverted blinks" when there's activity instead of a blink. cpu0 is also strange like mmc0.

There's a DT overlay parameter (act_led_activelow) to change the inversion, but I think that the expected behaviour should be the default.

Available triggers:

~$ cat /sys/class/leds/led0/trigger
none kbd-scrollock kbd-numlock kbd-capslock kbd-kanalock kbd-shiftlock kbd-altgrlock kbd-ctrllock kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock kbd-ctrlrlock [mmc0] timer oneshot heartbeat backlight gpio cpu0 default-on input

Change trigger from boot with overlay parameter: act_led_trigger

Change trigger immediately:

~$ echo cpu0 | sudo tee /sys/class/leds/led0/trigger

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MrEngman commented Dec 27, 2015

I didn't say it works "exactly" as a PWR led, but it does provide a good indication the Pi zer0 is powered. It appears to default to ON indicating the Pi is powered and flashes off when the SD card is being accessed/written.

My Pi zero sat next to me is idle at the moment and the ACT led is lit showing it is being powered. If it is changed to behave like the earlier Pi's that have a seperate PWR led then it would be off and I would not know if it was powered on or not.

Or is this a case of it ain't broke fix it so it is, like the changes to the wifi made over the last few months.

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clivem commented Dec 27, 2015

I think the original Zero LED behaviour should stay! I'm more interested in a visual indication that the Zero is being powered than its SDCARD is being accessed. What we have now is a good compromise, only having a single LED available to work with.

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notro commented Dec 27, 2015

How about this change which will turn the led on, but keep the correct behaviour for the other triggers?

 &leds {
    act_led: act {
        label = "led0";
-       linux,default-trigger = "mmc0";
+       linux,default-trigger = "default-on";
        gpios = <&gpio 47 1>;

I agree that it's useful to have a power indicator.

Some thoughts on using the act led:
If I were to make a product based around a Zero, I would have turned the led on using dt-blob.bin, indicating that the board is powered (firmware loaded and sdcard funtional).
Then I would have used the trigger "timer" in the dtb causing the led to blink during kernel loading.
At last I would have turned the led steady on when the board was fully funtional.

This way I would now that if the led never lights, there's a power/sdcard problem. If it lights up I know the firmware is loaded.
If it doesn't start blinking, the kernel has failed to load.
If it stays blinking, I know that my app or whatever hasn't started succesfully.
There is also one more diagnostic step, and that would be to switch to the hearbeat pattern as the first thing when userspace loads. To show that systemd has started.
But I have never shipped a product so I don't know how useful it is to pinpoint these different stages for diagnostic purposes.

Steve Glendinning and others added 21 commits Dec 28, 2015
smsc95xx is adjusting truesize when it shouldn't, and following a recent patch from Eric this is now triggering warnings.

This patch stops smsc95xx from changing truesize.

Signed-off-by: Steve Glendinning <>
The old arch-specific IRQ macros included a dsb to ensure the
write to clear the mailbox interrupt completed before returning
from the interrupt. The BCM2836 irqchip driver needs the same
precaution to avoid spurious interrupts.

Spurious interrupts are still possible for other reasons,
though, so trap them early.
Add a duplicate irq range with an offset on the hwirq's so the
driver can detect that enable_fiq() is used.
Tested with downstream dwc_otg USB controller driver.

Signed-off-by: Noralf Trønnes <>
Reviewed-by: Eric Anholt <>
Acked-by: Stephen Warren <>
Signed-off-by: Noralf Trønnes <>
Signed-off-by: Noralf Tronnes <>
Contrary to the documentation, the BCM2835 GPIO controller actually has
four interrupt lines - one each for the three IRQ groups and one common. Rather
confusingly, the GPIO interrupt groups don't correspond directly with the GPIO
control banks. Instead, GPIOs 0-27 generate IRQ GPIO0, 28-45 GPIO1 and
46-53 GPIO2.

Awkwardly, the GPIOS for IRQ GPIO1 straddle two 32-entry GPIO banks, so it is
cleaner to split out a function to process the interrupts for a single GPIO

This bug has only just been observed because GPIOs above 27 can only be
accessed on an old Raspberry Pi with the optional P5 header fitted, where
the pins are often used for I2S instead.
Although the GPIO controller can generate three interrupts (four counting
the common one), the device tree files currently only specify two. In the
absence of the third, simply don't register that interrupt (as opposed to
registering 0), which has the effect of making it impossible to generate
interrupts for GPIOs 46-53 which, since they share pins with the SD card
interface, is unlikely to be a problem.
The spi-bcm2835 driver automatically uses GPIO chip-selects due to
some unreliability of the native ones. In doing so it chooses the
same pins as the native chip-selects would use, but the existing
code always uses pins 7 and 8, wherever the SPI function is mapped.

