Re: [PATCH] usb: gadget: Add the console support for usb-to-serial port

[Peter Hurley]

On 11/18/2015 09:35 PM, Baolin Wang wrote:
> On 18 November 2015 at 23:32, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
>> Hi Baolin,
>>
>> On 11/16/2015 02:05 AM, Baolin Wang wrote:
>>> It dose not work when we want to use the usb-to-serial port based
>>> on one usb gadget as a console. Thus this patch adds the console
>>> initialization to support this request.
>>
>> I have some high level concerns.
>>
>> 1. I would defer registering the console until the port has at least been
>>    allocated in gserial_alloc_line(), and unregister when the line is freed.
>>    That also reduces many of the conditions that you shouldn't need to
>>    check, like port number range and so on.
> 
> The 'setup' callback of 'struct console' is just do some memory
> allocation and member initialization, that no need to defer
> registering the console in gserial_alloc_line(). But the
> 'gs_console_connect()' which will use the port need to be called in
> gserial_connect().

My point here was why are you registering the console before the port
table is even allocated and initialized?  The console can't possibly
perform i/o that early because the port doesn't even exist.
Which is why I suggested waiting until gserial_alloc_line() to
register the console.

A typical console setup() performs the cross-reference linking between
the console data structure and the port table. The reason for that
is the console needs to be cleaned up if the port is being torn down.

For example, in gserial_disconnect() the port->port_usb is reset to NULL,
and later in gserial_console_exit():

	if (port && port->port_usb) {
		....
		gs_request_free(req, ep);
	}

which means your leaking the request allocation when the port has been
disconnected.


>>    Further, consider deferring the console registration until gserial_connect();
>>    that would further simplify things. In this case, unregistration would
>>    happen at disconnect.
> 
> That sounds reasonable. I would try.
> 
>>
>> 2. Why are you using a kworker for actual device i/o when all of the i/o
>>    is done with interrupts disabled anyway?
>>    Getting rid of the work would eliminate using the 8K intermediate buffer
>>    as well.
> 
> If remove the kworker, there are some deadlocks to call the printk()
> when in the process of transferring data with usb endpoint. So we need
> a async kworker to complete the io or it can not work.

The commit log should detail the major design choices, including why you
used the kworker (because of re-entrancy issues with usb endpoint).



>> 3. If for some reason i/o deferral is really necessary, consider using a kthread
>>    kworker instead of the pooled kworker. The scheduler response will be _way_
>>    better.
>>
> 
> OK, make sense.
> 
>> 4. If for some reason i/o deferral is really necessary, use a circular buffer
>>    for the intermediate buffer, preferably lockless since there is only
>>    one producer and one consumer.
>>
> 
> Yeah, the circular buffer is better but more tricky. I would try.
> 
>> Some other review comments below; please ignore anything other reviewers
>> have already noted.
>>
>> Regards,
>> Peter Hurley
>>
>>> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxx>
>>> ---
>>>  drivers/usb/gadget/Kconfig             |    6 +
>>>  drivers/usb/gadget/function/u_serial.c |  238 ++++++++++++++++++++++++++++++++
>>>  2 files changed, 244 insertions(+)
>>>
>>> diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig
>>> index 33834aa..be5aab9 100644
>>> --- a/drivers/usb/gadget/Kconfig
>>> +++ b/drivers/usb/gadget/Kconfig
>>> @@ -127,6 +127,12 @@ config USB_GADGET_STORAGE_NUM_BUFFERS
>>>          a module parameter as well.
>>>          If unsure, say 2.
>>>
>>> +config U_SERIAL_CONSOLE
>>> +     bool "Serial gadget console support"
>>> +     depends on USB_G_SERIAL
>>> +     help
>>> +        It supports the serial gadget can be used as a console.
>>> +
>>>  source "drivers/usb/gadget/udc/Kconfig"
>>>
>>>  #
>>> diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
>>> index f7771d8..4ade527 100644
>>> --- a/drivers/usb/gadget/function/u_serial.c
>>> +++ b/drivers/usb/gadget/function/u_serial.c
>>> @@ -27,6 +27,7 @@
>>>  #include <linux/slab.h>
>>>  #include <linux/export.h>
>>>  #include <linux/module.h>
>>> +#include <linux/console.h>
>>>
>>>  #include "u_serial.h"
>>>
>>> @@ -79,6 +80,16 @@
>>>   */
>>>  #define QUEUE_SIZE           16
>>>  #define WRITE_BUF_SIZE               8192            /* TX only */
>>> +#define GS_BUFFER_SIZE               (4096)
>>> +#define GS_CONSOLE_BUF_SIZE  (2 * GS_BUFFER_SIZE)
>>> +
>>> +struct gscons_info {
>>> +     struct gs_port          *port;
>>> +     struct tty_driver       *tty_driver;
>>> +     struct work_struct      work;
>>> +     int                     buf_tail;
>>> +     char                    buf[GS_CONSOLE_BUF_SIZE];
>>> +};
>>
>> Make struct gscons_info a static declaration instead of
>> allocating it.
> 
> But I think the dynamic allocation is more reasonable with reducing
> some global variables.

