PCI: pciehp: Check link state before accessing device during removal

While removing a card, we can't assume the presence to mean that the access to card is OK. That is because the cause of removal may be a link down event, and the card may still be physically present. Thus, instead of presence, use the link state to decide whether or not it is OK to access the card devices.

Here are the problem symptoms:
During the removal of a card due to link down, sometimes the following error is seen (because pciehp_unconfigure_device() reads 0xFF from bridge control register as the link is down, which cause it to assume that the VGA bit is set):

pciehp 0000:21:05.0:pcie24: pcie_isr: intr_loc 100
pciehp 0000:21:05.0:pcie24: Data Link Layer State change
pciehp 0000:21:05.0:pcie24: slot(5): Link Down event
pciehp 0000:21:05.0:pcie24: Disabling domain:bus:device=0000:60:00 pciehp 0000:21:05.0:pcie24: pciehp_unconfigure_device: domain:bus:dev = 0000:60:00 pciehp 0000:21:05.0:pcie24: Cannot remove display device 0000:60:00.0

Ofcourse, when the link comes back up, the device addition fails too:

pciehp 0000:21:05.0:pcie24: pcie_isr: intr_loc 100
pciehp 0000:21:05.0:pcie24: Data Link Layer State change
pciehp 0000:21:05.0:pcie24: pciehp_check_link_active: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: slot(5): Link Up event
pciehp 0000:21:05.0:pcie24: Enabling domain:bus:device=0000:60:00
pciehp 0000:21:05.0:pcie24: pciehp_check_link_active: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: pciehp_check_link_status: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: Device 0000:60:00.0 already exists at 0000:60:00, cannot hot-add pciehp 0000:21:05.0:pcie24: Cannot add device at 0000:60:00

The problem is not seen with this patch applied. The device removal and insertion works as expected.

Signed-off-by Rajat Jain <[email protected]>

Signed-off-by Rajat Jain <[email protected]>

Signed-off-by Guenter Roeck <[email protected]>

---
v2: Use the already initialized "ctrl" instead of "p_slot->ctrl"

 drivers/pci/hotplug/pciehp_pci.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

drivers/pci/hotplug/pciehp_pci.c +4 -5

index 9e69403..911f85b 100644
@@ -77,7 +77,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
 {
        int rc = 0;
        u8 bctl = 0;
-       u8 presence = 0;
+       bool link_active = false;
        struct pci_dev *dev, *temp;
        struct pci_bus *parent = p_slot->ctrl->pcie->port->subordinate;
        u16 command;

@@ -85,7 +85,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
 
        ctrl_dbg(ctrl, "%s: domain:bus:dev = %04x:%02x:00\n",
                 __func__, pci_domain_nr(parent), parent->number);
-       pciehp_get_adapter_status(p_slot, &presence);
+       link_active = pciehp_check_link_active(ctrl);
 
        pci_lock_rescan_remove();
 

@@ -98,7 +98,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
        list_for_each_entry_safe_reverse(dev, temp, &parent->devices,
                                         bus_list) {
                pci_dev_get(dev);
-               if (dev->hdr_type == PCI_HEADER_TYPE_BRIDGE && presence) {
+               if (dev->hdr_type == PCI_HEADER_TYPE_BRIDGE && link_active) {
                        pci_read_config_byte(dev, PCI_BRIDGE_CONTROL, &bctl);
                        if (bctl & PCI_BRIDGE_CTL_VGA) {
                                ctrl_err(ctrl,

@@ -114,7 +114,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
                 * Ensure that no new Requests will be generated from
                 * the device.
                 */
-               if (presence) {
+               if (link_active) {
                        pci_read_config_word(dev, PCI_COMMAND, &command);
                        command &= ~(PCI_COMMAND_MASTER | PCI_COMMAND_SERR);
                        command |= PCI_COMMAND_INTX_DISABLE;
--

1.7.9.5

While removing a card, we can't assume the presence to mean that the access to card is OK. That is because the cause of removal may be a link down event, and the card may still be physically present. Thus, instead of presence, use the link state to decide whether or not it is OK to access the card devices.

