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

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Message-ID: <9969daf0-a16a-3785-28f8-80bd7dd0f81f@intel.com>
Date: Thu, 10 Aug 2023 13:02:37 +0200
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: Jesse Brandeburg <jesse.brandeburg@...el.com>,
	"intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH iwl-net v1] ice: fix receive buffer size miscalculation

On 8/10/23 02:23, 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.
> 
> Signed-off-by: Jesse Brandeburg <jesse.brandeburg@...el.com>

Nice catch!

> ---
>   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));

looks like %ICE_RLAN_CTX_DBUF_S is only used once in whole codebase, here.

Perhaps you could rename it
(even considering that it is targeted for -net),
and have it as ICE_RLAN_CTX_DBUF_DIV or similar?

Then it will be BIT(7) or just 128. Also, I would keep it as 32bit value.

>   
>   	/* use 32 byte descriptors */
>   	rlan_ctx.dsize = 1;

(BTW: you have missed my review on e1000 ML)
anyway, it's good
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>