Re: [RFC] raise the maximum number of usb-serial devices to 512

[Johan Hovold <jhovold@gmail.com>]

On Mon, Jun 03, 2013 at 07:49:59PM -0700, Greg KH wrote:
> On Mon, May 27, 2013 at 02:28:51PM +0200, Bjørn Mork wrote:
> > But, IMHO, a nicer approach would be to make the allocation completely
> > dynamic, using e.g. the idr subsystem. Static tables are always feel
> > like straight jackets to me, no matter how big they are :)
> 
> You are right, I didn't change the code to use idr (it predates idr by
> about a decade or so), because I thought we needed the "rage" logic that
> the usb-serial minor reservation does.
> 
> But I'm not so sure anymore, so here's a patch to change to use the idr
> code, and should remove all minor number limitations (well 65k is the
> limit the tty core should be setting I think.)
> 
> Tobias, can you test this patch out?  Note, I only compiled it, did not
> get the chance to actually run it, so it might not work at all.

I'm afraid this won't work in it's current form. Several drivers and
parts of usb-serial core still depend on the minor numbers being
consecutive for multi-port devices. There are also still references to
SERIAL_TTY_NO_MINOR (255) as well as SERIAL_TTY_MINORS that need to be
addressed.

> 
> thanks,
> 
> greg k-h
> 
> Subject: [PATCH] usb: serial: remove minor number limitation of 255
> 
> 
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
>  drivers/usb/serial/usb-serial.c |   86 +++++++++++++++++-----------------------
>  1 file changed, 38 insertions(+), 48 deletions(-)
> 
> diff --git a/drivers/usb/serial/usb-serial.c b/drivers/usb/serial/usb-serial.c
> index 4753c00..74b6f08 100644
> --- a/drivers/usb/serial/usb-serial.c
> +++ b/drivers/usb/serial/usb-serial.c
> @@ -37,6 +37,7 @@
>  #include <linux/usb.h>
>  #include <linux/usb/serial.h>
>  #include <linux/kfifo.h>
> +#include <linux/idr.h>
>  #include "pl2303.h"
>  
>  #define DRIVER_AUTHOR "Greg Kroah-Hartman <gregkh@linuxfoundation.org>"
> @@ -49,7 +50,7 @@
>     drivers depend on it.
>  */
>  
> -static struct usb_serial *serial_table[SERIAL_TTY_MINORS];
> +static DEFINE_IDR(serial_minors);
>  static DEFINE_MUTEX(table_lock);
>  static LIST_HEAD(usb_serial_driver_list);
>  
> @@ -61,59 +62,52 @@ static LIST_HEAD(usb_serial_driver_list);
>  struct usb_serial *usb_serial_get_by_index(unsigned index)
>  {
>  	struct usb_serial *serial;
> +	struct usb_serial_port *port;
>  
>  	mutex_lock(&table_lock);
> -	serial = serial_table[index];
> -
> -	if (serial) {
> -		mutex_lock(&serial->disc_mutex);
> -		if (serial->disconnected) {
> -			mutex_unlock(&serial->disc_mutex);
> -			serial = NULL;
> -		} else {
> -			kref_get(&serial->kref);
> -		}
> -	}
> +	port = idr_find(&serial_minors, index);
>  	mutex_unlock(&table_lock);
> +	if (!port)
> +		return NULL;
> +
> +	serial = port->serial;
> +	kref_get(&serial->kref);
>  	return serial;
>  }

We would still need to handle disconnect, and make sure to return with
the disc_mutex held unless disconnected.