Here are the problem symptoms:
During the removal of a card due to link down, sometimes the following error is seen (because pciehp_unconfigure_device() reads 0xFF from bridge control register as the link is down, which cause it to assume that the VGA bit is set):

pciehp 0000:21:05.0:pcie24: pcie_isr: intr_loc 100
pciehp 0000:21:05.0:pcie24: Data Link Layer State change
pciehp 0000:21:05.0:pcie24: slot(5): Link Down event
pciehp 0000:21:05.0:pcie24: Disabling domain:bus:device=0000:60:00 pciehp 0000:21:05.0:pcie24: pciehp_unconfigure_device: domain:bus:dev = 0000:60:00 pciehp 0000:21:05.0:pcie24: Cannot remove display device 0000:60:00.0

Ofcourse, when the link comes back up, the device addition fails too:

pciehp 0000:21:05.0:pcie24: pcie_isr: intr_loc 100
pciehp 0000:21:05.0:pcie24: Data Link Layer State change
pciehp 0000:21:05.0:pcie24: pciehp_check_link_active: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: slot(5): Link Up event
pciehp 0000:21:05.0:pcie24: Enabling domain:bus:device=0000:60:00
pciehp 0000:21:05.0:pcie24: pciehp_check_link_active: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: pciehp_check_link_status: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: Device 0000:60:00.0 already exists at 0000:60:00, cannot hot-add pciehp 0000:21:05.0:pcie24: Cannot add device at 0000:60:00

The problem is not seen with this patch applied. The device removal and insertion works as expected.

Signed-off-by Rajat Jain <[email protected]>

Signed-off-by Rajat Jain <[email protected]>

Signed-off-by Guenter Roeck <[email protected]>

---
v2: Use the already initialized "ctrl" instead of "p_slot->ctrl"

 drivers/pci/hotplug/pciehp_pci.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

drivers/pci/hotplug/pciehp_pci.c +2 -2

index 9e69403..911f85b 100644
@@ -77,7 +77,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
 {
 	int rc = 0;
 	u8 bctl = 0;
-	u8 presence = 0;
+	bool link_active = false;
 	struct pci_dev *dev, *temp;
 	struct pci_bus *parent = p_slot->ctrl->pcie->port->subordinate;
 	u16 command;

@@ -85,7 +85,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
 
 	ctrl_dbg(ctrl, "%s: domain:bus:dev = %04x:%02x:00\n",
 		 __func__, pci_domain_nr(parent), parent->number);
-	pciehp_get_adapter_status(p_slot, &presence);
+	link_active = pciehp_check_link_active(ctrl);
 
 	pci_lock_rescan_remove();
 

drivers/pci/hotplug/pciehp_pci.c +1 -1

@@ -114,7 +114,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
 		 * Ensure that no new Requests will be generated from
 		 * the device.
 		 */
-		if (presence) {
+		if (link_active) {
 			pci_read_config_word(dev, PCI_COMMAND, &command);
 			command &= ~(PCI_COMMAND_MASTER | PCI_COMMAND_SERR);
 			command |= PCI_COMMAND_INTX_DISABLE;

Signature · 1 line

1.7.9.5

Quoted 65 lines · Rajat Jain

While removing a card, we can't assume the presence to mean that the access to card is OK. That is because the cause of removal may be a link down event, and the card may still be physically present. Thus, instead of presence, use the link state to decide whether or not it is OK to access the card devices.

