[PATCH RFC v2 09/29] PCI/MSI: Factor out pci_get_msi_cap() interface

[Alexander Gordeev <agordeev@redhat.com>]

Device drivers can use this interface to obtain maximum number
of MSI interrupts the device supports and use that number i.e.
in a following call to pci_enable_msi_block() interface.

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
---
 Documentation/PCI/MSI-HOWTO.txt |   15 +++++++++++++++
 drivers/pci/msi.c               |   33 +++++++++++++++++++++------------
 include/linux/pci.h             |    6 ++++++
 3 files changed, 42 insertions(+), 12 deletions(-)

diff --git a/Documentation/PCI/MSI-HOWTO.txt b/Documentation/PCI/MSI-HOWTO.txt
index 845edb5..e7e3b20 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_get_msi_cap
+
+int pci_get_msi_cap(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 fdae3a4..14dc4d1 100644
--- a/drivers/pci/msi.c
+++ b/drivers/pci/msi.c
@@ -795,6 +795,21 @@ static int pci_msi_check_device(struct pci_dev *dev, int nvec, int type)
 	return 0;
 }
 
+int pci_get_msi_cap(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_get_msi_cap);
+
 /**
  * pci_enable_msi_block - configure device's MSI capability structure
  * @dev: device to configure
@@ -811,13 +826,10 @@ static int pci_msi_check_device(struct pci_dev *dev, int nvec, int type)
 int pci_enable_msi_block(struct pci_dev *dev, unsigned int nvec)
 {
 	int status, maxvec;
-	u16 msgctl;
 
-	if (!dev->msi_cap)
-		return -EINVAL;
-
-	pci_read_config_word(dev, dev->msi_cap + PCI_MSI_FLAGS, &msgctl);
-	maxvec = 1 << ((msgctl & PCI_MSI_FLAGS_QMASK) >> 1);
+	maxvec = pci_get_msi_cap(dev);
+	if (maxvec < 0)
+		return maxvec;
 	if (nvec > maxvec)
 		return maxvec;
 
@@ -842,13 +854,10 @@ EXPORT_SYMBOL(pci_enable_msi_block);
 int pci_enable_msi_block_auto(struct pci_dev *dev, unsigned int *maxvec)
 {
 	int ret, nvec;
-	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);
+	ret = pci_get_msi_cap(dev);
+	if (ret < 0)
+		return ret;
 
 	if (maxvec)
 		*maxvec = ret;
diff --git a/include/linux/pci.h b/include/linux/pci.h
index d6832de..df72084 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -1146,6 +1146,11 @@ struct msix_entry {
 
 
 #ifndef CONFIG_PCI_MSI
+static inline int pci_get_msi_cap(struct pci_dev *dev)
+{
+	return -ENOSYS;
+}
+
 static inline int pci_enable_msi_block(struct pci_dev *dev, unsigned int nvec)
 {
 	return -ENOSYS;
@@ -1187,6 +1192,7 @@ static inline int pci_msi_enabled(void)
 	return 0;
 }
 #else
+int pci_get_msi_cap(struct pci_dev *dev);
 int pci_enable_msi_block(struct pci_dev *dev, unsigned int nvec);
 int pci_enable_msi_block_auto(struct pci_dev *dev, unsigned int *maxvec);
 void pci_msi_shutdown(struct pci_dev *dev);
-- 
1.7.7.6