PCI/MSI: Add pci_msi_vec_count()

Device drivers can use this interface to obtain the maximum number of MSI
interrupts the device supports and use that number, e.g., in a subsequent
call to pci_enable_msi_block().
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Tejun Heo <tj@kernel.org>

Documentation/PCI/MSI-HOWTO.txt

@@ -169,6 +169,21 @@ on any interrupt for which it previously called request_irq().
 Failure to do so results in a BUG_ON(), leaving the device with
 MSI enabled and thus leaking its vector.
 
+4.2.5 pci_msi_vec_count
+
+int pci_msi_vec_count(struct pci_dev *dev)
+
+This function could be used to retrieve the number of MSI vectors the
+device requested (via the Multiple Message Capable register). The MSI
+specification only allows the returned value to be a power of two,
+up to a maximum of 2^5 (32).
+
+If this function returns a negative number, it indicates the device is
+not capable of sending MSIs.
+
+If this function returns a positive number, it indicates the maximum
+number of MSI interrupt vectors that could be allocated.
+
 4.3 Using MSI-X
 
 The MSI-X capability is much more flexible than the MSI capability.

drivers/pci/msi.c

@@ -842,6 +842,31 @@ static int pci_msi_check_device(struct pci_dev *dev, int nvec, int type)
 	return 0;
 }
 
+/**
+ * pci_msi_vec_count - Return the number of MSI vectors a device can send
+ * @dev: device to report about
+ *
+ * This function returns the number of MSI vectors a device requested via
+ * Multiple Message Capable register. It returns a negative errno if the
+ * device is not capable sending MSI interrupts. Otherwise, the call succeeds
+ * and returns a power of two, up to a maximum of 2^5 (32), according to the
+ * MSI specification.
+ **/
+int pci_msi_vec_count(struct pci_dev *dev)
+{
+	int ret;
+	u16 msgctl;
+
+	if (!dev->msi_cap)
+		return -EINVAL;
+
+	pci_read_config_word(dev, dev->msi_cap + PCI_MSI_FLAGS, &msgctl);
+	ret = 1 << ((msgctl & PCI_MSI_FLAGS_QMASK) >> 1);
+
+	return ret;
+}
+EXPORT_SYMBOL(pci_msi_vec_count);
+
 /**
  * pci_enable_msi_block - configure device's MSI capability structure
  * @dev: device to configure
@@ -858,13 +883,13 @@ static int pci_msi_check_device(struct pci_dev *dev, int nvec, int type)
 int pci_enable_msi_block(struct pci_dev *dev, int nvec)
 {
 	int status, maxvec;
-	u16 msgctl;
 
-	if (!dev->msi_cap || dev->current_state != PCI_D0)
+	if (dev->current_state != PCI_D0)
 		return -EINVAL;
 
-	pci_read_config_word(dev, dev->msi_cap + PCI_MSI_FLAGS, &msgctl);
-	maxvec = 1 << ((msgctl & PCI_MSI_FLAGS_QMASK) >> 1);
+	maxvec = pci_msi_vec_count(dev);
+	if (maxvec < 0)
+		return maxvec;
 	if (nvec > maxvec)
 		return maxvec;
 
@@ -889,13 +914,13 @@ EXPORT_SYMBOL(pci_enable_msi_block);
 int pci_enable_msi_block_auto(struct pci_dev *dev, int *maxvec)
 {
 	int ret, nvec;
-	u16 msgctl;
 
-	if (!dev->msi_cap || dev->current_state != PCI_D0)
+	if (dev->current_state != PCI_D0)
 		return -EINVAL;
 
-	pci_read_config_word(dev, dev->msi_cap + PCI_MSI_FLAGS, &msgctl);
-	ret = 1 << ((msgctl & PCI_MSI_FLAGS_QMASK) >> 1);
+	ret = pci_msi_vec_count(dev);
+	if (ret < 0)
+		return ret;
 
 	if (maxvec)
 		*maxvec = ret;

include/linux/pci.h

@@ -1154,6 +1154,11 @@ struct msix_entry {
 
 
 #ifndef CONFIG_PCI_MSI
+static inline int pci_msi_vec_count(struct pci_dev *dev)
+{
+	return -ENOSYS;
+}
+
 static inline int pci_enable_msi_block(struct pci_dev *dev, int nvec)
 {
 	return -ENOSYS;
@@ -1195,6 +1200,7 @@ static inline int pci_msi_enabled(void)
 	return 0;
 }
 #else
+int pci_msi_vec_count(struct pci_dev *dev);
 int pci_enable_msi_block(struct pci_dev *dev, int nvec);
 int pci_enable_msi_block_auto(struct pci_dev *dev, int *maxvec);
 void pci_msi_shutdown(struct pci_dev *dev);