Search the pinctrl group assigned to the driver for pins that
correspond to native chip-selects, and use those for GPIO chip-

Signed-off-by: Phil Elwell <>
The VideoCore bootloader passes in Serial number and
Revision number through Device Tree. Make these available to
userspace through /proc/cpuinfo.

Mainline status:

There is a commit in linux-next that standardize passing the serial
number through Device Tree (string: /serial-number):
ARM: 8355/1: arch: Show the serial number from devicetree in cpuinfo

There was an attempt to do the same with the revision number, but it
didn't get in:
[PATCH v2 1/2] arm: devtree: Set system_rev from DT revision

Signed-off-by: Noralf Trønnes <>
Code copied from spi-bcm2835. Get physical address from devicetree
instead of using hardcoded constant.

Signed-off-by: Matthias Reichl <>
Code ported from bcm2708-i2s driver in Raspberry Pi tree.

RPi commit 62c05a0 ("ASoC: BCM2708:
Add 24 bit support")

This adds 24 bit support to the I2S driver of the BCM2708.
Besides enabling the 24 bit flags, it includes two bug fixes:

MMAP is not supported. Claiming this leads to strange issues
when the format of driver and file do not match.

The datasheet states that the width extension bit should be set
for widths greater than 24, but greater or equal would be correct.
This follows from the definition of the width field.

Signed-off-by: Florian Meier <>

RPi commit 3e8c672 ("bcm2708-i2s:
Update bclk_ratio to more correct values")

Discussion about blck_ratio affecting sound quality:

Signed-off-by: Matthias Reichl <>
Code ported from bcm2708-i2s driver in Raspberry Pi tree.

RPi commit c14827e ("bcm2708: Allow
option card devices to be configured via DT")

Original work by Zoltan Szenczi, committed to RPi tree by
Phil Elwell.

Signed-off-by: Matthias Reichl <>
Code ported from bcm2708-i2s driver in Raspberry Pi tree.

RPi commit fd7d7a3 ("bcm2708:
Eliminate i2s debugfs directory error")

Qualify the two regmap ranges uses by bcm2708-i2s ('-i2s' and '-clk')
to avoid the name clash when registering debugfs entries.

Signed-off-by: Matthias Reichl <>
Code ported from bcm2708-i2s driver in Raspberry Pi tree.

RPi commit ba46b49 ("ASoC: Add
support for BCM2708")

This driver adds support for digital audio (I2S)
for the BCM2708 SoC that is used by the
Raspberry Pi. External audio codecs can be
connected to the Raspberry Pi via P5 header.

It relies on cyclic DMA engine support for BCM2708.

Signed-off-by: Florian Meier <>

Signed-off-by: Matthias Reichl <>
Code ported from bcm2708-i2s driver in Raspberry Pi tree.

RPi commit 7ee829f ("bcm2708-i2s:
Enable MMAP support via a DT property and overlay")

The i2s driver used to claim to support MMAP, but that feature was disabled
when some problems were found. Add the ability to enable this feature
through Device Tree, using the i2s-mmap overlay.

See: raspberrypi#1004

Signed-off-by: Matthias Reichl <>
Add slave transfer capability to BCM2835 dmaengine driver.
This patch is pulled from the bcm2708-dmaengine driver in the
Raspberry Pi repo. The work was done by Gellert Weisz.

Tested using the bcm2835-mmc driver from the same repo.

Signed-off-by: Noralf Trønnes <>
bcm2835-dma supports residue reporting at burst level but didn't report
this via the residue_granularity field.

Without this field set properly we get playback issues with I2S cards.

[by HiassofT, taken from bcm2708-dmaengine]
Signed-off-by: Noralf Trønnes <>
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NicoHood commented Jan 9, 2016

I updated the tutorial above now with a lot more details!

It works great, and with the patch @nmaas87 suggested everything works now at boot. Plug in your pi and it always connects with the same mac now.

@shrx I guess we dont. I just tried with the full install. Feel free to test the lite image. I guess it should work without any problems.

I guess its quite easy to use OTG now ;)
The power issue is still now solved though.
I also never tried to switch back to a non-zero pi. Not sure what will happen then with this kinda dev state.

