Restore optoe page to default 0 for active cables (sonic-net#550)

<!-- Provide a general summary of your changes in the Title above -->

#### Description
Restore the optoe's page to page ZERO before reading the Lower page > 127

#### Motivation and Context
When the module is busy with CDB command, the optoe kernel driver at the end of user page read request, tries to restore the page to Page ZERO and if this write fails because the module could be busy with CDB command (since these modules donot support background CDB mode). Because of this when Xcvrd tries to read offset (> 127) in Lower page, since optoe does not restore the page to 0 for any Lower page access, the page read to these offset will result in wrong page to be read (because of previous page select write failure) and Xcvrd parsing fails causing crash.

Now in optoe EEPROM read API we ensure to restore the Page to ZERO page if the Page select byte is non-zero

#### How Has This Been Tested?
TBD:
1. Test on Arista switch
2. Test on Cisco switch

#### Additional Information (Optional)

Author: mssonicbld

sonic_platform_base/sonic_xcvr/sfp_optoe_base.py

@@ -7,6 +7,10 @@
 
 from ..sfp_base import SfpBase
 
+SFP_OPTOE_PAGE_SELECT_OFFSET = 127
+SFP_OPTOE_UPPER_PAGE0_OFFSET = 128
+SFP_OPTOE_PAGE_SIZE = 128
+
 class SfpOptoeBase(SfpBase):
     def __init__(self):
         SfpBase.__init__(self)
@@ -261,6 +265,12 @@ def set_optoe_write_max(self, write_max):
         except (OSError, IOError):
             pass
 
+    def get_optoe_current_page(self):
+        return self.read_eeprom(SFP_OPTOE_PAGE_SELECT_OFFSET, 1)[0]
+
+    def set_page0(self):
+        self.write_eeprom(SFP_OPTOE_PAGE_SELECT_OFFSET, 1, bytearray([0x00]))
+
     def set_optoe_write_timeout(self, write_timeout):
         sys_path = self.get_eeprom_path()
         sys_path = sys_path.replace("eeprom", "write_timeout")
@@ -273,6 +283,12 @@ def set_optoe_write_timeout(self, write_timeout):
     def read_eeprom(self, offset, num_bytes):
         try:
             with open(self.get_eeprom_path(), mode='rb', buffering=0) as f:
+                if offset >= SFP_OPTOE_UPPER_PAGE0_OFFSET  and \
+                    offset < (SFP_OPTOE_UPPER_PAGE0_OFFSET+SFP_OPTOE_PAGE_SIZE) and \
+                        self.get_optoe_current_page() != 0:
+                    # Restoring the page to 0 helps in cases where the optoe driver failed to restore
+                    # the page when say the module was busy with CDB command processing
+                   self.set_page0()
                 f.seek(offset)
                 return bytearray(f.read(num_bytes))
         except (OSError, IOError):

tests/sonic_xcvr/test_sfp_optoe_base.py

@@ -2,7 +2,7 @@
 from mock import MagicMock 
 from mock import patch 
 import pytest 
-from sonic_platform_base.sonic_xcvr.sfp_optoe_base import SfpOptoeBase 
+from sonic_platform_base.sonic_xcvr.sfp_optoe_base import SfpOptoeBase, SFP_OPTOE_UPPER_PAGE0_OFFSET, SFP_OPTOE_PAGE_SELECT_OFFSET
 from sonic_platform_base.sonic_xcvr.api.public.c_cmis import CCmisApi
 from sonic_platform_base.sonic_xcvr.api.public.cmis import CmisApi
 from sonic_platform_base.sonic_xcvr.mem_maps.public.c_cmis import CCmisMemMap 
@@ -132,3 +132,13 @@ def test_set_power(self):
             exception_raised = True
         assert exception_raised
 
+    def test_default_page(self):
+        with patch("builtins.open", mock_open(read_data=b'\x01')) as mocked_file:
+            self.sfp_optoe_api.write_eeprom =  MagicMock(return_value=True)
+            self.sfp_optoe_api.get_optoe_current_page = MagicMock(return_value=0x10)
+            self.sfp_optoe_api.get_eeprom_path = MagicMock(return_value='/sys/class/eeprom')
+            data = self.sfp_optoe_api.read_eeprom(SFP_OPTOE_UPPER_PAGE0_OFFSET, 1)
+            mocked_file.assert_called_once_with("/sys/class/eeprom", mode='rb', buffering=0)
+            assert data == b'\x01'
+            self.sfp_optoe_api.write_eeprom.assert_called_once_with(SFP_OPTOE_PAGE_SELECT_OFFSET, 1, b'\x00')
+            self.sfp_optoe_api.get_optoe_current_page.assert_called_once()