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>

3 files changed, 54 insertions, 8 deletions

diff --git a/Documentation/PCI/MSI-HOWTO.txt b/Documentation/PCI/MSI-HOWTO.txt
index a4d174e95413..a8b41788dfde 100644
--- a/Documentation/PCI/MSI-HOWTO.txt
+++ b/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.
diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
index ce0d4eb91a22..ba6d0a9bdd39 100644
--- a/drivers/pci/msi.c
+++ b/drivers/pci/msi.c
@@ -843,6 +843,31 @@ static int pci_msi_check_device(struct pci_dev *dev, int nvec, int type)
 }
 
 /**
+ * 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
  * @nvec: number of interrupts 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;
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 7c34c3913bcb..6691de093f1c 100644
--- a/include/linux/pci.h
+++ b/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);