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Date:   Tue, 10 Mar 2020 19:24:01 +0500
From:   Александр Берсенев 
        <bay@...kerdom.ru>
To:     linux-usb@...r.kernel.org
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cdc_ncm: Implement the 32-bit version of NCM Transfer Block

пн, 9 мар. 2020 г. в 15:17, Oliver Neukum <oneukum@...e.com>:
>
> Am Freitag, den 06.03.2020, 01:33 +0500 schrieb Alexander Bersenev:
> > The NCM specification defines two formats of transfer blocks: with 16-bit
> > fields (NTB-16) and with 32-bit fields (NTB-32). Currently only NTB-16 is
> > implemented.
> >
> > This patch adds the support of NTB-32. The motivation behind this is that
> > some devices such as E5785 or E5885 from the current generation of Huawei
> > LTE routers do not support NTB-16. The previous generations of Huawei
> > devices are also use NTB-32 by default.
> >
> > Also this patch enables NTB-32 by default for Huawei devices
>
> Hi,
>
> do you really see no other option but to make the choice with yet
> anothet flag? The rest of the code looks good to me.
>

Hi,

The reason of yet another flag is that some Huawei devices, E5785 and E5885,
are incorrectly reporting that they support NTB-16. In fact they support only
NTB-32.

Historically the Huawei devices used NTB-32 by default and there
was a flag CDC_NCM_FLAG_RESET_NTB16 to work around the bug that
some Huawei E3372H devices come out of reset in NTB-32 mode even if
NTB-16 mode was set. This commit removes the
CDC_NCM_FLAG_RESET_NTB16 flag, that was specific to Huawei devices
and introduces the CDC_NCM_FLAG_PREFER_NTB32 flag.

The NTB-16 has lower, protocol overhead, but NTB-32 allows to transfer more
data per transfer block, up to 4GB, supporting both High Speed and
SuperSpeed data rates. So NTB-32 can be faster on devices with big buffers
and slower on devices with small buffers.

Anyway, for 4g modem devices there should not be much difference between
NDP-16 and NDP-32 because the 4g speeds are lower than the USB speed.
But also there may be the devices, that, vice versa,
buggy with NTB-32 and work well with NTB-16.

So having a flag to choose the preferred implementation is probably the best
option - it allows to keep older device to work as before, but if it
is found out that
the device works with NTB-32 better, the flag can be enabled for that device
or vendor.

Best,
Alexander Bersenev

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