soc:2010:cooldavid:journal:week2

Week 2 [ 31 May - 6 Jun 2010 ]: Discuss TCP and memory changes, update jme driver
jme driver
Trace memory related codes

Week 2 [ 31 May - 6 Jun 2010 ]: Discuss TCP and memory changes, update jme driver

jme driver

From Joshua Oreman
- jme_check_link(struct net_device *netdev, int testonly)
  - In this function, I believe the code and string data used in constructing linkmsg will make it into even non-debug versions of the executable. If that's true it would be better if you could perform all the manipulations in the final DBG() line, using the ?: operator and such. It's possible the compiler is clever enough to know what strcat() does and see that linkmsg[] is not used in non-debug, though, so this may be moot.

From Michael Brown

if (memcmp(addr, netdev→hw_addr, ETH_ALEN))
- No need for this compare; just always write the MAC address. (This code will fail if you set the MAC to a non-default value, then try to set it back.)

Code cleanups

gPXE dose not use any DMA address that is above 4G

#if __x86_64__
     rxdesc->desc1.flags     = RXFLAG_64BIT;
#endif

Use struct ethhdr and eth_ntoa().
Drivers should generally not try to parse the packets they transmit and receive.

This code will break if/when software VLAN support is added.

            DBG2("Received packet: "
                 "from %02x:%02x:%02x:%02x:%02x:%02x "
                 "to %02x:%02x:%02x:%02x:%02x:%02x "
                 "type %04x\n",
                 *(uint8_t *)(rxbi->data + 6),
                 *(uint8_t *)(rxbi->data + 7),
                 *(uint8_t *)(rxbi->data + 8),
                 *(uint8_t *)(rxbi->data + 9),
                 *(uint8_t *)(rxbi->data + 10),
                 *(uint8_t *)(rxbi->data + 11),
                 *(uint8_t *)(rxbi->data + 0),
                 *(uint8_t *)(rxbi->data + 1),
                 *(uint8_t *)(rxbi->data + 2),
                 *(uint8_t *)(rxbi->data + 3),
                 *(uint8_t *)(rxbi->data + 4),
                 *(uint8_t *)(rxbi->data + 5),
                 be16_to_cpu(*(uint16_t *)(rxbi->data + 12)));

%p should be used for printing out pointer values.

    DBG2("TX buffer address: %08lx(%08lx+%x)\n",
                    (unsigned long)iob->data, mapping, len);

This will behave poorly when memory runs out. Better is to have a refill routine that runs after the poll(), and always attempts to refill up to a specified level.

    rxdesc += idx;
    if (jme_make_new_rx_buf(rxring->bufinf + idx)) {
            DBG("Dropped packet due to memory allocation error.\n");
            netdev_rx_err(netdev, NULL, -ENOMEM);
    } else {

From me:
- The refill function did not work correctly. Because of the error packet was reused instead of making a hole. This patch fixed it

Trace memory related codes

Current heap size is fixed at 128K.
Trying to figure out a way to obtain more memory, and report it to OS.
See if it is possiable to know how much heap size we can use out of code and stack.

Schedule for memory and TCP modifications

Trace how the gPXE boot, relocate, and fill segment informations.
Try to understand how Michael Brown removes the even magabyte limit.
Start a discussion on the list about how to reasonably extend gPXE's usable memory.
Patch gPXE to support large(Several MB perhaps) heap memory.
Design/discuss a TCP receive window advertisement algorithm.
Patch gPXE with TCP receive queue, SACK Support, and Window scale.
Do some benchmark against different window size, and different network environment.
Post the result and discuss it on the gpxe mailling-list.

Some notes after trace

gPXE memory layout
Good articals for understanding Gate-A20
1. WIKIPEDIA - A20 line
2. A20 - a pain from the past

Expand heap

Expand gPXE heap