sfc: Correct reporting and validation of TX interrupt coalescing

The reported TX IRQ moderation is generated in a completely crazy way.
Make it simple and correct.

When channels are shared between RX and TX, TX IRQ moderation must be
the same as RX IRQ moderation, but must be specified as 0!  Allow it
to be either specified as the same, or left at its previous value
in which case it will be quietly overridden.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/sfc/efx.c b/drivers/net/ethernet/sfc/efx.c
index 097ed8b..e0157c0 100644
--- a/drivers/net/ethernet/sfc/efx.c
+++ b/drivers/net/ethernet/sfc/efx.c
@@ -1585,6 +1585,24 @@
 	}
 }
 
+void efx_get_irq_moderation(struct efx_nic *efx, unsigned int *tx_usecs,
+			    unsigned int *rx_usecs, bool *rx_adaptive)
+{
+	*rx_adaptive = efx->irq_rx_adaptive;
+	*rx_usecs = efx->irq_rx_moderation * EFX_IRQ_MOD_RESOLUTION;
+
+	/* If channels are shared between RX and TX, so is IRQ
+	 * moderation.  Otherwise, IRQ moderation is the same for all
+	 * TX channels and is not adaptive.
+	 */
+	if (efx->tx_channel_offset == 0)
+		*tx_usecs = *rx_usecs;
+	else
+		*tx_usecs =
+			efx->channel[efx->tx_channel_offset]->irq_moderation *
+			EFX_IRQ_MOD_RESOLUTION;
+}
+
 /**************************************************************************
  *
  * Hardware monitor