Re: [net-next,v17,03/14] net: homa: create shared Homa header files

[Paolo Abeni <pabeni@redhat.com>]

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---
net: homa: create shared Homa header files

homa_impl.h defines "struct homa", which contains overall information
about the Homa transport, plus various odds and ends that are used
throughout the Homa implementation. homa_stub.h is a temporary header
file that provides stubs for facilities that have omitted for this first
patch series. This file will go away once Homa is fully upstreamed.

> diff --git a/net/homa/homa_impl.h b/net/homa/homa_impl.h
> new file mode 100644
> index 000000000000..f55a7b80431d
> --- /dev/null
> +++ b/net/homa/homa_impl.h

[ ... ]

> @@ -0,0 +1,512 @@

[ ... ]

> +/**
> + * homa_set_doff() - Fills in the doff TCP header field for a Homa packet.
> + * @h:     Packet header whose doff field is to be set.
> + * @size:  Size of the "header", bytes (must be a multiple of 4). This
> + *         information is used only for TSO; it's the number of bytes
> + *         that should be replicated in each segment. The bytes after
> + *         this will be distributed among segments.
> + */
> +static inline void homa_set_doff(struct homa_data_hdr *h, int size)
> +{
> +	/* Drop the 2 low-order bits from size and set the 4 high-order
> +	 * bits of doff from what's left.
> +	 */
> +	h->common.doff = size << 2;
> +}

Does the comment in homa_set_doff() accurately describe what the code
does? The comment says "Drop the 2 low-order bits from size" which would
be a right shift (size >> 2), but the code performs a left shift
(size << 2).

Looking at similar usage patterns in TCP where the doff field stores
header length in 4-byte word units, the code appears correct - it's
converting a byte size to 4-byte word units by shifting left.

The comment should probably say "Shift size left by 2 bits to store in
4-byte word units" or "Multiply size by 4 and store in doff field" to
match what the code actually does.

[ ... ]