netdev - Re: [PATCH net-next v5 08/13] net: wwan: t7xx: Add data path interface

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Date:   Mon, 4 Apr 2022 17:10:03 -0700
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 (刘海军) 
        <haijun.liu@...iatek.com>, amir.hanania@...el.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        dinesh.sharma@...el.com, eliot.lee@...el.com,
        ilpo.johannes.jarvinen@...el.com, moises.veleta@...el.com,
        pierre-louis.bossart@...el.com, muralidharan.sethuraman@...el.com,
        Soumya.Prakash.Mishra@...el.com, sreehari.kancharla@...el.com,
        madhusmita.sahu@...el.com
Subject: Re: [PATCH net-next v5 08/13] net: wwan: t7xx: Add data path
 interface


On 4/4/2022 4:50 PM, Sergey Ryazanov wrote:
> Hello Ricardo,
>
> On Tue, Apr 5, 2022 at 2:29 AM Martinez, Ricardo
> <ricardo.martinez@...ux.intel.com> wrote:
>> On 3/6/2022 6:58 PM, Sergey Ryazanov wrote:
>>> On Thu, Feb 24, 2022 at 1:35 AM Ricardo Martinez
>>> <ricardo.martinez@...ux.intel.com> wrote:
>>>> From: Haijun Liu <haijun.liu@...iatek.com>
>>>>
>>>> Data Path Modem AP Interface (DPMAIF) HIF layer provides methods
>>>> for initialization, ISR, control and event handling of TX/RX flows.
>>>>
>>>> DPMAIF TX
>>>> Exposes the `dmpaif_tx_send_skb` function which can be used by the
>>>> network device to transmit packets.
>>>> The uplink data management uses a Descriptor Ring Buffer (DRB).
>>>> First DRB entry is a message type that will be followed by 1 or more
>>>> normal DRB entries. Message type DRB will hold the skb information
>>>> and each normal DRB entry holds a pointer to the skb payload.
>>>>
>>>> DPMAIF RX
>>>> The downlink buffer management uses Buffer Address Table (BAT) and
>>>> Packet Information Table (PIT) rings.
>>>> The BAT ring holds the address of skb data buffer for the HW to use,
>>>> while the PIT contains metadata about a whole network packet including
>>>> a reference to the BAT entry holding the data buffer address.
>>>> The driver reads the PIT and BAT entries written by the modem, when
>>>> reaching a threshold, the driver will reload the PIT and BAT rings.
>> ...
>>>> +static int t7xx_dpmaif_add_skb_to_ring(struct dpmaif_ctrl *dpmaif_ctrl, struct sk_buff *skb)
>>>> +{
>>>> +       unsigned short cur_idx, drb_wr_idx_backup;
>>>> ...
>>>> +       txq = &dpmaif_ctrl->txq[skb_cb->txq_number];
>>>> ...
>>>> +       cur_idx = txq->drb_wr_idx;
>>>> +       drb_wr_idx_backup = cur_idx;
>>>> ...
>>>> +       for (wr_cnt = 0; wr_cnt < payload_cnt; wr_cnt++) {
>>>> ...
>>>> +               bus_addr = dma_map_single(dpmaif_ctrl->dev, data_addr, data_len, DMA_TO_DEVICE);
>>>> +               if (dma_mapping_error(dpmaif_ctrl->dev, bus_addr)) {
>>>> +                       dev_err(dpmaif_ctrl->dev, "DMA mapping fail\n");
>>>> +                       atomic_set(&txq->tx_processing, 0);
>>>> +
>>>> +                       spin_lock_irqsave(&txq->tx_lock, flags);
>>>> +                       txq->drb_wr_idx = drb_wr_idx_backup;
>>>> +                       spin_unlock_irqrestore(&txq->tx_lock, flags);
>>> What is the purpose of locking here?
>> The intention is to protect against concurrent access of drb_wr_idx by t7xx_txq_drb_wr_available()
> t7xx_txq_drb_wr_available() only reads the drb_wr_idx field, why would
> you serialize that concurrent read?

Good point, drb_wr_idx has only 1 writer which is this thread.

>>>> +                       return -ENOMEM;
>>>> +               }
>>>> ...
>>>> +       }
>>>> ...
>>>> +}