Re: [PATCH net-next v3 08/12] net: ethernet: oa_tc6: implement transmit path to transfer tx ethernet frames

Parthiban.Veerasooran
2024-03-19T12:55:13Z

Hi Andrew,

On 07/03/24 10:38 pm, Andrew Lunn wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>
>> @@ -55,6 +77,14 @@
>> (OA_TC6_CTRL_MAX_REGISTERS *\
>> OA_TC6_CTRL_REG_VALUE_SIZE) +\
>> OA_TC6_CTRL_IGNORED_SIZE)
>> +#define OA_TC6_CHUNK_PAYLOAD_SIZE 64
>> +#define OA_TC6_DATA_HEADER_SIZE 4
>> +#define OA_TC6_CHUNK_SIZE (OA_TC6_DATA_HEADER_SIZE +\
>> + OA_TC6_CHUNK_PAYLOAD_SIZE)
>> +#define OA_TC6_TX_SKB_QUEUE_SIZE 100
>
> So you keep up to 100 packets in a queue. If use assume typical MTU
> size packets, that is 1,238,400 bits. At 10Mbps, that is 120ms of
> traffic. That is quite a lot of latency when a high priority packet is
> added to the tail of the queue and needs to wait for all the other
> packets to be sent first.
>
> Chunks are 64 bytes. So in practice, you only ever need two
> packets. You need to be able to fill a chunk with the final part of
> one packet, and the beginning of the next. So i would try using a much
> smaller queue size. That will allow Linux queue disciplines to give
> you the high priority packets first which you send with low latency.
Thanks for the detailed explanation. If I understand you correctly,

1. The tx skb queue size (OA_TC6_TX_SKB_QUEUE_SIZE) should be 2 to avoid
the latency when a high priority packet added.

2. Need to implement the handling part of the below case,
In case if one packet ends in a chunk and that chunk still having some
space left to accommodate some bytes from the next packet if available
from network layer.

Will implement in the next version.
>
>> +static void oa_tc6_add_tx_skb_to_spi_buf(struct oa_tc6 *tc6)
>> +{
>> + enum oa_tc6_data_start_valid_info start_valid = OA_TC6_DATA_START_INVALID;
>> + enum oa_tc6_data_end_valid_info end_valid = OA_TC6_DATA_END_INVALID;
>> + __be32 *tx_buf = tc6->spi_data_tx_buf + tc6->spi_data_tx_buf_offset;
>> + u16 remaining_length = tc6->tx_skb->len - tc6->tx_skb_offset;
>> + u8 *tx_skb_data = tc6->tx_skb->data + tc6->tx_skb_offset;
>> + u8 end_byte_offset = 0;
>> + u16 length_to_copy;
>> +
>> + /* Set start valid if the current tx chunk contains the start of the tx
>> + * ethernet frame.
>> + */
>> + if (!tc6->tx_skb_offset)
>> + start_valid = OA_TC6_DATA_START_VALID;
>> +
>> + /* If the remaining tx skb length is more than the chunk payload size of
>> + * 64 bytes then copy only 64 bytes and leave the ongoing tx skb for
>> + * next tx chunk.
>> + */
>> + length_to_copy = min_t(u16, remaining_length, OA_TC6_CHUNK_PAYLOAD_SIZE);
>> +
>> + /* Copy the tx skb data to the tx chunk payload buffer */
>> + memcpy(tx_buf + 1, tx_skb_data, length_to_copy);
>> + tc6->tx_skb_offset += length_to_copy;
>
> You probably need a call to skb_linearize() somewhere. You assume the
> packet data is contiguous. It can in fact be split into multiple
> segments. skb_linearize() will convert it to a single buffer.
Ah ok. Then probably I have to add the below code in the
oa_tc6_start_xmit() function before adding the skb into the transmit queue.

if (skb_linearize(skb)) {
dev_kfree_skb_any(skb);
tc6->netdev->stats.tx_dropped++;
return NETDEV_TX_OK;
}

>
>> +static int oa_tc6_try_spi_transfer(struct oa_tc6 *tc6)
>> +{
>> + int ret;
>> +
>> + while (true) {
>> + u16 spi_length = 0;
>> +
>> + tc6->spi_data_tx_buf_offset = 0;
>> +
>> + if (tc6->tx_skb || !skb_queue_empty(&tc6->tx_skb_q))
>> + spi_length = oa_tc6_prepare_spi_tx_buf_for_tx_skbs(tc6);
>> +
>> + if (spi_length == 0)
>> + break;
>> +
>> + ret = oa_tc6_spi_transfer(tc6, OA_TC6_DATA_HEADER, spi_length);
>> + if (ret) {
>> + netdev_err(tc6->netdev,
>> + "SPI data transfer failed. Restart the system: %d\n",
>> + ret);
>
> What does Restart the system mean?
Hmm, actually if SPI transfer failed then it can be hardware failure or
poor SPI connection. Now I realize that just restarting the system will
not help. I will remove "Restart the system:" as it is not the correct info.
>
>> +static int oa_tc6_spi_thread_handler(void *data)
>> +{
>> + struct oa_tc6 *tc6 = data;
>> + int ret;
>> +
>> + while (likely(!kthread_should_stop())) {
>> + /* This kthread will be waken up if there is a tx skb */
>> + wait_event_interruptible(tc6->spi_wq,
>> + !skb_queue_empty(&tc6->tx_skb_q) ||
>> + kthread_should_stop());
>> + ret = oa_tc6_try_spi_transfer(tc6);
>
> Shouldn't you check why you have been woken up? It seems more logical
> to test here for kthread_should_stop() rather than have
> oa_tc6_try_spi_transfer() handle there is not actually a packet to be
> sent.
Ok, then I will add the below code before the oa_tc6_try_spi_transfer().

if (kthread_should_stop())
break;

Best regards,
Parthiban V
>
> Andrew
>