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clivem commented Jan 9, 2016

@notro LOL. You're making me nostalgic! I remember how cool I thought it was when I plugged my first BeagleBone into a PC and got a serial console....... But the fact doesn't change, without dwc2 receiving some love, while it may be the answer for world peace in device mode, it leaves something to be desired, (that's the politest way of putting it), in host mode. wifi dongles suddenly disconnecting from USB bus is the tip of an iceberg. Heaven help you, (train crash leading to toxic chemical spill), trying to use it for USB audio! ;)

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notro commented Jan 9, 2016

I remember how cool I thought it was when I plugged my first BeagleBone into a PC and got a serial console

Me too, it was so easy to get started.

Your comment jogged my Solution-Finder-Unit and I realised that my goal might not be so far away after all.
config.txt is easily available for editing, so a user could add: dtoverlay=dwc2
Then the service could do:

if zero() and dwc2() and device_mode() then
    modprobe g_serial

A keyboard plugged in after boot will still work, and if this behaviour is not desirable, the service can just be disabled.

So here's project for me when I'm done banging my head agaist getting Pi2 mainline support working without issues (unless someone can't wait that long and finds a way to actually implement this).

Heaven help you, (train crash leading to toxic chemical spill), trying to use it for USB audio! ;)

My focus on an easily available serial console gave me tunnel vision, so I'm glad there's a review process :-)

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NicoHood commented Jan 9, 2016

Just a small question:
I cannot use ping without sudo on the g_ether module. Is this normal?

pi@raspberrypizero:~$ ping
ping: icmp open socket: Operation not permitted

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nmaas87 commented Jan 9, 2016

@NicoHood Not a problem because of g_ether I think. Test if the SUID bit is set correctly on your ping command, as described here:
On my Minibian Jessie - it worked right out of the box :)
Edit: It did work right out of the box (ping without root access, however, the SUID bit was not set on my PiZero (I have no idea how this could work, because network access is always a kind of root thing...). On my Ubuntu Box, however, the SUID bit is set on /bin/ping :).

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NicoHood commented Jan 14, 2016

I now also tried the usb gadget. It seems to work fine, I tried to install an OS into the emulated usb drive, but it failed after booting. But it was used on a slow "random" sd card, I will try this again on a better one.

The question is now, how to enable the gadgets by default on a zero, but disable them in a pi2? We currently have to edit the config.txt. Is there a way to add something like if inside those files, or can the be done with a kernel fix?

Also can we automatically enable the dwc2 module if g_ether is loaded for example? Would this work too? Then people do not have to type both of them in the /etc/modules.

I really would like to see this feature more flexible across different pi models. This is really awesome. And some of the responsible pi people should solve the power issue by providing a schematic, so we know if connecting a PSU in device mode hurts any device.

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NicoHood commented Jan 15, 2016

I just tried again (with a different sd card, from the beginning) and after applying:

echo "device_tree=bcm2708-rpi-zero.dtb" | sudo tee -a /boot/config.txt

The raspi stops booting and the rainbow screen only shows up. If i remove the line (with an sd card reader) it boots up again.

Using g_ether without those line does not work, but the line itself now also doesnt work anymore. So what has changed?

I first setup the device with a pi2, however the 4.4 kernel all boots fine on a zero, just the device tree setting makes it not boot up anymore.

I put in my old sd card and I noticed that bcm2708-rpi-zero.dtb is just missing on the boot partition. Maybe because I did the rpi-upgrade on the pi2, not the zero? I guess this is a bug?

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t3chguy commented Jan 15, 2016

@NicoHood no, the zero specific .dtb is no more as I found out. Just dtoverlay=dwc2 is enough after sudo BRANCH=next rpi-update, then either manually load one of the g_ modules or stick


inside /etc/modules

NicoHood referenced this pull request in Hexxeh/rpi-firmware Jan 15, 2016
firmware: Fix for h264 picture corruption with german tv
See: raspberrypi/firmware#499

firmware: TC358762: Avoid hanging when no DISPLAY_I2C_PORT defined in blob
See: raspberrypi/firmware#526

firmware: video_render: Ensure pixel_x/pixel_y don't overflow calculations
See: raspberrypi/firmware#525

firmware: arm_loader: Set uart0_clkrate before merging overlays
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notro commented Jan 16, 2016

What's the link to the Raspian github issue tracker? I can't find it.
I have a proposal on how to easily enable a usb console.

