EMBEDDED.md

Using PAPPL on Embedded Linux

PAPPL has been developed for use on embedded Linux as well as traditional desktop/server environments.

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)) {