[lwip-users] RFC: PBUF_FLAG_* must be orthogonal

[Leon Woestenberg]

Hello,

The PBUF_FLAG_* constants are only used within pbuf.c.

However, the defines are not really orthogonal bit flags at this time, but
actually are used as "pbuf types", each value a different type.

In order to optimize the code base for all different cases, I propose the
change of the PBUF_FLAG_* meaning as follows:

PBUF_FLAG_ROM: payload is in ROM (otherwise, pbuf is in RAM)
PBUF_FLAG_REF: payload is referenced externally (if not set, payload is
internal to pbuf)
PBUF_FLAG_POOL: pbuf is from pool (otherwise, dynamically allocated)

Now, depending on the argument given to pbuf_alloc():

PBUF_RAM type pbuf has flag 0.
PBUF_ROM type pbuf has flags PBUF_FLAG_ROM | PBUF_FLAG_REF
PBUF_REF type pbuf has flags
PBUF_FLAG_REF
PBUF_POOL type pbuf has flags PBUF_FLAG_POOL.

but more combinations are possible:

PBUF_FLAG_POOL | PBUF_FLAG_REF
PBUF_FLAG_POOL | PBUF_FLAG_REF | PBUF_FLAG_ROM

which allows for an improved pbuf pool system.

As said, changing the flag implementation will not affect other modules, as
they
are pbuf internal.

Regards,

Leon.