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lurch commented Jan 16, 2016

What's the link to the Raspian github issue tracker?

I guess is the nearest match?

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popcornmix commented Jan 16, 2016

Correct. For raspbian build of default debian packages it is:

For raspberry pi specific packages and init scripts (not from debian) then:

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notro commented Jan 16, 2016

Thanks. RPi-Distro/repo#17

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NicoHood commented Jan 16, 2016

@t3chguy thanks! I updated the tutorial above. echo "dtoverlay=dwc2" | sudo tee -a /boot/config.txt is now all you need.

Is there a chance to load this overlay by default or is there any disadvantage for this?

There is also another bug that /etc/modprobe.d/g_ether.conf is not recognized sometimes. I think its mostly after first loading g_ether. If you remove it and load it again it will be applied.

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lurch commented Jan 16, 2016

Is there a chance to load this overlay by default or is there any disadvantage for this?

See the messages from @clivem earlier in this discussion.

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NicoHood commented Jan 16, 2016

So its unstable for host mode on some devices and it will be kept like this?

Cant we load/unload those overlays at runtime? (Sorry i never did such kernel things before).
Is there any chance to see this somehow improved in the future or is this just the limitation we have?

@notro notro closed this Jan 27, 2016
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braian87b commented Mar 15, 2016

Hey, what about using with usbip client as host on Raspberry Pi Zero (as usbip server running in another rasbperry pi with a device connected), then wirelessly you can plug Raspberry Pi Zero on a computer and see the device if it where connected directly to the computer. Can this be done?

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lurch commented Mar 15, 2016

@braian87b I've read your comment several times, and I'm still not quite sure what you're asking for?

I think what you're suggesting is having a PiZero in gadget-mode connected over USB to a PC, and then having the PiZero communicating wirelessly with another Raspberry Pi, and then having a USB device you connect to this second Raspberry Pi "tunnelled" wirelessly to the PiZero, and then exposing it as a 'virtual' USB device to the PC which the PiZero is connected to?

However the PiZero only has one USB OTG port, which can operate in either host or device mode. So if you've got the Pi connected in device-mode to a PC, there's then no 'spare' USB host port into which to plug a Wifi dongle. There are ways of communicating with a wifi chip over the Pi's GPIO pins e.g. but that only runs at 1mbps, whereas USB2 runs at up to 480mbps (even USB1 is 12mbps).

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braian87b commented Mar 16, 2016

@lurch Right... sorry, I didn't notice... thanks!

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dtaht commented Jun 9, 2016

I was catching up on this discussion in light of some discussion elsewhere ( )

and I see my pi2 had in 4.4.12+/

for dwc2.ko

but not in my latest update (4.4.12-v7). Does the OTG stuff work on the pi2 or pi3?

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lurch commented Jun 9, 2016

Does the OTG stuff work on the pi2 or pi3?

No. The USB hub in the LAN9514 'gets in the way' and means that the single USB port in the SoC on those devices can only work in host (master) mode. It's only because the PiZero has a single USB port (i.e. no USB hub) that it's also able to (optionally) work in device (slave) mode.

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Dygear commented Mar 29, 2017

Anyone happen to have documentation on how to use the g_serial option. With only

  • Raspberry Pi Zero (W),
  • Formatted SD Card with Jessie Lite
  • A USB Cable.

I want to just connect it to my desktop computer and connect to it via ttyACM0: USB ACM device.

The CHIP does this out of the box, so I was hoping to get the same sort of interface out of the Pi Zero. Can someone point me in the right direction for that?

(I'm willing to be a guinea pig as I have 2 Pi Zero Ws and 1 Pi Zero - All of them I intend on using as headless servers. So never having to get a USB keyboard and Mini HDMI to HDMI connector is a bonus.)

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t3chguy commented Mar 29, 2017

@Dygear you can use this for reference

You can ignore rpi-update and run all those commands in a chroot of the mounted sd card (or even looped iso/img)

You will have to create an appropriate symlink so systemd starts the tty/getty, and ignore the interfaces related commands and replace g_ether with g_serial

I have notes on my laptop, I'll update when I get home

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bastijr commented Mar 15, 2019

I now need PI zero to implement a function: use USB Wire to connect PIzero and iPhone so that iPhone is the host, PIzero is the slave? I want to known if the funtion has been finished? Is there open source code?

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pelwell commented Mar 15, 2019

This is a question, not an issue. Please open a topic in the Raspberry Pi forums, stating what functionality the PiZero should provide - mass storage, etc.

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