RE: [PATCH net-next 2/2] net: lan966x: Stop using packing library

[David Laight <David.Laight@ACULAB.COM>]

From: Horatiu Vultur
> Sent: 12 March 2023 20:24
> 
> When a frame is injected from CPU, it is required to create an IFH(Inter
> frame header) which sits in front of the frame that is transmitted.
> This IFH, contains different fields like destination port, to bypass the
> analyzer, priotity, etc. Lan966x it is using packing library to set and
> get the fields of this IFH. But this seems to be an expensive
> operations.
> If this is changed with a simpler implementation, the RX will be
> improved with ~5Mbit while on the TX is a much bigger improvement as it
> is required to set more fields. Below are the numbers for TX.
...
> +static void lan966x_ifh_set(u8 *ifh, size_t val, size_t pos, size_t length)
> +{
> +	u32 v = 0;
> +
> +	for (int i = 0; i < length ; i++) {
> +		int j = pos + i;
> +		int k = j % 8;
> +
> +		if (i == 0 || k == 0)
> +			v = ifh[IFH_LEN_BYTES - (j / 8) - 1];
> +
> +		if (val & (1 << i))
> +			v |= (1 << k);
> +
> +		if (i == (length - 1) || k == 7)
> +			ifh[IFH_LEN_BYTES - (j / 8) - 1] = v;
> +	}
> +}
> +

It has to be possible to do much better that that.
Given  that 'pos' and 'length' are always constants it looks like
each call should reduce to (something like):
	ifh[k] |= val << n;
	ifk[k + 1] |= val >> (8 - n);
	...
It might be that the compiler manages to do this, but I doubt it.

	David

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