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

[<Parthiban.Veerasooran@microchip.com>]

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
>