[PATCH net 1/4] net: ti: icssg-prueth: Fix AF_XDP fill ring alloc and wakeup condition

[Meghana Malladi <m-malladi@ti.com>]

emac_rx_packet_zc() calls prueth_rx_alloc_zc() with count (frames
received in the current NAPI poll) as the allocation budget.  Two
problems arise from this:

1. When the CPPI5 descriptor pool is exhausted (avail_desc == 0,
   FDQ already holds the maximum number of descriptors), count > 0
   still triggers allocation attempts that all fail, spamming the
   kernel log with "rx push: failed to allocate descriptor" at
   high packet rates.

2. The XSK wakeup condition "ret < count" is wrong when avail_desc
   is zero: ret == 0 and count can be up to 64, so the condition is
   always true.  This causes ~200 spurious ndo_xsk_wakeup() calls
   per second even when the FDQ is already full, wasting CPU cycles
   in repeated NAPI invocations that process zero frames.

Fix both by introducing alloc_budget = min(budget, avail_desc):
- When avail_desc == 0 no allocation is attempted, avoiding pool
  exhaustion errors.  The wakeup condition "ret < alloc_budget"
  evaluates to 0 < 0 == false, correctly clearing the wakeup flag
  so the hardware IRQ re-arms NAPI without spurious kicks.
- In steady state avail_desc == count <= budget, so alloc_budget
  == count and behaviour is unchanged.
- After a dry-ring stall (count == 0, avail_desc > 0), alloc_budget
  > 0 causes new descriptors to be posted to the FDQ so the hardware
  can resume receiving immediately.

Fixes: 7a64bb388df3 ("net: ti: icssg-prueth: Add AF_XDP zero copy for RX")
Signed-off-by: Meghana Malladi <m-malladi@ti.com>
---
 drivers/net/ethernet/ti/icssg/icssg_common.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/drivers/net/ethernet/ti/icssg/icssg_common.c b/drivers/net/ethernet/ti/icssg/icssg_common.c
index a28a608f9bf4..55a696912811 100644
--- a/drivers/net/ethernet/ti/icssg/icssg_common.c
+++ b/drivers/net/ethernet/ti/icssg/icssg_common.c
@@ -927,6 +927,7 @@ static int emac_rx_packet_zc(struct prueth_emac *emac, u32 flow_id,
 	struct cppi5_host_desc_t *desc_rx;
 	struct prueth_swdata *swdata;
 	dma_addr_t desc_dma, buf_dma;
+	int avail_desc, alloc_budget;
 	struct xdp_buff *xdp;
 	int xdp_status = 0;
 	int count = 0;
@@ -993,16 +994,13 @@ static int emac_rx_packet_zc(struct prueth_emac *emac, u32 flow_id,
 	if (xdp_status & ICSSG_XDP_REDIR)
 		xdp_do_flush();
 
-	/* Allocate xsk buffers from the pool for the "count" number of
-	 * packets processed in order to be able to receive more packets.
-	 */
-	ret = prueth_rx_alloc_zc(emac, count);
+	avail_desc = k3_cppi_desc_pool_avail(rx_chn->desc_pool);
+	alloc_budget = min_t(int, budget, avail_desc);
+
+	ret = prueth_rx_alloc_zc(emac, alloc_budget);
 
 	if (xsk_uses_need_wakeup(rx_chn->xsk_pool)) {
-		/* If the user space doesn't provide enough buffers then it must
-		 * explicitly wake up the kernel when new buffers are available
-		 */
-		if (ret < count)
+		if (ret < alloc_budget)
 			xsk_set_rx_need_wakeup(rx_chn->xsk_pool);
 		else
 			xsk_clear_rx_need_wakeup(rx_chn->xsk_pool);
-- 
2.43.0