netdev - Re: [PATCH v2 06/14] net: wwan: t7xx: Add AT and MBIM WWAN ports

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Date:   Tue, 11 Jan 2022 20:29:10 -0800
From:   "Martinez, Ricardo" <ricardo.martinez@...ux.intel.com>
To:     Sergey Ryazanov <ryazanov.s.a@...il.com>
Cc:     netdev@...r.kernel.org, linux-wireless@...r.kernel.org,
        Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        Johannes Berg <johannes@...solutions.net>,
        Loic Poulain <loic.poulain@...aro.org>,
        M Chetan Kumar <m.chetan.kumar@...el.com>,
        chandrashekar.devegowda@...el.com,
        Intel Corporation <linuxwwan@...el.com>,
        chiranjeevi.rapolu@...ux.intel.com, haijun.liu@...iatek.com,
        amir.hanania@...el.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        dinesh.sharma@...el.com, eliot.lee@...el.com,
        mika.westerberg@...ux.intel.com, moises.veleta@...el.com,
        pierre-louis.bossart@...el.com, muralidharan.sethuraman@...el.com,
        Soumya.Prakash.Mishra@...el.com, sreehari.kancharla@...el.com,
        suresh.nagaraj@...el.com
Subject: Re: [PATCH v2 06/14] net: wwan: t7xx: Add AT and MBIM WWAN ports

Hi Sergey,

On 12/6/2021 6:41 PM, Martinez, Ricardo wrote:
>
> On 12/1/2021 12:45 PM, Sergey Ryazanov wrote:
>> Hello Ricardo,
>>
>> On Wed, Dec 1, 2021 at 9:14 AM Martinez, Ricardo
>> <ricardo.martinez@...ux.intel.com> wrote:
>>> On 11/9/2021 4:06 AM, Sergey Ryazanov wrote:
>>>> On Mon, Nov 1, 2021 at 6:57 AM Ricardo Martinez wrote:
>>>>> ...
>>>>>    static struct t7xx_port md_ccci_ports[] = {
>>>>> +       {CCCI_UART2_TX, CCCI_UART2_RX, DATA_AT_CMD_Q, 
>>>>> DATA_AT_CMD_Q, 0xff,
>>>>> +        0xff, ID_CLDMA1, PORT_F_RX_CHAR_NODE, &wwan_sub_port_ops, 
>>>>> 0, "ttyC0", WWAN_PORT_AT},
>>>>> +       {CCCI_MBIM_TX, CCCI_MBIM_RX, 2, 2, 0, 0, ID_CLDMA1,
>>>>> +        PORT_F_RX_CHAR_NODE, &wwan_sub_port_ops, 10, "ttyCMBIM0", 
>>>>> WWAN_PORT_MBIM},
>>>>> ...
>>>>> +               if (count + CCCI_H_ELEN > txq_mtu &&
>>>>> +                   (port_ccci->tx_ch == CCCI_MBIM_TX ||
>>>>> +                    (port_ccci->tx_ch >= CCCI_DSS0_TX && 
>>>>> port_ccci->tx_ch <= CCCI_DSS7_TX)))
>>>>> +                       multi_packet = DIV_ROUND_UP(count, txq_mtu 
>>>>> - CCCI_H_ELEN);
>>>> I am just wondering, the chip does support MBIM message fragmentation,
>>>> but does not support AT commands stream (CCCI_UART2_TX) fragmentation.
>>>> Is that the correct conclusion from the code above?
>>> Yes, that is correct.
>> Are you sure that the modem does not support AT command fragmentation?
>> The AT commands interface is a stream of chars by its nature. It is
>> designed to work over serial lines. Some modem configuration software
>> even writes commands to a port in a char-by-char manner, i.e. it
>> writes no more than one char at a time to the port.
>>
>> The mechanism that is implemented in the driver to split user input
>> into individual messages is not a true fragmentation mechanism since
>> it does not preserve the original user input length. It just cuts the
>> user input into individual messages and sends them to the modem
>> independently. So, the modem firmware has no way to distinguish
>> whether the user input has been "fragmented" by the user or the
>> driver. How, then, does the modem firmware deal with an AT command
>> "fragmented" by a user? Will the modem firmware ignore the AT command
>> that is received in the char-by-char manner?
>
> The modem supports char-by-char AT command over serial lines, I'll get 
> more information
>
> about why splitting long commands is supported only for MBIM and not 
> for AT commands.
>
The next version will allow AT command fragmentation as there is modem 
is able to handle

fragments as you pointed out. This will make the WWAN Tx callback a bit 
simpler.