Here are the problem symptoms:
During the removal of a card due to link down, sometimes the following error is seen (because pciehp_unconfigure_device() reads 0xFF from bridge control register as the link is down, which cause it to assume that the VGA bit is set):

pciehp 0000:21:05.0:pcie24: pcie_isr: intr_loc 100
pciehp 0000:21:05.0:pcie24: Data Link Layer State change
pciehp 0000:21:05.0:pcie24: slot(5): Link Down event
pciehp 0000:21:05.0:pcie24: Disabling domain:bus:device=0000:60:00 pciehp 0000:21:05.0:pcie24: pciehp_unconfigure_device: domain:bus:dev = 0000:60:00 pciehp 0000:21:05.0:pcie24: Cannot remove display device 0000:60:00.0

Ofcourse, when the link comes back up, the device addition fails too:

pciehp 0000:21:05.0:pcie24: pcie_isr: intr_loc 100
pciehp 0000:21:05.0:pcie24: Data Link Layer State change
pciehp 0000:21:05.0:pcie24: pciehp_check_link_active: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: slot(5): Link Up event
pciehp 0000:21:05.0:pcie24: Enabling domain:bus:device=0000:60:00
pciehp 0000:21:05.0:pcie24: pciehp_check_link_active: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: pciehp_check_link_status: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: Device 0000:60:00.0 already exists at 0000:60:00, cannot hot-add pciehp 0000:21:05.0:pcie24: Cannot add device at 0000:60:00

The problem is not seen with this patch applied. The device removal and insertion works as expected.

Signed-off-by Rajat Jain <[email protected]>

Signed-off-by Rajat Jain <[email protected]>

Signed-off-by Guenter Roeck <[email protected]>

---
v2: Use the already initialized "ctrl" instead of "p_slot->ctrl"

 drivers/pci/hotplug/pciehp_pci.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

drivers/pci/hotplug/pciehp_pci.c +2 -2

index 9e69403..911f85b 100644
@@ -77,7 +77,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
 {
 	int rc = 0;
 	u8 bctl = 0;
-	u8 presence = 0;
+	bool link_active = false;
 	struct pci_dev *dev, *temp;
 	struct pci_bus *parent = p_slot->ctrl->pcie->port->subordinate;
 	u16 command;

@@ -85,7 +85,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
 
 	ctrl_dbg(ctrl, "%s: domain:bus:dev = %04x:%02x:00\n",
 		 __func__, pci_domain_nr(parent), parent->number);
-	pciehp_get_adapter_status(p_slot, &presence);
+	link_active = pciehp_check_link_active(ctrl);
 
 	pci_lock_rescan_remove();
 

drivers/pci/hotplug/pciehp_pci.c int pciehp_unconfigure_device(struct slot *p_slot) · line 104

Quoted 9 lines

drivers/pci/hotplug/pciehp_pci.c +1 -1

@@ -98,7 +98,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
 	list_for_each_entry_safe_reverse(dev, temp, &parent->devices,
 					 bus_list) {
 		pci_dev_get(dev);
-		if (dev->hdr_type == PCI_HEADER_TYPE_BRIDGE && presence) {
+		if (dev->hdr_type == PCI_HEADER_TYPE_BRIDGE && link_active) {
 			pci_read_config_byte(dev, PCI_BRIDGE_CONTROL, &bctl);
 			if (bctl & PCI_BRIDGE_CTL_VGA) {
 				ctrl_err(ctrl,

Why do we even have this code to check for VGA devices? I looked (briefly) and couldn't find anything in the spec that prohibits removal of VGA devices.

If we do need it (and it looks like most or all hotplug drivers copied it), isn't there still a race? Can't we have the following sequence?

- pciehp_check_link_active() # returns true
- Link goes down
- pci_read_config_byte() # fails because link is down

Quoted 66 lines · Bjorn Helgaas

Quoted 65 lines · Rajat Jain

While removing a card, we can't assume the presence to mean that the access to card is OK. That is because the cause of removal may be a link down event, and the card may still be physically present. Thus, instead of presence, use the link state to decide whether or not it is OK to access the card devices.

