linux-kernel - Re: [PATCH net-next v1] net: stmmac: Drop useless code related to ethtool rx-copybreak

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Message-ID: <b2ae6b80-83e3-4b22-8301-c91569c89494@intel.com>
Date: Wed, 18 Dec 2024 16:48:38 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Furong Xu <0x1207@...il.com>
CC: <netdev@...r.kernel.org>, <linux-stm32@...md-mailman.stormreply.com>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
	"Simon Horman" <horms@...nel.org>, Alexandre Torgue
	<alexandre.torgue@...s.st.com>, Jose Abreu <joabreu@...opsys.com>, Andrew
 Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, "Maxime Coquelin" <mcoquelin.stm32@...il.com>,
	<xfr@...look.com>
Subject: Re: [PATCH net-next v1] net: stmmac: Drop useless code related to
 ethtool rx-copybreak

From: Furong Xu <0x1207@...il.com>
Date: Wed, 18 Dec 2024 16:34:07 +0800

> After commit 2af6106ae949 ("net: stmmac: Introducing support for Page
> Pool"), the driver always copies frames to get a better performance,
> zero-copy for RX frames is no more, then these code turned to be
> useless and users of ethtool may get confused about the unhandled
> rx-copybreak parameter.
> 
> This patch mostly reverts
> commit 22ad38381547 ("stmmac: do not perform zero-copy for rx frames")
> 
> Signed-off-by: Furong Xu <0x1207@...il.com>

The patch itself is fine, *but*

> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac.h  |  2 -
>  .../ethernet/stmicro/stmmac/stmmac_ethtool.c  | 39 -------------------
>  .../net/ethernet/stmicro/stmmac/stmmac_main.c |  5 ---
>  3 files changed, 46 deletions(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac.h b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> index 1d86439b8a14..b8d631e559c0 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> @@ -126,7 +126,6 @@ struct stmmac_rx_queue {
>  	unsigned int cur_rx;
>  	unsigned int dirty_rx;
>  	unsigned int buf_alloc_num;
> -	u32 rx_zeroc_thresh;
>  	dma_addr_t dma_rx_phy;

If sizeof(dma_addr_t) == 8, you're clearly introducing a 4-byte hole
here. Perhaps you could reshuffle the struct a bit to avoid this.

It's always good to inspect the .kos with pahole after modifying
structures to make sure there are no regressions.

>  	u32 rx_tail_addr;
>  	unsigned int state_saved;

Thanks,
Olek