>  
> -static struct usb_serial *get_free_serial(struct usb_serial *serial,
> -					int num_ports, unsigned int *minor)
> +static int get_free_port(struct usb_serial_port *port)
>  {
> -	unsigned int i, j;
> -	int good_spot;
> -
> -	dev_dbg(&serial->interface->dev, "%s %d\n", __func__, num_ports);
> +	int i;
>  
> -	*minor = 0;
>  	mutex_lock(&table_lock);
> -	for (i = 0; i < SERIAL_TTY_MINORS; ++i) {
> -		if (serial_table[i])
> -			continue;
> +	i = idr_alloc(&serial_minors, port, 0, 0, GFP_KERNEL);
> +	if (i < 0)

Missing mutex unlock (as already Dave noted).

> +		return -EEXIST;
> +	port->number = i;
> +	mutex_unlock(&table_lock);
> +	return i;
> +}
>  
> -		good_spot = 1;
> -		for (j = 1; j <= num_ports-1; ++j)
> -			if ((i+j >= SERIAL_TTY_MINORS) || (serial_table[i+j])) {
> -				good_spot = 0;
> -				i += j;
> -				break;
> -			}
> -		if (good_spot == 0)
> -			continue;
> +static int get_free_serial(struct usb_serial *serial, int num_ports,
> +			   unsigned int *minor)
> +{
> +	unsigned int i;
> +	unsigned int x;
>  
> -		*minor = i;
> -		j = 0;
> -		dev_dbg(&serial->interface->dev, "%s - minor base = %d\n", __func__, *minor);
> -		for (i = *minor; (i < (*minor + num_ports)) && (i < SERIAL_TTY_MINORS); ++i) {
> -			serial_table[i] = serial;
> -			serial->port[j++]->number = i;
> -		}
> -		mutex_unlock(&table_lock);
> -		return serial;
> +	dev_dbg(&serial->interface->dev, "%s %d\n", __func__, num_ports);
> +
> +	*minor = 0xffffffff;
> +	for (i = 0; i < num_ports; ++i) {
> +		x = get_free_port(serial->port[i]);
> +		if (x < 0)
> +			goto error;
> +		if (*minor == 0xffffffff)
> +			*minor = x;
>  	}
> -	mutex_unlock(&table_lock);
> -	return NULL;
> +	return 0;
> +error:
> +	// FIXME unwind the already allocated minors
> +	return -ENODEV;
>  }

As mentioned above, usb-serial core and several drivers currently depend
on us returning the first minor in a consecutive range. It's mostly used
to determine the per device port index, so storing that index in the port
structure could possibly be sufficient.

>  
>  static void return_serial(struct usb_serial *serial)
> @@ -122,7 +116,7 @@ static void return_serial(struct usb_serial *serial)
>  
>  	mutex_lock(&table_lock);
>  	for (i = 0; i < serial->num_ports; ++i)
> -		serial_table[serial->minor + i] = NULL;
> +		idr_remove(&serial_minors, serial->port[i]->number);
>  	mutex_unlock(&table_lock);
>  }
>  
> @@ -1041,7 +1035,7 @@ static int usb_serial_probe(struct usb_interface *interface,
>  	 */
>  	serial->disconnected = 1;
>  
> -	if (get_free_serial(serial, num_ports, &minor) == NULL) {
> +	if (get_free_serial(serial, num_ports, &minor)) {
>  		dev_err(ddev, "No more free serial devices\n");
>  		goto probe_error;
>  	}
> @@ -1225,7 +1219,6 @@ static struct usb_driver usb_serial_driver = {
>  
>  static int __init usb_serial_init(void)
>  {
> -	int i;
>  	int result;
>  
>  	usb_serial_tty_driver = alloc_tty_driver(SERIAL_TTY_MINORS);
> @@ -1233,9 +1226,6 @@ static int __init usb_serial_init(void)
>  		return -ENOMEM;
>  
>  	/* Initialize our global data */

You could remove the above comment as well.

> -	for (i = 0; i < SERIAL_TTY_MINORS; ++i)
> -		serial_table[i] = NULL;
> -
>  	result = bus_register(&usb_serial_bus_type);
>  	if (result) {
>  		pr_err("%s - registering bus driver failed\n", __func__);

Thanks,
Johan