Here are the problem symptoms:
During the removal of a card due to link down, sometimes the following error is seen (because pciehp_unconfigure_device() reads 0xFF from bridge control register as the link is down, which cause it to assume that the VGA bit is set):

pciehp 0000:21:05.0:pcie24: pcie_isr: intr_loc 100
pciehp 0000:21:05.0:pcie24: Data Link Layer State change
pciehp 0000:21:05.0:pcie24: slot(5): Link Down event
pciehp 0000:21:05.0:pcie24: Disabling domain:bus:device=0000:60:00 pciehp 0000:21:05.0:pcie24: pciehp_unconfigure_device: domain:bus:dev = 0000:60:00 pciehp 0000:21:05.0:pcie24: Cannot remove display device 0000:60:00.0

Ofcourse, when the link comes back up, the device addition fails too:

pciehp 0000:21:05.0:pcie24: pcie_isr: intr_loc 100
pciehp 0000:21:05.0:pcie24: Data Link Layer State change
pciehp 0000:21:05.0:pcie24: pciehp_check_link_active: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: slot(5): Link Up event
pciehp 0000:21:05.0:pcie24: Enabling domain:bus:device=0000:60:00
pciehp 0000:21:05.0:pcie24: pciehp_check_link_active: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: pciehp_check_link_status: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: Device 0000:60:00.0 already exists at 0000:60:00, cannot hot-add pciehp 0000:21:05.0:pcie24: Cannot add device at 0000:60:00

The problem is not seen with this patch applied. The device removal and insertion works as expected.

Signed-off-by Rajat Jain <[email protected]>

Signed-off-by Rajat Jain <[email protected]>

Signed-off-by Guenter Roeck <[email protected]>

---
v2: Use the already initialized "ctrl" instead of "p_slot->ctrl"

 drivers/pci/hotplug/pciehp_pci.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

drivers/pci/hotplug/pciehp_pci.c +2 -2

index 9e69403..911f85b 100644
@@ -77,7 +77,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
 {
    int rc = 0;
    u8 bctl = 0;
-   u8 presence = 0;
+   bool link_active = false;
    struct pci_dev *dev, *temp;
    struct pci_bus *parent = p_slot->ctrl->pcie->port->subordinate;
    u16 command;

@@ -85,7 +85,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
 
    ctrl_dbg(ctrl, "%s: domain:bus:dev = %04x:%02x:00\n",
             __func__, pci_domain_nr(parent), parent->number);
-   pciehp_get_adapter_status(p_slot, &presence);
+   link_active = pciehp_check_link_active(ctrl);
 
    pci_lock_rescan_remove();
 

Quoted 14 lines

drivers/pci/hotplug/pciehp_pci.c int pciehp_unconfigure_device(struct slot *p_slot) · line 104

Quoted 9 lines

drivers/pci/hotplug/pciehp_pci.c +1 -1

@@ -98,7 +98,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
    list_for_each_entry_safe_reverse(dev, temp, &parent->devices,
                                     bus_list) {
            pci_dev_get(dev);
-           if (dev->hdr_type == PCI_HEADER_TYPE_BRIDGE && presence) {
+           if (dev->hdr_type == PCI_HEADER_TYPE_BRIDGE && link_active) {
                    pci_read_config_byte(dev, PCI_BRIDGE_CONTROL, &bctl);
                    if (bctl & PCI_BRIDGE_CTL_VGA) {
                            ctrl_err(ctrl,

Why do we even have this code to check for VGA devices? I looked (briefly) and couldn't find anything in the spec that prohibits removal of VGA devices.

For my part I don't know. I only know that I had to integrate the patch into our images since I hit the problem repeatedly. Usually I wait with integrating Rajat's patches until you accept them, but this one was too disruptive.

I would argue that while the patch may not be perfect, at least it improves the situation substantially.

Quoted 7 lines

If we do need it (and it looks like most or all hotplug drivers copied it), isn't there still a race? Can't we have the following sequence?

- pciehp_check_link_active() # returns true
- Link goes down
- pci_read_config_byte() # fails because link is down

I would guess so. Question is how to address it. Read the configuration byte first, then check if the link is down ? Check if link is still up after reading the configuration byte ? Add a note that there may be a potential race condition and do nothing until it is actually seen ?

