Re: [PATCH v6 RESEND net-next 2/5] net: enetc: add Tx checksum offload for i.MX95 ENETC

[Simon Horman <horms@kernel.org>]

On Wed, Dec 04, 2024 at 01:29:29PM +0800, Wei Fang wrote:
> In addition to supporting Rx checksum offload, i.MX95 ENETC also supports
> Tx checksum offload. The transmit checksum offload is implemented through
> the Tx BD. To support Tx checksum offload, software needs to fill some
> auxiliary information in Tx BD, such as IP version, IP header offset and
> size, whether L4 is UDP or TCP, etc.
> 
> Same as Rx checksum offload, Tx checksum offload capability isn't defined
> in register, so tx_csum bit is added to struct enetc_drvdata to indicate
> whether the device supports Tx checksum offload.
> 
> Signed-off-by: Wei Fang <wei.fang@nxp.com>
> Reviewed-by: Frank Li <Frank.Li@nxp.com>
> Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>

...

> diff --git a/drivers/net/ethernet/freescale/enetc/enetc_hw.h b/drivers/net/ethernet/freescale/enetc/enetc_hw.h
> index 4b8fd1879005..590b1412fadf 100644
> --- a/drivers/net/ethernet/freescale/enetc/enetc_hw.h
> +++ b/drivers/net/ethernet/freescale/enetc/enetc_hw.h
> @@ -558,7 +558,12 @@ union enetc_tx_bd {
>  		__le16 frm_len;
>  		union {
>  			struct {
> -				u8 reserved[3];
> +				u8 l3_start:7;
> +				u8 ipcs:1;
> +				u8 l3_hdr_size:7;
> +				u8 l3t:1;
> +				u8 resv:5;
> +				u8 l4t:3;
>  				u8 flags;
>  			}; /* default layout */

Hi Wei,

Given that little-endian types are used elsewhere in this structure
I am guessing that the layout above works for little-endian hosts
but will not work on big-endian hosts.

If so, I would suggest an alternate approach of using a single 32-bit
word and accessing it using a combination of FIELD_PREP() and FIELD_GET()
using masks created using GENMASK() and BIT().

Or, less desirably IMHO, by providing an alternate layout for
the embedded struct for big endian systems.

...