freebsd stable - Jul 2013 - Fwd: ixgbe Jumbo race condition leading to Deadlock

In 82599, for a Jumbo packet of 9.5 K ( which consumes 5 descriptors of
2048 bytes each ), when does the Descriptor write back happen ? Does it
happen per Descriptor or once per aggregated Descriptors ? Is it possible
that all descriptors except last one to be written back and when you read
RDH register, I get the last pending descriptor waiting inside 82599.
We are using srrctl |= IXGBE_SRRCTL_DESCTYPE_ADV_ONEBUF;

In my setup, I am seeing that, I don't see EOP set even when I read 5
descriptors. Checking DD will return me an incomplete packet. What should I
do in such a case ?

References from Data sheet:
-> Checking through DD bits eliminates a potential race condition: all
descriptor data is posted internally prior to incrementing the head
register and a read of the head register could potentially pass the
descriptor waiting inside the 82599.

Regards,
Kaushal

The "potential race condition" as the data sheet puts it, is only when
you
are
trying to manage your RX ring by reading the RDH register, this is a bad
idea
anyway, none of our (Intel) drivers do this. Using the DD bit is what you
want
to do. The DD bit is set when the descriptor is written back, and that
happens
when the DMA is complete.

The packet is incomplete until the descriptor with EOP set, in my code an
mbuf chain is created, and as each new descriptor is processed the pointer
to the head of the whole chain is kept in rxbuf->fmp, thus when you get to
the EOP descriptor you will be ready to send the whole chain to the stack.

Its good that you are using ONEBUF since packet split has hardware issues
on 82599.

Are you developing a new driver, or simply having issues using mine?

Regards,

Jack



On Sun, Jul 7, 2013 at 2:24 AM, Kaushal Bhandankar <kaushalgoa at
gmail.com>wrote:
> In 82599, for a Jumbo packet of 9.5 K ( which consumes 5 descriptors of
> 2048 bytes each ), when does the Descriptor write back happen ? Does it
> happen per Descriptor or once per aggregated Descriptors ? Is it possible
> that all descriptors except last one to be written back and when you read
> RDH register, I get the last pending descriptor waiting inside 82599.
> We are using srrctl |= IXGBE_SRRCTL_DESCTYPE_ADV_ONEBUF;
>
> In my setup, I am seeing that, I don't see EOP set even when I read 5
> descriptors. Checking DD will return me an incomplete packet. What should I
> do in such a case ?
>
> References from Data sheet:
> -> Checking through DD bits eliminates a potential race condition: all
> descriptor data is posted internally prior to incrementing the head
> register and a read of the head register could potentially pass the
> descriptor waiting inside the 82599.
>
> Regards,
> Kaushal
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