Quoted 16 lines · Bjorn Helgaas

Quoted 6 lines · Guenter Roeck

Quoted 5 lines · Bjorn Helgaas

On Thu, Nov 20, 2014 at 02:54:24PM -0800, Rajat Jain wrote:

If we do need it (and it looks like most or all hotplug drivers copied it), isn't there still a race? Can't we have the following sequence?

Quoted 8 lines

Quoted 4 lines

- pciehp_check_link_active() # returns true
- Link goes down
- pci_read_config_byte() # fails because link is down

I would guess so. Question is how to address it. Read the configuration byte first, then check if the link is down ? Check if link is still up after reading the configuration byte ? Add a note that there may be a potential race condition and do nothing until it is actually seen ?

Quoted 5 lines

I think we should just read PCI_BRIDGE_CONTROL and look for a 0xff value. That's not a legal value for the register, so if we see it, it should be pretty safe to assume the link is down or the device is not present at all.

Something like

        if (bctl != 0xff && (bctl & PCI_BRIDGE_CTL_VGA)) {

in addition to Rajat's changes ?

I think it would be good to keep the change Rajat proposed, ie to check the link state instead of presence. Question then is if you'd want a new revision of Rajat's patch or another patch on top of it with the bctl related change.

Quoted 85 lines · Guenter Roeck

Quoted 66 lines · Bjorn Helgaas

Quoted 65 lines · Rajat Jain

While removing a card, we can't assume the presence to mean that the access to card is OK. That is because the cause of removal may be a link down event, and the card may still be physically present. Thus, instead of presence, use the link state to decide whether or not it is OK to access the card devices.

Here are the problem symptoms:
During the removal of a card due to link down, sometimes the following error is seen (because pciehp_unconfigure_device() reads 0xFF from bridge control register as the link is down, which cause it to assume that the VGA bit is set):

pciehp 0000:21:05.0:pcie24: pcie_isr: intr_loc 100
pciehp 0000:21:05.0:pcie24: Data Link Layer State change
pciehp 0000:21:05.0:pcie24: slot(5): Link Down event
pciehp 0000:21:05.0:pcie24: Disabling domain:bus:device=0000:60:00 pciehp 0000:21:05.0:pcie24: pciehp_unconfigure_device: domain:bus:dev = 0000:60:00 pciehp 0000:21:05.0:pcie24: Cannot remove display device 0000:60:00.0

Ofcourse, when the link comes back up, the device addition fails too:

pciehp 0000:21:05.0:pcie24: pcie_isr: intr_loc 100
pciehp 0000:21:05.0:pcie24: Data Link Layer State change
pciehp 0000:21:05.0:pcie24: pciehp_check_link_active: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: slot(5): Link Up event
pciehp 0000:21:05.0:pcie24: Enabling domain:bus:device=0000:60:00
pciehp 0000:21:05.0:pcie24: pciehp_check_link_active: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: pciehp_check_link_status: lnk_status = 6011 pciehp 0000:21:05.0:pcie24: Device 0000:60:00.0 already exists at 0000:60:00, cannot hot-add pciehp 0000:21:05.0:pcie24: Cannot add device at 0000:60:00

The problem is not seen with this patch applied. The device removal and insertion works as expected.

Signed-off-by Rajat Jain <[email protected]>

Signed-off-by Rajat Jain <[email protected]>

Signed-off-by Guenter Roeck <[email protected]>

---
v2: Use the already initialized "ctrl" instead of "p_slot->ctrl"

 drivers/pci/hotplug/pciehp_pci.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

drivers/pci/hotplug/pciehp_pci.c +2 -2

index 9e69403..911f85b 100644
@@ -77,7 +77,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
 {
    int rc = 0;
    u8 bctl = 0;
-   u8 presence = 0;
+   bool link_active = false;
    struct pci_dev *dev, *temp;
    struct pci_bus *parent = p_slot->ctrl->pcie->port->subordinate;
    u16 command;