But introduces a failure mode that doesn't exist with the
static definition.


>>>  /* circular buffer */
>>>  struct gs_buf {
>>> @@ -118,6 +129,7 @@ struct gs_port {
>>>
>>>       /* REVISIT this state ... */
>>>       struct usb_cdc_line_coding port_line_coding;    /* 8-N-1 etc */
>>> +     struct usb_request      *console_req;
>>>  };
>>>
>>>  static struct portmaster {
>>> @@ -1054,6 +1066,7 @@ gs_port_alloc(unsigned port_num, struct usb_cdc_line_coding *coding)
>>>
>>>       port->port_num = port_num;
>>>       port->port_line_coding = *coding;
>>> +     port->console_req = NULL;
>>>
>>>       ports[port_num].port = port;
>>>  out:
>>> @@ -1143,6 +1156,227 @@ err:
>>>  }
>>>  EXPORT_SYMBOL_GPL(gserial_alloc_line);
>>>
>>> +#ifdef CONFIG_U_SERIAL_CONSOLE
>>> +
>>> +static struct usb_request *gs_request_new(struct usb_ep *ep, int buffer_size)
>>                                                                 ^^^^^^^^^^^^^^^
>> With only a single caller that uses a symbolic constant, is the
>> 'buffer_size' parameter necessary?
>>
> 
> Yeah, I'll remove the 'buffer_size' parameter.
> 
>>
>>> +{
>>> +     struct usb_request *req = usb_ep_alloc_request(ep, GFP_ATOMIC);
>>> +
>>
>> Remove this newline.
> 
> OK.
> 
>>
>>> +     if (!req)
>>> +             return NULL;
>>> +
>>> +     /* now allocate buffers for the requests */
>>
>> Unnecessary comment.
> 
> OK.
> 
>>
>>> +     req->buf = kmalloc(buffer_size, GFP_ATOMIC);
>>> +     if (!req->buf) {
>>> +             usb_ep_free_request(ep, req);
>>> +             return NULL;
>>> +     }
>>> +
>>> +     return req;
>>> +}
>>> +
>>> +static void gs_request_free(struct usb_request *req, struct usb_ep *ep)
>>> +{
>>> +     if (req) {
>>> +             kfree(req->buf);
>>> +             usb_ep_free_request(ep, req);
>>> +     }
>>> +}
>>> +
>>> +static void gs_complete_out(struct usb_ep *ep, struct usb_request *req)
>>> +{
>>> +     if (req->status != 0 && req->status != -ECONNRESET)
>>> +             return;
>>> +}
>>> +
>>> +static struct console gserial_cons;
>>> +static int gs_console_connect(void)
>>
>> Pass the console * as parameter, instead of forward declaring the console.
>> Or initialize info directly from the static struct gscons_info address.
> 
> Make sense.
> 
>>
>>> +{
>>> +     struct gscons_info *info = gserial_cons.data;
>>> +     int port_num = gserial_cons.index;
>>> +     struct usb_request *req;
>>> +     struct gs_port *port;
>>> +     struct usb_ep *ep;
>>> +
>>> +     if (port_num >= MAX_U_SERIAL_PORTS || port_num < 0) {
>>> +             pr_err("%s: port num [%d] exceeds the range.\n",
>>> +                    __func__, port_num);
>>> +             return -ENXIO;
>>> +     }
>>> +
>>> +     port = ports[port_num].port;
>>> +     if (!port) {
>>> +             pr_err("%s: serial line [%d] not allocated.\n",
>>> +                    __func__, port_num);
>>> +             return -ENODEV;
>>> +     }
>>> +
>>> +     if (!port->port_usb) {
>>> +             pr_err("%s: no port usb.\n", __func__);
>>> +             return -ENODEV;
>>> +     }
>>> +
>>> +     ep = port->port_usb->in;
>>> +     if (!ep) {
>>> +             pr_err("%s: no usb endpoint.\n", __func__);
>>> +             return -ENXIO;
>>> +     }
>>
>> Looking at the caller, gserial_connect(), none of the error
>> conditions above look possible.
> 
> That's right. I'll remove these checks.
> 
>>
>>
>>> +
>>> +     req = port->console_req;
>>> +     if (!req) {
>>> +             req = gs_request_new(ep, GS_BUFFER_SIZE);
>>
>> Where is port->console_req assigned to?
> 
> The connect may be called several times, if the req is allocated at
> one time, there is no need to assign it.

