netdev - Re: [PATCH net-next v4 08/13] net: wwan: t7xx: Add data path interface

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Date:   Tue, 15 Feb 2022 18:17:53 -0800
From:   "Martinez, Ricardo" <ricardo.martinez@...ux.intel.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     Netdev <netdev@...r.kernel.org>, linux-wireless@...r.kernel.org,
        kuba@...nel.org, davem@...emloft.net, johannes@...solutions.net,
        ryazanov.s.a@...il.com, loic.poulain@...aro.org,
        m.chetan.kumar@...el.com, chandrashekar.devegowda@...el.com,
        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,
        moises.veleta@...el.com, pierre-louis.bossart@...el.com,
        muralidharan.sethuraman@...el.com, Soumya.Prakash.Mishra@...el.com,
        sreehari.kancharla@...el.com
Subject: Re: [PATCH net-next v4 08/13] net: wwan: t7xx: Add data path
 interface


On 2/8/2022 12:19 AM, Ilpo Järvinen wrote:
> On Thu, 13 Jan 2022, Ricardo Martinez 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.
...
>
>> +	spin_lock_irqsave(&txq->tx_lock, flags);
>> +	cur_idx = txq->drb_wr_idx;
>> +	drb_wr_idx_backup = cur_idx;
>> +
>> +	txq->drb_wr_idx += send_cnt;
>> +	if (txq->drb_wr_idx >= txq->drb_size_cnt)
>> +		txq->drb_wr_idx -= txq->drb_size_cnt;
>> +
>> +	t7xx_setup_msg_drb(dpmaif_ctrl, txq->index, cur_idx, skb->len, 0, skb->cb[TX_CB_NETIF_IDX]);
>> +	t7xx_record_drb_skb(dpmaif_ctrl, txq->index, cur_idx, skb, 1, 0, 0, 0, 0);
>> +	spin_unlock_irqrestore(&txq->tx_lock, flags);
>> +
>> +	cur_idx = t7xx_ring_buf_get_next_wr_idx(txq->drb_size_cnt, cur_idx);
>> +
>> +	for (wr_cnt = 0; wr_cnt < payload_cnt; wr_cnt++) {
>> +		if (!wr_cnt) {
>> +			data_len = skb_headlen(skb);
>> +			data_addr = skb->data;
>> +			is_frag = false;
>> +		} else {
>> +			skb_frag_t *frag = info->frags + wr_cnt - 1;
>> +
>> +			data_len = skb_frag_size(frag);
>> +			data_addr = skb_frag_address(frag);
>> +			is_frag = true;
>> +		}
>> +
>> +		if (wr_cnt == payload_cnt - 1)
>> +			is_last_one = true;
>> +
>> +		/* TX mapping */
>> +		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);
> Hmm, can txq's drb_wr_idx get updated (or cleared) by something else
> in between these critical sections?
drb_wr_idx cannot be modified inbetween, but it can be used to calculate 
the number of DRBs available, which shouldn't be a problem.
The function is reserving the DRBs at the beginning, in the rare case of 
error it will release them.
...
> +		txq_id = t7xx_select_tx_queue(dpmaif_ctrl);
> +		if (txq_id >= 0) {
> t7xx_select_tx_queue used to do que_started check (in v2) but it
> doesn't anymore so this if is always true these days. I'm left to
> wonder though if it was ok to drop that que_started check?

The que_started check wasn't supposed to be dropped, I'll add it back.

...

>> +/* SKB control buffer indexed values */
>> +#define TX_CB_NETIF_IDX		0
>> +#define TX_CB_QTYPE		1
>> +#define TX_CB_DRB_CNT		2
> The normal way of storing a struct to skb->cb area is:
>
> struct t7xx_skb_cb {
> 	u8	netif_idx;
> 	u8	qtype;
> 	u8	drb_cnt;
> };
>
> #define T7XX_SKB_CB(__skb)	((struct t7xx_skb_cb *)&((__skb)->cb[0]))
>
> However, there's only a single txqt/qtype (TXQ_TYPE_DEFAULT) in the
> patchset? And it seems to me that drb_cnt is a value that could be always
> derived using t7xx_get_drb_cnt_per_skb() from the skb rather than
> stored?

The next iteration will contain t7xx_tx_skb_cb and t7xx_rx_skb_cb 
structures.

Also, q_number is going to be used instead of qtype.

Only one queue is used but I think we can keep this code generic as it 
is straight forward (not like the drb_lack case), any thoughts?

>> +#define DRB_PD_DATA_LEN		((u32)GENMASK(31, 16))
> Drop the cast?

The cast was added to avoid a compiler warning about truncated bits.

I'll move it to the place where it is required:

drb->header &= cpu_to_le32(~(u32)DRB_PD_DATA_LEN);

...