@@ -85,7 +85,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
 
    ctrl_dbg(ctrl, "%s: domain:bus:dev = %04x:%02x:00\n",
             __func__, pci_domain_nr(parent), parent->number);
-   pciehp_get_adapter_status(p_slot, &presence);
+   link_active = pciehp_check_link_active(ctrl);
 
    pci_lock_rescan_remove();
 

Quoted 14 lines

drivers/pci/hotplug/pciehp_pci.c int pciehp_unconfigure_device(struct slot *p_slot) · line 104

Quoted 9 lines

drivers/pci/hotplug/pciehp_pci.c +1 -1

@@ -98,7 +98,7 @@ int pciehp_unconfigure_device(struct slot *p_slot)
    list_for_each_entry_safe_reverse(dev, temp, &parent->devices,
                                     bus_list) {
            pci_dev_get(dev);
-           if (dev->hdr_type == PCI_HEADER_TYPE_BRIDGE && presence) {
+           if (dev->hdr_type == PCI_HEADER_TYPE_BRIDGE && link_active) {
                    pci_read_config_byte(dev, PCI_BRIDGE_CONTROL, &bctl);
                    if (bctl & PCI_BRIDGE_CTL_VGA) {
                            ctrl_err(ctrl,

Why do we even have this code to check for VGA devices? I looked (briefly) and couldn't find anything in the spec that prohibits removal of VGA devices.

For my part I don't know. I only know that I had to integrate the patch into our images since I hit the problem repeatedly. Usually I wait with integrating Rajat's patches until you accept them, but this one was too disruptive.

Quoted 2 lines

I would argue that while the patch may not be perfect, at least it improves the situation substantially.

I don't think removing the VGA checks is the way to fix the problem you're seeing. But I do want to investigate this code since we're in the area.

Quoted 11 lines

Quoted 7 lines

If we do need it (and it looks like most or all hotplug drivers copied it), isn't there still a race? Can't we have the following sequence?

- pciehp_check_link_active() # returns true
- Link goes down
- pci_read_config_byte() # fails because link is down

I would guess so. Question is how to address it. Read the configuration byte first, then check if the link is down ? Check if link is still up after reading the configuration byte ? Add a note that there may be a potential race condition and do nothing until it is actually seen ?

I think we should just read PCI_BRIDGE_CONTROL and look for a 0xff value. That's not a legal value for the register, so if we see it, it should be pretty safe to assume the link is down or the device is not present at all.

Quoted 22 lines · Guenter Roeck

Quoted 16 lines · Bjorn Helgaas

Quoted 6 lines · Guenter Roeck

Quoted 5 lines · Bjorn Helgaas

On Thu, Nov 20, 2014 at 02:54:24PM -0800, Rajat Jain wrote:

If we do need it (and it looks like most or all hotplug drivers copied it), isn't there still a race? Can't we have the following sequence?

Quoted 8 lines

Quoted 4 lines

- pciehp_check_link_active() # returns true
- Link goes down
- pci_read_config_byte() # fails because link is down

I would guess so. Question is how to address it. Read the configuration byte first, then check if the link is down ? Check if link is still up after reading the configuration byte ? Add a note that there may be a potential race condition and do nothing until it is actually seen ?

Quoted 5 lines

I think we should just read PCI_BRIDGE_CONTROL and look for a 0xff value. That's not a legal value for the register, so if we see it, it should be pretty safe to assume the link is down or the device is not present at all.

Quoted 8 lines

Something like

        if (bctl != 0xff && (bctl & PCI_BRIDGE_CTL_VGA)) {

in addition to Rajat's changes ?

I think it would be good to keep the change Rajat proposed, ie to check the link state instead of presence. Question then is if you'd want a new revision of Rajat's patch or another patch on top of it with the bctl related change.

Why do we need the link state or the presence check? It seems like those are sort of a 90% solution, and doing them provides the illusion of value but without real value. If we think that checking for 0xff is a 100% solution, we should rely on that and not bother with anything else.