netdev - Re: [PATCH net] net: xilinx: axienet: Fix IRQ coalescing packet count overflow

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Message-ID: <20240904160013.GX4792@kernel.org>
Date: Wed, 4 Sep 2024 17:00:13 +0100
From: Simon Horman <horms@...nel.org>
To: Sean Anderson <sean.anderson@...ux.dev>
Cc: Radhey Shyam Pandey <radhey.shyam.pandey@....com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, Michal Simek <michal.simek@....com>,
	linux-arm-kernel@...ts.infradead.org,
	Ariane Keller <ariane.keller@....ee.ethz.ch>,
	linux-kernel@...r.kernel.org,
	Daniel Borkmann <daniel@...earbox.net>,
	Andy Chiu <andy.chiu@...ive.com>
Subject: Re: [PATCH net] net: xilinx: axienet: Fix IRQ coalescing packet
 count overflow

On Tue, Sep 03, 2024 at 02:00:59PM -0400, Sean Anderson wrote:
> If coalesce_count is greater than 255 it will not fit in the register and
> will overflow. Clamp it to 255 for more-predictable results.

Hi Sean,

Can this occur in practice?

> 
> Signed-off-by: Sean Anderson <sean.anderson@...ux.dev>
> Fixes: 8a3b7a252dca ("drivers/net/ethernet/xilinx: added Xilinx AXI Ethernet driver")

nit: I think it is usual for the order of these tags to be reversed.

> ---
> 
>  drivers/net/ethernet/xilinx/xilinx_axienet_main.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> index 9aeb7b9f3ae4..5f27fc1c4375 100644
> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> @@ -252,7 +252,8 @@ static u32 axienet_usec_to_timer(struct axienet_local *lp, u32 coalesce_usec)
>  static void axienet_dma_start(struct axienet_local *lp)
>  {
>  	/* Start updating the Rx channel control register */
> -	lp->rx_dma_cr = (lp->coalesce_count_rx << XAXIDMA_COALESCE_SHIFT) |
> +	lp->rx_dma_cr = (min(lp->coalesce_count_rx, 255) <<
> +			 XAXIDMA_COALESCE_SHIFT) |
>  			XAXIDMA_IRQ_IOC_MASK | XAXIDMA_IRQ_ERROR_MASK;

nit: it would be nice to avoid using a naked 255 here.
     Perhaps: #define XAXIDMA_COALESCE_MAX 0xff

>  	/* Only set interrupt delay timer if not generating an interrupt on
>  	 * the first RX packet. Otherwise leave at 0 to disable delay interrupt.
> @@ -264,7 +265,8 @@ static void axienet_dma_start(struct axienet_local *lp)
>  	axienet_dma_out32(lp, XAXIDMA_RX_CR_OFFSET, lp->rx_dma_cr);
>  
>  	/* Start updating the Tx channel control register */
> -	lp->tx_dma_cr = (lp->coalesce_count_tx << XAXIDMA_COALESCE_SHIFT) |
> +	lp->tx_dma_cr = (min(lp->coalesce_count_tx, 255) <<
> +			 XAXIDMA_COALESCE_SHIFT) |
>  			XAXIDMA_IRQ_IOC_MASK | XAXIDMA_IRQ_ERROR_MASK;
>  	/* Only set interrupt delay timer if not generating an interrupt on
>  	 * the first TX packet. Otherwise leave at 0 to disable delay interrupt.
> -- 
> 2.35.1.1320.gc452695387.dirty
> 
>