netdev - RE: [Intel-wired-lan] [PATCH iwl-net] i40e: remove redundant memory barrier when cleaning Tx descs

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Message-ID: <IA1PR11MB6241633A18C6497E503DEB278B08A@IA1PR11MB6241.namprd11.prod.outlook.com>
Date: Fri, 12 Sep 2025 05:00:40 +0000
From: "Rinitha, SX" <sx.rinitha@...el.com>
To: "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
	"intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "Nguyen, Anthony L"
	<anthony.l.nguyen@...el.com>, "Kitszel, Przemyslaw"
	<przemyslaw.kitszel@...el.com>, "Karlsson, Magnus"
	<magnus.karlsson@...el.com>, "Fijalkowski, Maciej"
	<maciej.fijalkowski@...el.com>
Subject: RE: [Intel-wired-lan] [PATCH iwl-net] i40e: remove redundant memory
 barrier when cleaning Tx descs

> -----Original Message-----
> From: Intel-wired-lan <intel-wired-lan-bounces@...osl.org> On Behalf Of Maciej Fijalkowski
> Sent: 22 August 2025 20:46
> To: intel-wired-lan@...ts.osuosl.org
> Cc: netdev@...r.kernel.org; Nguyen, Anthony L <anthony.l.nguyen@...el.com>; Kitszel, Przemyslaw <przemyslaw.kitszel@...el.com>; Karlsson, Magnus <magnus.karlsson@...el.com>; Fijalkowski, Maciej <maciej.fijalkowski@...el.com>
> Subject: [Intel-wired-lan] [PATCH iwl-net] i40e: remove redundant memory barrier when cleaning Tx descs
>
> i40e has a feature which writes to memory location last descriptor successfully sent. Memory barrier in i40e_clean_tx_irq() was used to avoid forward-reading descriptor fields in case DD bit was not set.
> Having mentioned feature in place implies that such situation will not happen as we know in advance how many descriptors HW has dealt with.
>
> Besides, this barrier placement was wrong. Idea is to have this protection *after* reading DD bit from HW descriptor, not before.
> Digging through git history showed me that indeed barrier was before DD bit check, anyways the commit introducing i40e_get_head() should have wiped it out altogether.
>
> Also, there was one commit doing s/read_barrier_depends/smp_rmb when get head feature was already in place, but it was only theoretical based on ixgbe experiences, which is different in these terms as that driver has to read DD bit from HW descriptor.
>
> Fixes: 1943d8ba9507 ("i40e/i40evf: enable hardware feature head write back")
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> ---
> drivers/net/ethernet/intel/i40e/i40e_txrx.c | 3 ---
> 1 file changed, 3 deletions(-)
>

Tested-by: Rinitha S <sx.rinitha@...el.com> (A Contingent worker at Intel)