[RFC PATCH] uio: uio_pci_generic: Add support for MSI interrupts

[Guenter Roeck <linux@roeck-us.net>]

Enable support for MSI interrupts if the device supports it.
Since MSI interrupts are edge triggered, it is no longer necessary to
disable interrupts in the kernel and re-enable them from user-space.
Instead, clearing the interrupt condition in the user space application
automatically re-enables the interrupt.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
An open question is if we can just do this unconditionally
or if there should be some flag to enable it. A module parameter, maybe ?

 Documentation/DocBook/uio-howto.tmpl |   23 ++++++++++++++++++++---
 drivers/uio/uio_pci_generic.c        |   15 ++++++++++++---
 2 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/Documentation/DocBook/uio-howto.tmpl b/Documentation/DocBook/uio-howto.tmpl
index 9561815..69b54e0 100644
--- a/Documentation/DocBook/uio-howto.tmpl
+++ b/Documentation/DocBook/uio-howto.tmpl
@@ -46,6 +46,12 @@ GPL version 2.
 
 <revhistory>
 	<revision>
+	<revnumber>0.10</revnumber>
+	<date>2013-06-26</date>
+	<authorinitials>gr</authorinitials>
+	<revremark>Added MSI support to uio_pci_generic.</revremark>
+	</revision>
+	<revision>
 	<revnumber>0.9</revnumber>
 	<date>2009-07-16</date>
 	<authorinitials>mst</authorinitials>
@@ -935,15 +941,26 @@ and look in the output for failure reasons
 <sect1 id="uio_pci_generic_internals">
 <title>Things to know about uio_pci_generic</title>
 	<para>
-Interrupts are handled using the Interrupt Disable bit in the PCI command
+Interrupts are handled either as MSI interrupts (if the device supports it) or
+as legacy INTx interrupts.
+	</para>
+	<para>
+uio_pci_generic automatically configures a device to use MSI interrupts
+if the device supports it. If an MSI interrupt is received, the user space
+driver is notified. Since MSI interrupts are edge sensitive, the user space
+driver needs to clear the interrupt condition in the device before blocking
+and waiting for more interrupts.
+	</para>
+	<para>
+Legacy interrupts are handled using the Interrupt Disable bit in the PCI command
 register and Interrupt Status bit in the PCI status register.  All devices
 compliant to PCI 2.3 (circa 2002) and all compliant PCI Express devices should
 support these bits.  uio_pci_generic detects this support, and won't bind to
 devices which do not support the Interrupt Disable Bit in the command register.
 	</para>
 	<para>
-On each interrupt, uio_pci_generic sets the Interrupt Disable bit.
-This prevents the device from generating further interrupts
+If legacy interrupts are used, uio_pci_generic sets the Interrupt Disable bit on
+each interrupt. This prevents the device from generating further interrupts
 until the bit is cleared. The userspace driver should clear this
 bit before blocking and waiting for more interrupts.
 	</para>
diff --git a/drivers/uio/uio_pci_generic.c b/drivers/uio/uio_pci_generic.c
index 14aa10c..3366fdb 100644
--- a/drivers/uio/uio_pci_generic.c
+++ b/drivers/uio/uio_pci_generic.c
@@ -32,6 +32,7 @@
 struct uio_pci_generic_dev {
 	struct uio_info info;
 	struct pci_dev *pdev;
+	bool have_msi;
 };
 
 static inline struct uio_pci_generic_dev *
@@ -46,7 +47,7 @@ static irqreturn_t irqhandler(int irq, struct uio_info *info)
 {
 	struct uio_pci_generic_dev *gdev = to_uio_pci_generic_dev(info);
 
-	if (!pci_check_and_mask_intx(gdev->pdev))
+	if (!gdev->have_msi && !pci_check_and_mask_intx(gdev->pdev))
 		return IRQ_NONE;
 
 	/* UIO core will signal the user process. */
@@ -58,6 +59,7 @@ static int probe(struct pci_dev *pdev,
 {
 	struct uio_pci_generic_dev *gdev;
 	int err;
+	bool have_msi = false;
 
 	err = pci_enable_device(pdev);
 	if (err) {
@@ -73,7 +75,9 @@ static int probe(struct pci_dev *pdev,
 		return -ENODEV;
 	}
 
-	if (!pci_intx_mask_supported(pdev)) {
+	if (!pci_enable_msi(pdev)) {
+		have_msi = true;
+	} else if (!pci_intx_mask_supported(pdev)) {
 		err = -ENODEV;
 		goto err_verify;
 	}
@@ -84,10 +88,11 @@ static int probe(struct pci_dev *pdev,
 		goto err_alloc;
 	}
 
+	gdev->have_msi = have_msi;
 	gdev->info.name = "uio_pci_generic";
 	gdev->info.version = DRIVER_VERSION;
 	gdev->info.irq = pdev->irq;
-	gdev->info.irq_flags = IRQF_SHARED;
+	gdev->info.irq_flags = have_msi ? 0 : IRQF_SHARED;
 	gdev->info.handler = irqhandler;
 	gdev->pdev = pdev;
 
@@ -99,6 +104,8 @@ static int probe(struct pci_dev *pdev,
 err_register:
 	kfree(gdev);
 err_alloc:
+	if (have_msi)
+		pci_disable_msi(pdev);
 err_verify:
 	pci_disable_device(pdev);
 	return err;
@@ -109,6 +116,8 @@ static void remove(struct pci_dev *pdev)
 	struct uio_pci_generic_dev *gdev = pci_get_drvdata(pdev);
 
 	uio_unregister_device(&gdev->info);
+	if (gdev->have_msi)
+		pci_disable_msi(pdev);
 	pci_disable_device(pdev);
 	kfree(gdev);
 }
-- 
1.7.9.5