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

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

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