i40e: Add a check to see if MFS is set

A customer was chain-booting to provision his systems and one of the
steps was setting MFS. MFS isn't cleared by normal warm reboots
(clearing requires a GLOBR) and there was no indication of why Jumbo
Frame receives were failing.

Add a warning if MFS is set to anything lower than the default.
Signed-off-by: NTodd Fujinaka <todd.fujinaka@intel.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40e/i40e_main.c

@@ -15290,6 +15290,14 @@ static int i40e_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
 			i40e_stat_str(&pf->hw, err),
 			i40e_aq_str(&pf->hw, pf->hw.aq.asq_last_status));
 
+	/* make sure the MFS hasn't been set lower than the default */
+#define MAX_FRAME_SIZE_DEFAULT 0x2600
+	val = (rd32(&pf->hw, I40E_PRTGL_SAH) &
+	       I40E_PRTGL_SAH_MFS_MASK) >> I40E_PRTGL_SAH_MFS_SHIFT;
+	if (val < MAX_FRAME_SIZE_DEFAULT)
+		dev_warn(&pdev->dev, "MFS for port %x has been set below the default: %x\n",
+			 i, val);
+
 	/* Add a filter to drop all Flow control frames from any VSI from being
 	 * transmitted. By doing so we stop a malicious VF from sending out
 	 * PAUSE or PFC frames and potentially controlling traffic for other