[lwip-devel] [bug #49914] lwip_sendmsg uses PBUF_REF pbufs

[Dirk Ziegelmeier]

Follow-up Comment #8, bug #49914 (project lwip):

Zero-copy depends on the application:

1)
- application uses raw API
- application creates a PBUF and generates/assembles its data into the PBUF
- application hands PBUF to lwIP
- lwIP prepends UDP/IP/ethernet header
- A scatter-gather capable DMA netif increases refcount on pbuf and enqueues
it in the ETH HW DMA chain
- ETH HW sends pbuf
- netif decreases refcount on PBUF, pbuf is released

2)
- application uses socket API
- application has data to send in a contiguous buffer anyway
- application calls lwip_send()
- lwIP creates a ref PBUF with pointer to application data
- lwIP prepends PBUF with UDP/IP/ethernet header
- netif synchronously copies data into ETH HW buffer for sending (not DMA
capable)


Problems:
- Systems with large amount of resources usually have an OS -> mostly socket
API is used. But these systems often have a DMA capable ETH HW -> copy needed
in driver for DMA

- Systems with low amount of resources usually have no OS -> raw API is used.
These systems often do NOT have DMA capable ETH HW -> copy needed in driver
for into ETH HW buffer


-> the only systems that can really do zero-copy TX are:

1) large systems with OS and socket API that have non-DMA HW (are there
systems like that?)

2) systems where RAW API is used (no matter if OS is running or not) and
DMA-capable HW. Whether to use RAW API or socket API depends on application
demands, portability requirements and programming effort


Comments to this summary? Is this correct or am I missing something?

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