netdev - Re: [PATCH net-next v10 4/7] net: tn40xx: add basic Tx handling

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Date: Sat, 15 Jun 2024 20:33:04 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Hans-Frieder Vogt <hfdevel@....net>
Cc: FUJITA Tomonori <fujita.tomonori@...il.com>, kuba@...nel.org,
	netdev@...r.kernel.org, horms@...nel.org, jiri@...nulli.us,
	pabeni@...hat.com, linux@...linux.org.uk, naveenm@...vell.com,
	jdamato@...tly.com
Subject: Re: [PATCH net-next v10 4/7] net: tn40xx: add basic Tx handling

> /* Sizes of tx desc (including padding if needed) as function of the SKB's
>  * frag number
>  * 7 - is number of lwords in txd with one phys buffer
>  * 3 - is number of lwords used for every additional phys buffer
>  * for (i = 0; i < TN40_MAX_PBL; i++) {
>  *    lwords = 7 + (i * 3);
>  *        if (lwords & 1)
>  *            lwords++;    pad it with 1 lword
>  *        tn40_txd_sizes[i].qwords = lwords >> 1;
>  *        tn40_txd_sizes[i].bytes = lwords << 2;
>  * }
>  */
> static struct {
>     u16 bytes;
>     u16 qwords;        /* qword = 64 bit */
> } const tn40_txd_sizes[] = {
>     { 0x20, 0x04 },
>     { 0x28, 0x05 },
>     { 0x38, 0x07 },

Nice comment section. Please keep it to help explain the table.

I did wounder if calculating the value as needed would be any
slower/faster than doing a table access. Doing arithmetic is very
cheap compared to a cache miss for a table lookup. Something which
could be bench marked and optimised later.

     Andrew