Hello Ricardo,
On Tue, Apr 5, 2022 at 2:29 AM Martinez, Ricardo
<ricardo.martinez@xxxxxxxxxxxxxxx> wrote:
> On 3/6/2022 6:58 PM, Sergey Ryazanov wrote:
>> On Thu, Feb 24, 2022 at 1:35 AM Ricardo Martinez
>> <ricardo.martinez@xxxxxxxxxxxxxxx> wrote:
>>> From: Haijun Liu <haijun.liu@xxxxxxxxxxxx>
>>>
>>> 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?
>>> + return -ENOMEM;
>>> + }
>>> ...
>>> + }
>>> ...
>>> +}
--
Sergey
