Jesse Brandeburg wrote:
On 5/2/06, Ben Greear <[EMAIL PROTECTED]> wrote:
In commit: a292ca6efbc1f259ddfb9c902367f2588e0e8b0f
to e1000_main.c, there is the change below.
I am curious why the skb_put no longer subtracts ETHERNET_FCS_SIZE
from the length. Is the idea that we will now always include the
FCS at the end of the skb?
This is a long and hairy story behind this, but there is a bit called
SECRC that controls hardware stripping of the CRC. In *this* driver
we turned that bit on, so that we didn't have to mess with skb->len -=
4 and the nasty negative unwrap if we were using multiple descriptors
for rx.
Since then, we've removed multiple descriptor rx.
And after that, I've discovered that some BMCs are very unhappy if we
strip CRCs using SECRC.
So, my related question is: is it okay if we just always leave the CRC
on the end of the data? It doesn't seem to harm anything.
I believe it might mess up bridging logic if it tries to send the entire
skb to a peer interface, ie cause an extra 4 bytes to be written.
I know that this 'save-crc' feature did mess up my particular bridge-like
thing, but that could have been just bugs in my code.
Also, if the CRC data is there, but it is never 'put' into the skb, then
it will be correctly ignored by the stacks, including bridging, and hacks
such as my own that simply increase the skb-put by 4 bytes will work.
My personal preference is to set a flag in the skb struct indicating whether
or not the crc is appended (and skb_put). Then, bridging code can ignore it if
needed,
and sniffers and such can get the CRC in user-land. To remain backwards compat,
at least the skb-put of the crc logic should default to OFF so that we don't
break any existing user-land bridging logic. I have the ethtool API logic
written to
twiddle this save-crc behaviour if someone decides this is worthy of the kernel.
Well, its a changing picture. I had planned to eventually enable the
hardware to strip the CRC if we aren't connected to some kind of
offboard management. We'll get there in steps.
So, as of 2.6.16.13, is the hardware stripping (SERC) enabled? Could
you also let me know where this bit is defined in case I want to twiddle
it myself (a quick grep for SERC in 2.6.16.13 yields nothing.)
Thanks,
Ben
--
Ben Greear <[EMAIL PROTECTED]>
Candela Technologies Inc http://www.candelatech.com
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