netdev - Re: [PATCH iwl-net v1] ice: fix receive buffer size miscalculation

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Date: Thu, 10 Aug 2023 14:33:16 -0700
From: Tony Nguyen <anthony.l.nguyen@...el.com>
To: Jesse Brandeburg <jesse.brandeburg@...el.com>,
	<intel-wired-lan@...ts.osuosl.org>
CC: <netdev@...r.kernel.org>
Subject: Re: [PATCH iwl-net v1] ice: fix receive buffer size miscalculation



On 8/9/2023 5:23 PM, Jesse Brandeburg wrote:
> The driver is misconfiguring the hardware for some values of MTU such that
> it could use multiple descriptors to receive a packet when it could have
> simply used one.
> 
> Change the driver to use a round-up instead of the result of a shift, as
> the shift can truncate the lower bits of the size, and result in the
> problem noted above. It also aligns this driver with similar code in i40e.
> 
> The insidiousness of this problem is that everything works with the wrong
> size, it's just not working as well as it could, as some MTU sizes end up
> using two or more descriptors, and there is no way to tell that is
> happening without looking at ice_trace or a bus analyzer.

This should have a Fixes: ?

> Signed-off-by: Jesse Brandeburg <jesse.brandeburg@...el.com>
> ---
>   drivers/net/ethernet/intel/ice/ice_base.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/ice/ice_base.c b/drivers/net/ethernet/intel/ice/ice_base.c
> index b678bdf96f3a..074bf9403cd1 100644
> --- a/drivers/net/ethernet/intel/ice/ice_base.c
> +++ b/drivers/net/ethernet/intel/ice/ice_base.c
> @@ -435,7 +435,8 @@ static int ice_setup_rx_ctx(struct ice_rx_ring *ring)
>   	/* Receive Packet Data Buffer Size.
>   	 * The Packet Data Buffer Size is defined in 128 byte units.
>   	 */
> -	rlan_ctx.dbuf = ring->rx_buf_len >> ICE_RLAN_CTX_DBUF_S;
> +	rlan_ctx.dbuf = DIV_ROUND_UP(ring->rx_buf_len,
> +				     BIT_ULL(ICE_RLAN_CTX_DBUF_S));
>   
>   	/* use 32 byte descriptors */
>   	rlan_ctx.dsize = 1;