Re: [PATCH] net/ixgbe: fix min Rx/Tx descriptors

[Bruce Richardson <bruce.richardson@intel.com>]

On Fri, Feb 21, 2025 at 08:23:18AM +0000, Mingjin Ye wrote:
> The minimum free packet threshold (tx_free_thresh) and the minimum RS bit
> threshold (tx_rs_thresh) both have a default value of 32. Therefore, the
> default minimum number of ring descriptors value is 64.
> 
> Fixes: dee5f1fd5fc7 ("ixgbe: get queue info and descriptor limits")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Mingjin Ye <mingjinx.ye@intel.com>
> ---
>  doc/guides/nics/ixgbe.rst            | 2 +-
>  drivers/net/intel/ixgbe/ixgbe_rxtx.h | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/doc/guides/nics/ixgbe.rst b/doc/guides/nics/ixgbe.rst
> index c5c6a6c34b..10a0cdd270 100644
> --- a/doc/guides/nics/ixgbe.rst
> +++ b/doc/guides/nics/ixgbe.rst
> @@ -76,7 +76,7 @@ Scattered packets are not supported in this mode.
>  If an incoming packet is greater than the maximum acceptable length of one "mbuf" data size (by default, the size is 2 KB),
>  vPMD for RX would be disabled.
>  
> -By default, IXGBE_MAX_RING_DESC is set to 8192 and RTE_PMD_IXGBE_RX_MAX_BURST is set to 32.
> +By default, IXGBE_MAX_RING_DESC is set to 8192 and RTE_PMD_IXGBE_RX_MAX_BURST is set to 64.
>  

Was this part of the patch included by mistake? The RX_MAX_BURST value is
still set to 32 in the code.

However, in terms of doc updates, I believe the text above this point is
incorrect. It states that scattered packets are not supported in vector
drivers, which is no longer correct, as we support them for vector Rx.
Similarly, I think the pre-conditions for use of burst/vector mode are
incorrect compared to what is checked in the code.

>  Windows Prerequisites and Pre-conditions
>  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> diff --git a/drivers/net/intel/ixgbe/ixgbe_rxtx.h b/drivers/net/intel/ixgbe/ixgbe_rxtx.h
> index 278f665108..54569c7ade 100644
> --- a/drivers/net/intel/ixgbe/ixgbe_rxtx.h
> +++ b/drivers/net/intel/ixgbe/ixgbe_rxtx.h
> @@ -26,7 +26,7 @@
>   * descriptors should meet the following condition:
>   *      (num_ring_desc * sizeof(rx/tx descriptor)) % 128 == 0
>   */
> -#define	IXGBE_MIN_RING_DESC	32
> +#define	IXGBE_MIN_RING_DESC	64
>  #define	IXGBE_MAX_RING_DESC	8192
>  
>  #define RTE_PMD_IXGBE_TX_MAX_BURST 32
> -- 
> 2.25.1
>