Re: [dpdk-dev] [PATCH v3 07/10] vmxnet3: support jumbo frames

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 9 Mar 2015 23:32:48 +0000
Yong Wang <yongwang@vmware.com> wrote:

> >
> >I didn’t see where mtu is used to calculate how many rx descriptors will
> >be needed for each packet.  Furthermore, as pointed out by the following
> >code comments, the device requires the first rx buffer of a packet be of
> >type VMXNET3_RXD_BTYPE_HEAD with the remaining buffers of type
> >VMXNET3_RXD_BTYPE_NODY.  This needs to be taken care of when populating rx
> >rings in vmxnet3_post_rx_bufs(). Currently we don’t do this because no
> >scatter-rx is supported and only one descriptor is needed for a packet
> >(thus all types should be HEAD). Otherwise, the device will complain with  
> 
> To clarify, in this case only the 1st ring will be used and thus all types
> will be HEAD.
> 
> >error returned.  For the 2nd rx ring, type needs to be BODY for all
> >descriptors still.

Looking in more detail, the code as submitted looks fine:

* The # of rx descriptors needed for each packet is responsibility of
  the application. Before and after this patch the # of Rx descriptors
  for each ring (head/body) is determined by vmxnet3_rx_queue_setup.
  And the driver always allocates the same number of descriptors
  for both Rx rings.

  One could argue that was a bug in the original driver, since it needlessly
  allocated mbufs for the second ring and never used them. I suppose
  as a optimization in future, the second ring could be sized as 0
  and no buffers allocated unless doing rx-scatter (this is what the
  Linux driver does. But that change is independent of this.

* Buffers are correctly reallocated as needed when consumed.

* The code works for bulk transfer and performance tests in jumbo mode.

I see no requirement to change this patch.