that's sure. thank you Dan.
And - what do you think guys if I try to implement something like a sysfs file 
to change dynamically the buffer size? Yes, it seems not so simple, but I might 
try, following th Bjorn example in the cdc_ncm driver.
I might try at some point.
Enrico.

On Wed, 18 Jun 2014, Dan Williams wrote:

==Date: Wed, 18 Jun 2014 16:03:17
==From: Dan Williams <dcbw-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
==To: Bjørn Mork <bjorn-yOkvZcmFvRU@public.gmane.org>
==Cc: netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-usb-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
==    Enrico Mioso <mrkiko.rs-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
==Subject: Re: [PATCH net,
==    stable] net: huawei_cdc_ncm: increase command buffer size
==
==On Wed, 2014-06-18 at 14:21 +0200, Bjørn Mork wrote:
==> Messages from the modem exceeding 256 bytes cause communication
==> failure.
==> 
==> The WDM protocol is strictly "read on demand", meaning that we only
==> poll for unread data after receiving a notification from the modem.
==> Since we have no way to know how much data the modem has to send,
==> we must make sure that the buffer we provide is "big enough".
==> Message truncation does not work. Truncated messages are left unread
==> until the modem has another message to send.  Which often won't
==> happen until the userspace application has given up waiting for the
==> final part of the last message, and therefore sends another command.
==> 
==> With a proper CDC WDM function there is a descriptor telling us
==> which buffer size the modem uses. But with this vendor specific
==> implementation there is no known way to calculate the exact "big
==> enough" number.  It is an unknown property of the modem firmware.
==> Experience has shown that 256 is too small.  The discussion of
==> this failure ended up concluding that 512 might be too small as
==> well. So 1024 seems like a reasonable value for now.
==> 
==> Fixes: 41c47d8cfd68 ("net: huawei_cdc_ncm: Introduce the huawei_cdc_ncm driver")
==> Cc: Enrico Mioso <mrkiko.rs-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
==> Reported-by: Dan Williams <dcbw-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
==> Signed-off-by: Bjørn Mork <bjorn-yOkvZcmFvRU@public.gmane.org>
==
==Tested-by: Dan Williams <dcbw-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
==
=='<CR><LF>^SYSCFGEX:
==("00","01","02","03","99"),((400380,"GSM900/GSM1800/WCDMA2100"),(6a80000,"GSM850/GSM1900/WCDMA850/AWS/WCDMA1900"),(3fffffff,"All bands")),(0-2),(0-4),((1081b,"LTE_B1/LTE_B2/LTE_B4/LTE_B5/LTE_B12/LTE_B17"),(7fffffffffffffff,"All bands"))<CR><LF><CR><LF>OK<CR><LF>'
==
==I get the last "<LF>" now :)
==
==> ---
==> 
==> The problem is a showstopper for anyone hitting it, so I believe this
==> fix should go into all maintained stable kernels with this driver.
==> That is anything based on v3.13 or newer.
==> 
==> Thanks,
==> Bjørn
==> 
==> 
==>  drivers/net/usb/huawei_cdc_ncm.c | 7 ++++---
==>  1 file changed, 4 insertions(+), 3 deletions(-)
==> 
==> diff --git a/drivers/net/usb/huawei_cdc_ncm.c b/drivers/net/usb/huawei_cdc_ncm.c
==> index f9822bc75425..5d95a13dbe2a 100644
==> --- a/drivers/net/usb/huawei_cdc_ncm.c
==> +++ b/drivers/net/usb/huawei_cdc_ncm.c
==> @@ -84,12 +84,13 @@ static int huawei_cdc_ncm_bind(struct usbnet *usbnet_dev,
==>  	ctx = drvstate->ctx;
==>  
==>  	if (usbnet_dev->status)
==> -		/* CDC-WMC r1.1 requires wMaxCommand to be "at least 256
==> -		 * decimal (0x100)"
==> +		/* The wMaxCommand buffer must be big enough to hold
==> +		 * any message from the modem. Experience has shown
==> +		 * that some replies are more than 256 bytes long
==>  		 */
==>  		subdriver = usb_cdc_wdm_register(ctx->control,
==>  						 &usbnet_dev->status->desc,
==> -						 256, /* wMaxCommand */
==> +						 1024, /* wMaxCommand */
==>  						 huawei_cdc_ncm_wdm_manage_power);
==>  	if (IS_ERR(subdriver)) {
==>  		ret = PTR_ERR(subdriver);
==
==
==