netdev - Re: [EXT] Re: [PATCH net-next v2 3/4] octeon_ep: implement xmit

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Date: Thu, 26 Oct 2023 07:57:54 +0000
From: Shinas Rasheed <srasheed@...vell.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Haseeb Gani
	<hgani@...vell.com>, Vimlesh Kumar <vimleshk@...vell.com>,
        "egallen@...hat.com" <egallen@...hat.com>,
        "mschmidt@...hat.com"
	<mschmidt@...hat.com>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "horms@...nel.org" <horms@...nel.org>,
        "davem@...emloft.net"
	<davem@...emloft.net>,
        "wizhao@...hat.com" <wizhao@...hat.com>,
        "konguyen@...hat.com" <konguyen@...hat.com>,
        Veerasenareddy Burru
	<vburru@...vell.com>,
        Sathesh B Edara <sedara@...vell.com>,
        Eric Dumazet
	<edumazet@...gle.com>
Subject: Re: [EXT] Re: [PATCH net-next v2 3/4] octeon_ep: implement xmit_more
 in transmit

Hi Jakub, 

Again, thanks for your review.

> Does this guarantee that a full-sized skb can be accommodated?
>> I assume by full-sized skb you mean a non-linear skb with MAX_SG_FRAGS elements in frags array.  Yes, this can be accommodated and the hardware uses separate SG list memory to siphon these skb frags instead of obtaining them from the same tx hardware queue. What I'm trying to say is, this means that a single tx descriptor will be enough for a full-sized skb as well, as hardware can pick up SG frags from separate memory and doesn't require separate descriptors.

>If so - consider stopping stopping the queue when the condition is not true.
>> We do stop the queue if tx queue is full, as in octep_iq_full_check earlier on.

>The recommended way of implementing 'driver flow control'
is to stop the queue once next packet may not fit, and then use
netif_xmit_stopped() when deciding whether we need to flush or we can
trust xmit_more. see 
https://urldefense.proofpoint.com/v2/url?u=https-3A__www.kernel.org_doc_html_next_networking_driver.html-23transmit-2Dpath-2Dguidelines&d=DwICAg&c=nKjWec2b6R0mOyPaz7xtfQ&r=1OxLD4y-oxrlgQ1rjXgWtmLz1pnaDjD96sDq-cKUwK4&m=FyJHTb5Z2u9DTFSYPU38S5kPcP5KvwGWzY-DPcqOl1gdnm7ToZhTFpyvhLMqh1hd&s=dBMmwfWKAi0UH3nrz7j9kYnAodDjuN3LZ5tC2aL_Prs&e= 

>> In the skeleton code above, as I understand each tx desc holds a skb frag and hence there can be possibility of receiving a full-sized skb, stopping the queue but on receiving another normal skb we should observe our queue to be stopped. This doesn't arise in our case as even if the skb is full-sized, it will only use a single tx descriptor so we can be sure if queue has been stopped, the write index will only be updated once posted (and read) tx descriptors are processed in napi context and queues awoken.

Please correct me if I'm wrong anywhere (sorry if so) to further my understanding, and again thanks for your time!