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Using PAPPL on Embedded Linux

PAPPL has been developed for use on embedded Linux as well as traditional desktop/server environments. Beyond the reduced resource usage, PAPPL also supports providing USB access to a printer via the Linux USB gadget driver.

RaspbianOS

The Raspberry Pi Zero, Zero2, 4B, and 5 single board computers all support USB gadget mode. To develop using the stock RaspbianOS distribution you'll need to make some configuration changes:

  1. In "/boot/config.txt", comment out the line reading otg_mode=1 and add a line at the end reading dtoverlay=dwc2.
  2. In "/boot/cmdline.txt", add modules-load=dwc2,libcomposite between the rootwait and quiet options.
  3. Run systemctl disable usb-gadget.target to disable systemd's incomplete, incompatible, and barely documented USB gadget support.
  4. Reboot.

I also find it useful to enable ssh by creating an empty file named "/boot/ssh".

To try gadget mode with the PAPPL test suite, connect your computer to the Raspberry Pi's USB Micro B data port (Zero/Zero2) or USB-C data/power port, build the PAPPL software, and run the following command:

sudo testsuite/testpappl -U -c -1 -L debug -l -

The -U option enables USB gadget mode. Once the program is running, your computer will see a composite USB device that offers a legacy USB printer interface (7-1-2) and three IPP-USB (7-1-4) interfaces.

You can enable additional gadgets with various --usb-xxx options:

  • --usb-ethernet: Enable an Ethernet gadget.
  • --usb-product-id PRODUCT-ID: Set the USB product ID - default is 0x8011.
  • --usb-readonly DISK-IMAGE: Enable the storage gadget with a read-only disk image.
  • --usb-removable DISK-IMAGE: Enable the storage gadget with a read-write and removable disk image.
  • --usb-serial: Enable a serial gadget.
  • --usb-storage DISK-IMAGE: Enable the storage gadget with a read-write disk image.
  • --usb-vendor-id VENDOR-ID: Set the USB vendor ID - default is 0x1209.

Yocto Recipe

The recipes-pappl project provides a Yocto recipe for the current stable release of PAPPL. You can add it to your meta layer with:

git submodule add https://github.com/michaelrsweet/recipes-pappl.git

USB Printer Gadget Kernel Patch

Until Linux kernel 5.12, there is a bug in the USB printer gadget functional driver (the one that uses configfs) that can prevent the gadget from working due to a lack of buffers with the default configuration. Since udev is so helpful in keeping kernel drivers loaded, it can become impossible to unload the f_printer module in order to get a new configuration of the q_len parameter applied.

The following kernel patch applies cleanly to 4.19 through 5.11, was submitted to (and approved by) the Linux USB kernel developers, and is included in Linux kernel 5.12 and later. You'll need to use this patch if you want to use PAPPL to provide a USB printer interface to your project with an version of Linux kernel prior to 5.12.

diff --git a/drivers/usb/gadget/function/f_printer.c b/drivers/usb/gadget/function/f_printer.c
index 9c7ed2539ff7..4f3161005e4f 100644
--- a/drivers/usb/gadget/function/f_printer.c
+++ b/drivers/usb/gadget/function/f_printer.c
@@ -50,6 +50,8 @@
#define GET_PORT_STATUS		1
#define SOFT_RESET		2

+#define DEFAULT_Q_LEN		10 /* same as legacy g_printer gadget */
+
static int major, minors;
static struct class *usb_gadget_class;
static DEFINE_IDA(printer_ida);
@@ -1317,6 +1319,9 @@ static struct usb_function_instance *gprinter_alloc_inst(void)
	opts->func_inst.free_func_inst = gprinter_free_inst;
	ret = &opts->func_inst;

+	/* Make sure q_len is initialized, otherwise the bound device can't support read/write! */
+	opts->q_len = DEFAULT_Q_LEN;
+
	mutex_lock(&printer_ida_lock);

	if (ida_is_empty(&printer_ida)) {