sfc: reduce the number of requested xdp ev queues

Without this change the driver tries to allocate too many queues,
breaching the number of available msi-x interrupts on machines
with many logical cpus and default adapter settings:

Insufficient resources for 12 XDP event queues (24 other channels, max 32)

Which in turn triggers EINVAL on XDP processing:

sfc 0000:86:00.0 ext0: XDP TX failed (-22)
Signed-off-by: Ivan Babrou <ivan@cloudflare.com>
Acked-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://lore.kernel.org/r/20210120212759.81548-1-ivan@cloudflare.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/sfc/efx_channels.c

@@ -17,6 +17,7 @@
 #include "rx_common.h"
 #include "nic.h"
 #include "sriov.h"
+#include "workarounds.h"
 
 /* This is the first interrupt mode to try out of:
  * 0 => MSI-X
@@ -137,6 +138,7 @@ static int efx_allocate_msix_channels(struct efx_nic *efx,
 {
 	unsigned int n_channels = parallelism;
 	int vec_count;
+	int tx_per_ev;
 	int n_xdp_tx;
 	int n_xdp_ev;
 
@@ -149,9 +151,9 @@ static int efx_allocate_msix_channels(struct efx_nic *efx,
 	 * multiple tx queues, assuming tx and ev queues are both
 	 * maximum size.
 	 */
-
+	tx_per_ev = EFX_MAX_EVQ_SIZE / EFX_TXQ_MAX_ENT(efx);
 	n_xdp_tx = num_possible_cpus();
-	n_xdp_ev = DIV_ROUND_UP(n_xdp_tx, EFX_MAX_TXQ_PER_CHANNEL);
+	n_xdp_ev = DIV_ROUND_UP(n_xdp_tx, tx_per_ev);
 
 	vec_count = pci_msix_vec_count(efx->pci_dev);
 	if (vec_count < 0)