Re: [PATCH 4/4] ethdev: check support for rx_queue_count and descriptor_done fns

[Thomas Monjalon <thomas.monjalon@6wind.com>]

Neil, Bruce,
Can we move forward?

2015-07-06 17:11, Thomas Monjalon:
> Neil, your ABI expertise is required for this patch.
> 
> 2015-06-15 11:14, Bruce Richardson:
> > On Fri, Jun 12, 2015 at 01:32:56PM -0400, Roger B. Melton wrote:
> > > Hi Bruce,  Comment in-line.  Regards, Roger
> > > 
> > > On 6/12/15 7:28 AM, Bruce Richardson wrote:
> > > >The functions rte_eth_rx_queue_count and rte_eth_descriptor_done are
> > > >supported by very few PMDs. Therefore, it is best to check for support
> > > >for the functions in the ethdev library, so as to avoid crashes
> > > >at run-time if the application goes to use those APIs. The performance
> > > >impact of this change should be very small as this is a predictable
> > > >branch in the function.
> > > >
> > > >Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> > > >---
> > > >  lib/librte_ether/rte_ethdev.h | 8 ++++++--
> > > >  1 file changed, 6 insertions(+), 2 deletions(-)
> > > >
> > > >diff --git a/lib/librte_ether/rte_ethdev.h b/lib/librte_ether/rte_ethdev.h
> > > >index 827ca3e..9ad1b6a 100644
> > > >--- a/lib/librte_ether/rte_ethdev.h
> > > >+++ b/lib/librte_ether/rte_ethdev.h
> > > >@@ -2496,6 +2496,8 @@ rte_eth_rx_burst(uint8_t port_id, uint16_t queue_id,
> > > >   *  The queue id on the specific port.
> > > >   * @return
> > > >   *  The number of used descriptors in the specific queue.
> > > >+ *  NOTE: if function is not supported by device this call
> > > >+ *        returns (uint32_t)-ENOTSUP
> > > >   */
> > > >  static inline uint32_t
> > > 
> > > Why not change the return type to int32_t?
> > > In this way, the caller isn't required to make the assumption that a large
> > > queue count indicates an error.  < 0 means error, other wise it's a valid
> > > queue count.
> > > 
> > > This approach would be consistent with other APIs.
> > > 
> > 
> > Yes, good point, I should see about that. One thing I'm unsure of, though, is
> > does this count as ABI breakage? I don't see how it should break any older
> > apps, since the return type is the same size, but I'm not sure as we are 
> > changing the return type of the function.
> > 
> > Neil, can you perhaps comment here? Is changing uint32_t to int32_t ok, from
> > an ABI point of view?
> > 
> > Regards,
> > /Bruce
> 
>