On 10/11/22 15:36, Ferruh Yigit wrote:
On 10/11/2022 11:48 AM, Viacheslav Galaktionov wrote:
Hi!
We're looking to implement CRC offload in our driver and we're having
difficulties understanding what
the feature changes from the application's point of view. If we
enable the KEEP_CRC offload, then the
NIC is supposed to preserve the CRC in the packet, that much is
clear. But we checked other drivers
and it seems common for PMDs to remove the CRC from the final mbufs.
Why is that?
We couldn't find any place where the CRC would be stored after
removal, so it looks like the application
doesn't have access to this piece of data. And if so, what's the
point of having this feature if the CRC
is discarded either way?
We're probably missing something and would really appreciate any help
with this.
Hi Viacheslav,
As you said default behavior is to strip the CRC from packet, even
some devices doesn't support having CRC in the packet it is removed by
HW automatically. In this case application can't access to the CRC.
For the devices that has capability to keep CRC, KEEP_CRC offload
should enable having CRC as part of the packet. There is no special
field to store the CRC.
I'm asking because I'm seeing a common pattern in the code base: if the
hardware didn't remove the CRC,
the driver does this itself. Grepping the code for "crc_len" will show
you what I mean. One of the most apparent
examples of this happening can be seen in drivers/net/e1000/em_rxtx.c:
/*
* This is the last buffer of the received packet.
* If the CRC is not stripped by the hardware:
* - Subtract the CRC length from the total packet length.
* - If the last buffer only contains the whole CRC or a part
* of it, free the mbuf associated to the last buffer.
* If part of the CRC is also contained in the previous
* mbuf, subtract the length of that CRC part from the
* data length of the previous mbuf.
*/
I don't understand why this is necessary, and whether this is just a
particularity of this driver or how the feature
is supposed to be implemented everywhere. I haven't checked every
driver, but it seems like a lot of them do
something similar to this.
