[PATCH] dma/idxd: fix burst capacity calculation

[Bruce Richardson <bruce.richardson@intel.com>]

When the maximum burst size supported by HW is less than the available
ring space, incorrect capacity was returned when there was already some
jobs queued up for submission. This was because the capacity calculation
failed to subtract the number of already-enqueued jobs from the max
burst size. After subtraction is done, ensure that any negative values
(which should never occur if the user respects the reported limits), are
clamped to zero.

Fixes: 9459de4edc99 ("dma/idxd: add burst capacity")
Cc: kevin.laatz@intel.com
Cc: stable@dpdk.org

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---
 drivers/dma/idxd/idxd_common.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/dma/idxd/idxd_common.c b/drivers/dma/idxd/idxd_common.c
index fc11b11337..4442d1cbbd 100644
--- a/drivers/dma/idxd/idxd_common.c
+++ b/drivers/dma/idxd/idxd_common.c
@@ -485,7 +485,9 @@ idxd_burst_capacity(const void *dev_private, uint16_t vchan __rte_unused)
 		write_idx += idxd->desc_ring_mask + 1;
 	used_space = write_idx - idxd->ids_returned;

-	return RTE_MIN((idxd->desc_ring_mask - used_space), idxd->max_batch_size);
+	const int ret = RTE_MIN((idxd->desc_ring_mask - used_space),
+			(idxd->max_batch_size - idxd->batch_size));
+	return ret < 0 ? 0 : (uint16_t)ret;
 }

 int
--
2.32.0