netdev - Re: [Intel-wired-lan][PATCH iwl-net] idpf: set completion tag for "empty" bufs associated with a packet

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Message-ID: <20241017090428.GS2162@kernel.org>
Date: Thu, 17 Oct 2024 10:04:28 +0100
From: Simon Horman <horms@...nel.org>
To: Joshua Hay <joshua.a.hay@...el.com>
Cc: intel-wired-lan@...ts.osuosl.org, aleksander.lobakin@...el.com,
	madhu.chittim@...el.com, netdev@...r.kernel.org
Subject: Re: [Intel-wired-lan][PATCH iwl-net] idpf: set completion tag for
 "empty" bufs associated with a packet

On Mon, Oct 07, 2024 at 01:24:35PM -0700, Joshua Hay wrote:
> Commit d9028db618a6 ("idpf: convert to libeth Tx buffer completion")
> inadvertently removed code that was necessary for the tx buffer cleaning
> routine to iterate over all buffers associated with a packet.
> 
> When a frag is too large for a single data descriptor, it will be split
> across multiple data descriptors. This means the frag will span multiple
> buffers in the buffer ring in order to keep the descriptor and buffer
> ring indexes aligned. The buffer entries in the ring are technically
> empty and no cleaning actions need to be performed. These empty buffers
> can precede other frags associated with the same packet. I.e. a single
> packet on the buffer ring can look like:
> 
> 	buf[0]=skb0.frag0
> 	buf[1]=skb0.frag1
> 	buf[2]=empty
> 	buf[3]=skb0.frag2
> 
> The cleaning routine iterates through these buffers based on a matching
> completion tag. If the completion tag is not set for buf2, the loop will
> end prematurely. Frag2 will be left uncleaned and next_to_clean will be
> left pointing to the end of packet, which will break the cleaning logic
> for subsequent cleans. This consequently leads to tx timeouts.
> 
> Assign the empty bufs the same completion tag for the packet to ensure
> the cleaning routine iterates over all of the buffers associated with
> the packet.
> 
> Fixes: d9028db618a6 ("idpf: convert to libeth Tx buffer completion")
> Signed-off-by: Joshua Hay <joshua.a.hay@...el.com>
> Acked-by: Alexander Lobakin <aleksander.lobakin@...el.com>
> Reviewed-by: Madhu chittim <madhu.chittim@...el.com>

Thanks for the detailed description.

Reviewed-by: Simon Horman <horms@...nel.org>