You've missed my point: where is

		port->console_req = ?????


>>> +             if (!req) {
>>> +                     pr_err("%s: request fail.\n", __func__);
>>
>> Remove redundant error message; the allocator has already done so.
> 
> OK.
> 
>>
>>> +                     return -ENOMEM;
>>> +             }
>>> +             req->complete = gs_complete_out;
>>> +     }
>>> +
>>> +     info->port = port;
>>> +
>>> +     pr_debug("%s: port[%d] console connect!\n", __func__, port_num);
>>> +     return 0;
>>> +}
>>> +
>>> +static void gs_console_work(struct work_struct *work)
>>> +{
>>> +     struct gscons_info *info = container_of(work, struct gscons_info, work);
>>> +     struct gs_port *port = info->port;
>>> +     struct usb_request *req;
>>> +     struct usb_ep *ep;
>>> +     int xfer, ret, count;
>>> +     char *p;
>>> +
>>> +     if (!port || !port->port_usb)
>>> +             return;
>>> +
>>> +     req = port->console_req;
>>> +     ep = port->port_usb->in;
>>> +     if (!req || !ep)
>>> +             return;
>>> +
>>> +     spin_lock_irq(&port->port_lock);
>>> +     count = info->buf_tail;
>>> +     p = info->buf;
>>> +
>>> +     while (count > 0 && !port->write_busy) {
>>> +             if (count > GS_BUFFER_SIZE)
>>> +                     xfer = GS_BUFFER_SIZE;
>>> +             else
>>> +                     xfer = count;
>>> +
>>> +             memcpy(req->buf, p, xfer);
>>> +             req->length = xfer;
>>> +
>>> +             port->write_busy = true;
>>> +             spin_unlock(&port->port_lock);
>>> +             ret = usb_ep_queue(ep, req, GFP_ATOMIC);
>>> +             spin_lock(&port->port_lock);
>>> +             port->write_busy = false;
>>> +             if (ret < 0)
>>> +                     break;
>>> +
>>> +             p += xfer;
>>> +             count -= xfer;
>>> +     }
>>> +
>>> +     info->buf_tail -= count;

I'm not seeing how info->buf_tail is ever reset to 0.

	count = info->buf_tail

	while (count > 0) {
		....
		count -= xfer;
	}

At loop exit count == 0, so

	info->buf_tail -= count;

never decreases buf_tail?


>>> +     spin_unlock_irq(&port->port_lock);
>>> +}
>>> +
>>> +static int gs_console_setup(struct console *co, char *options)
>>> +{
>>> +     struct gscons_info *gscons_info;
>>> +
>>> +     gscons_info = kzalloc(sizeof(struct gscons_info), GFP_KERNEL);
>>> +     if (!gscons_info)
>>> +             return -ENOMEM;
>>> +
>>> +     gscons_info->port = NULL;
>>
>> Redundant.
> 
> Will remove it.
> 
>>
>>> +     gscons_info->tty_driver = gs_tty_driver;
>>> +     INIT_WORK(&gscons_info->work, gs_console_work);
>>> +     gscons_info->buf_tail = 0;
>>
>> Redundant.
> 
> Will remove it.
> 
>>
>>> +     co->data = gscons_info;
>>> +
>>> +     return 0;
>>> +}
>>> +
>>> +static void gs_console_write(struct console *co,
>>> +                          const char *buf, unsigned count)
>>> +{
>>> +     struct gscons_info *info = co->data;
>>> +     int avail, xfer;
>>> +     char *p;
>>> +
>>> +     avail = GS_CONSOLE_BUF_SIZE - info->buf_tail;
>>> +     if (count > avail)
>>> +             xfer = avail;
>>> +     else
>>> +             xfer = count;
>>
>>         xfer = min(count, GS_CONSOLE_BUF_SIZE - info->buf_tail);
> 
> Yeah, that's right.
> 
>>
>>> +
>>> +     p = &info->buf[info->buf_tail];
>>> +     memcpy(p, buf, xfer);
>>> +     info->buf_tail += xfer;
>>
>> What is preventing concurrently running work and this from
>> using/modifying the info->buf and info->buf_tail simultaneously?
> 
> Like I said above, it will meet deadlocks if running the work
> directly, then introduce the kworker.

