Re: [PATCH] cdc_ncm: Implement the 32-bit version of NCM Transfer Block

пн, 9 мар. 2020 г. в 15:17, Oliver Neukum [off-list ref]:

Am Freitag, den 06.03.2020, 01:33 +0500 schrieb Alexander Bersenev:

quoted

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