Under normal circumstances GenetSimpleNetworkTransmit won't be
called unless the rest of the network stack detects the link is
up. So, during normal operation when the adapter is initialized
the link naturally transitions to link up, and then is ready for
activity later in the boot sequence. If that hasn't happened by
the time PXE runs then it will itself wait for the link.

OTOH, the normal distro PXE sequence involves PXE loading shim
which in turn loads grub, which tries to read machine specific
configs, modules, and grub.cfg in order to prepare the boot menu.
Then, once a grub selection is picked, it might try to load the
kernel+initrd.

In this sequence the network stack is shutdown and restarted
multiple times. Grub though, starts up, notices its been network
booted, reads saved network parameters and immediately tries to
transmit data assuming the link is still up.

When that happens grub will print "couldn't send network packet"
and if that lasts long enough it fails to load grub.cfg and the
user gets dropped to the grub prompt because no one in the path
bothers to assure the link state has transitioned back up.

For reference: https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fpftf%2FRPi4%2Fissues%2F113&data=04%7C01%7Cawarkentin%40vmware.com%7Cadf7e9a53f9b4c6cff9608d90043c562%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C637541113365821315%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=Gdg67x%2Fv75%2B5r9bWhz8SvB%2B4VBTgZ6sEZ9SeYor%2B02E%3D&reserved=0

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
---
 .../Drivers/Net/BcmGenetDxe/SimpleNetwork.c        | 24 ++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/Silicon/Broadcom/Drivers/Net/BcmGenetDxe/SimpleNetwork.c b/Silicon/Broadcom/Drivers/Net/BcmGenetDxe/SimpleNetwork.c
index 1bda18f157..29c76d8495 100644
--- a/Silicon/Broadcom/Drivers/Net/BcmGenetDxe/SimpleNetwork.c
+++ b/Silicon/Broadcom/Drivers/Net/BcmGenetDxe/SimpleNetwork.c
@@ -13,6 +13,7 @@
 #include <Library/DebugLib.h>
 #include <Library/DmaLib.h>
 #include <Library/NetLib.h>
+#include <Library/UefiBootServicesTableLib.h>
 #include <Protocol/SimpleNetwork.h>
 
 #include "BcmGenetDxe.h"
@@ -590,9 +591,28 @@ GenetSimpleNetworkTransmit (
 
   if (!Genet->SnpMode.MediaPresent) {
     //
-    // Don't bother transmitting if there's no link.
+    // We should only really get here if the link was up
+    // and is now down due to a stop/shutdown sequence, and
+    // the app (grub) doesn't bother to check link state
+    // because it was up a moment before.
+    // Lets wait a bit for the link to resume, rather than
+    // failing to send. In the case of grub it works either way
+    // but we can't be sure that is universally true, and
+    // hanging for a couple seconds is nicer than a screen of
+    // grub send failure messages.
     //
-    return EFI_NOT_READY;
+    int retries = 1000;
+    DEBUG ((DEBUG_INFO, "%a: Waiting 10s for link\n", __FUNCTION__));
+    do {
+      gBS->Stall (10000);
+      Status = GenericPhyUpdateConfig (&Genet->Phy);
+    } while (EFI_ERROR (Status) && retries--);
+    if (EFI_ERROR (Status)) {
+      DEBUG ((DEBUG_ERROR, "%a: no link\n", __FUNCTION__));
+      return EFI_NOT_READY;
+    } else {
+      Genet->SnpMode.MediaPresent = TRUE;
+    }
   }
 
   if (HeaderSize != 0) {
--
2.13.7