Hi Jerin,
<snipped>
> > > To understand it better, Could you share more details on feedback
> > > mechanism on your application enqueue
> > >
> > > app_enqueue_v1(.., nb_seg)
> > > {
> > > /* Not enough space, Let application handle it by
> > > dropping or resubmitting */
> > > if (rte_dmadev_burst_capacity() < nb_seg)
> > > return -ENOSPC;
> > >
> > > do rte_dma_op() in loop without checking error;
> > > return 0; // Success
> > > }
> > >
> > > vs
> > > app_enqueue_v2(.., nb_seg)
> > > {
> > > int rc;
> > >
> > > rc |= rte_dma_op() in loop without checking error;
> > > return rc; // return the actual status to application
> > > if Not enough space, Let application handle it by dropping or
> > > resubmitting */ }
> > >
> > > Is app_enqueue_v1() and app_enqueue_v2() logically the same from
> > > application PoV. Right?
> > >
> > > If not, could you explain, the version you are planning to do for
> > > app_enqueue()
> >
> > The exact version can be found here :
> >
> http://patchwork.ozlabs.org/project/openvswitch/patch/20210907120021.4
> > 093604-2-sunil.pa...@intel.com/ Unfortunately, both versions are not
> > same in our case because of the SW fallback we have for ring full
> > scenario's.
> > For a packet with 8 nb_segs, if the ring has only space for 4 , we
> > would avoid this packet with app_enqueue_v1 while going ahead with an
> enqueue with app_enqueue_v2, resulting in a mix of DMA and CPU copies
> for a packet which we would want to avoid.
>
> Thanks for RFC link. Usage is clear now, Since you are checking the space in
> the completion handler, I am not sure, what is hard to get the remaining
> space, Will following logic work to find the empty space. If not, please
> discuss
> the issue with this approach. I am trying to avoid yet another fastpath API
> and complication in driver as there is element checking space in the submit
> queue and completion queue at least in our hardware.
>
> max_count = nb_desc; (power of 2)
> mask = max_count - 1;
>
> for (i = 0; I < n; i++) {
> submit_idx = rte_dma_copy();
> }
> rc = rte_dma_completed(…, &completed_idx..);
> space_in_queue = mask - ((submit_idx – completed_idx) & mask);
>
Unfortunately, space left in the device (as calculated by the app) still can
mean there is no space in the device :|
i.e its pmd dependent.
As Jiayu mentioned before:
> The fact is that it's very hard for apps to calculate the available space of
> a DMA ring.
> For DSA, the available space is decided by three factors: the number of
> available slots
> in SW ring, the max batching size of a batch descriptor, and if there are
> available batch
> descriptors. The first one is configured by SW, and apps can calculate it.
> But the second
> depends on DSA HW, and the third one is hided in DSA driver which is not
> visible to apps.
> Considering the complexity of different DMA HW, I think the best way is to
> hide all details
> inside DMA dev and provide this check capacity API for apps.
<snipped>
Thanks and regards,
Sunil