You've missed my point here:

CPU 0                                 CPU 1
--------------------------------      -------------------------------
gs_console_write()                    gs_console_work()

   info->buf_tail += xfer                info->buf_tail -= count;


Neither of these operations are atomic so what will the value of
info->buf_tail be? For example:

   A <= LOAD(info->buf_tail)
                                         B <= LOAD(info->buf_tail)
   A <= A + xfer                         B <= B - count
   STORE(A, info->buf_tail)
                                         STORE(B, info->buf_tail)

The result is as if info->buf_tail += xfer never happened.


>>> +
>>> +     schedule_work(&info->work);
>>> +}
>>> +
>>> +static struct tty_driver *gs_console_device(struct console *co, int *index)
>>> +{
>>> +     struct gscons_info *info = co->data;
>>> +
>>> +     *index = co->index;
>>> +     return info->tty_driver;
>>> +}
>>> +
>>> +static struct console gserial_cons = {
>>> +     .name =         "ttyGS",
>>> +     .write =        gs_console_write,
>>> +     .device =       gs_console_device,
>>> +     .setup =        gs_console_setup,
>>> +     .flags =        CON_PRINTBUFFER,
>>> +     .index =        -1,
>>> +};
>>> +
>>> +static void gserial_console_init(void)
>>> +{
>>> +     register_console(&gserial_cons);
>>> +}
>>> +
>>> +static void gserial_console_exit(void)
>>> +{
>>> +     struct gscons_info *info = gserial_cons.data;
>>> +     struct gs_port *port = info->port;
>>> +     struct usb_request *req;
>>> +     struct usb_ep *ep;
>>> +
>>> +     if (port && port->port_usb) {
>>> +             req = port->console_req;
>>> +             ep = port->port_usb->in;
>>> +             gs_request_free(req, ep);
>>> +     }
>>> +
>>> +     kfree(info);
>>> +     unregister_console(&gserial_cons);
>>> +}
>>> +
>>> +#else
>>> +
>>> +static int gs_console_connect(void)
>>> +{
>>> +     return 0;
>>> +}
>>> +
>>> +static void gserial_console_init(void)
>>> +{
>>> +}
>>> +
>>> +static void gserial_console_exit(void)
>>> +{
>>> +}
>>> +
>>> +#endif
>>> +
>>>  /**
>>>   * gserial_connect - notify TTY I/O glue that USB link is active
>>>   * @gser: the function, set up with endpoints and descriptors
>>> @@ -1219,6 +1453,7 @@ int gserial_connect(struct gserial *gser, u8 port_num)
>>>                       gser->disconnect(gser);
>>>       }
>>>
>>> +     status = gs_console_connect();
>>>       spin_unlock_irqrestore(&port->port_lock, flags);
>>>
>>>       return status;
>>> @@ -1320,6 +1555,8 @@ static int userial_init(void)
>>>               goto fail;
>>>       }
>>>
>>> +     gserial_console_init();
>>> +
>>>       pr_debug("%s: registered %d ttyGS* device%s\n", __func__,
>>>                       MAX_U_SERIAL_PORTS,
>>>                       (MAX_U_SERIAL_PORTS == 1) ? "" : "s");
>>> @@ -1334,6 +1571,7 @@ module_init(userial_init);
>>>
>>>  static void userial_cleanup(void)
>>>  {
>>> +     gserial_console_exit();
>>>       tty_unregister_driver(gs_tty_driver);
>>>       put_tty_driver(gs_tty_driver);
>>>       gs_tty_driver = NULL;
>>>
>>
> 
> 
> 

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