[efi] Do not unconditionally raise back to internal TPL

Most TPL manipulation is handled by efi_raise_tpl()/efi_restore_tpl()
pairs.  The exceptions are the places where we need to temporarily
drop to a lower TPL in order to allow a timer interrupt to occur.

These currently assume that they are called only from code that is
already running at the internal TPL (generally TPL_CALLBACK).  This
assumption is not always correct.  In particular, the call from
_efi_start() to efi_driver_reconnect_all() takes place after the SNP
devices have been released and so will be running at the external TPL.

Create an efi_drop_tpl()/efi_undrop_tpl() pair to abstract away the
temporary lowering of the TPL, and ensure that the TPL is always
raised back to its original level rather than being unconditionally
raised to the internal TPL.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/efi/efi.h

@@ -84,6 +84,12 @@ struct efi_saved_tpl {
 	EFI_TPL previous;
 };
 
+/** An EFI dropped task priority level */
+struct efi_dropped_tpl {
+	/** Current TPL */
+	EFI_TPL current;
+};
+
 /** An EFI protocol used by iPXE */
 struct efi_protocol {
 	/** GUID */
@@ -408,6 +414,8 @@ extern EFI_STATUS efi_init ( EFI_HANDLE image_handle,
 			     EFI_SYSTEM_TABLE *systab );
 extern void efi_raise_tpl ( struct efi_saved_tpl *tpl );
 extern void efi_restore_tpl ( struct efi_saved_tpl *tpl );
+extern void efi_drop_tpl ( struct efi_dropped_tpl *tpl );
+extern void efi_undrop_tpl ( struct efi_dropped_tpl *tpl );
 extern int efi_open_untyped ( EFI_HANDLE handle, EFI_GUID *protocol,
 			      void **interface );
 extern int efi_open_unsafe_untyped ( EFI_HANDLE handle, EFI_GUID *protocol,