netdev - Re: [PATCH net-next v2 2/6] tsnep: Add XDP TX support

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Date:   Thu, 8 Dec 2022 20:57:15 +0100
From:   Gerhard Engleder <gerhard@...leder-embedded.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net,
        kuba@...nel.org, edumazet@...gle.com, pabeni@...hat.com,
        ast@...nel.org, daniel@...earbox.net, hawk@...nel.org,
        john.fastabend@...il.com
Subject: Re: [PATCH net-next v2 2/6] tsnep: Add XDP TX support

On 08.12.22 15:10, Maciej Fijalkowski wrote:
>> @@ -65,7 +71,11 @@ struct tsnep_tx_entry {
>>   
>>   	u32 properties;
>>   
>> -	struct sk_buff *skb;
>> +	enum tsnep_tx_type type;
>> +	union {
>> +		struct sk_buff *skb;
>> +		struct xdp_frame *xdpf;
>> +	};
>>   	size_t len;
>>   	DEFINE_DMA_UNMAP_ADDR(dma);
>>   };
>> diff --git a/drivers/net/ethernet/engleder/tsnep_main.c b/drivers/net/ethernet/engleder/tsnep_main.c
>> index a28fde9fb060..b97cfd5fa1fa 100644
>> --- a/drivers/net/ethernet/engleder/tsnep_main.c
>> +++ b/drivers/net/ethernet/engleder/tsnep_main.c
>> @@ -310,10 +310,11 @@ static void tsnep_tx_activate(struct tsnep_tx *tx, int index, int length,
>>   	struct tsnep_tx_entry *entry = &tx->entry[index];
>>   
>>   	entry->properties = 0;
>> -	if (entry->skb) {
>> +	if (entry->skb || entry->xdpf) {
> 
> i think this change is redundant, you could keep a single check as skb and
> xdpf ptrs share the same memory, but i guess this makes it more obvious

Yes it is actually redundant. I thought it is not a good idea to rely on
the union in the code.

>> +/* This function requires __netif_tx_lock is held by the caller. */
>> +static int tsnep_xdp_xmit_frame_ring(struct xdp_frame *xdpf,
>> +				     struct tsnep_tx *tx, bool dma_map)
>> +{
>> +	struct skb_shared_info *shinfo = xdp_get_shared_info_from_frame(xdpf);
>> +	unsigned long flags;
>> +	int count = 1;
>> +	struct tsnep_tx_entry *entry;
>> +	int length;
>> +	int i;
>> +	int retval;
>> +
>> +	if (unlikely(xdp_frame_has_frags(xdpf)))
>> +		count += shinfo->nr_frags;
>> +
>> +	spin_lock_irqsave(&tx->lock, flags);
>> +
>> +	if (tsnep_tx_desc_available(tx) < (MAX_SKB_FRAGS + 1 + count)) {
> 
> Wouldn't count + 1 be sufficient to check against the descs available?
> if there are frags then you have already accounted them under count
> variable so i feel like MAX_SKB_FRAGS is redundant.

In the standard TX path tsnep_xmit_frame_ring() would stop the queue if
less than MAX_SKB_FRAGS + 1 descriptors are available. I wanted to keep
that stop queue logic in tsnep_xmit_frame_ring() by ensuring that XDP
never exceeds this limit (similar to STMMAC_TX_THRESH of stmmac).

So this line checks if enough descriptors are available and that the
queue would not have been stopped by tsnep_xmit_frame_ring().

I could improve the comment below.

>> +		/* prevent full TX ring due to XDP */
>> +		spin_unlock_irqrestore(&tx->lock, flags);
>> +
>> +		return -EBUSY;
>> +	}

Gerhard