lguest: documentation III: Drivers

Documentation: The Drivers

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 69 insertions, 3 deletions

diff --git a/drivers/lguest/lguest_bus.c b/drivers/lguest/lguest_bus.c
index 9a22d199502e..55a7940ca732 100644
--- a/drivers/lguest/lguest_bus.c
+++ b/drivers/lguest/lguest_bus.c
@@ -46,6 +46,10 @@ static struct device_attribute lguest_dev_attrs[] = {
         __ATTR_NULL
 };
 
+/*D:130 The generic bus infrastructure requires a function which says whether a
+ * device matches a driver.  For us, it is simple: "struct lguest_driver"
+ * contains a "device_type" field which indicates what type of device it can
+ * handle, so we just cast the args and compare: */
 static int lguest_dev_match(struct device *_dev, struct device_driver *_drv)
 {
         struct lguest_device *dev = container_of(_dev,struct lguest_device,dev);
@@ -53,6 +57,7 @@ static int lguest_dev_match(struct device *_dev, struct device_driver *_drv)
 
         return (drv->device_type == lguest_devices[dev->index].type);
 }
+/*:*/
 
 struct lguest_bus {
         struct bus_type bus;
@@ -71,11 +76,24 @@ static struct lguest_bus lguest_bus = {
         }
 };
 
+/*D:140 This is the callback which occurs once the bus infrastructure matches
+ * up a device and driver, ie. in response to add_lguest_device() calling
+ * device_register(), or register_lguest_driver() calling driver_register().
+ *
+ * At the moment it's always the latter: the devices are added first, since
+ * scan_devices() is called from a "core_initcall", and the drivers themselves
+ * called later as a normal "initcall".  But it would work the other way too.
+ *
+ * So now we have the happy couple, we add the status bit to indicate that we
+ * found a driver.  If the driver truly loves the device, it will return
+ * happiness from its probe function (ok, perhaps this wasn't my greatest
+ * analogy), and we set the final "driver ok" bit so the Host sees it's all
+ * green. */
 static int lguest_dev_probe(struct device *_dev)
 {
         int ret;
-        struct lguest_device *dev = container_of(_dev,struct lguest_device,dev);
-        struct lguest_driver *drv = container_of(dev->dev.driver,
+        struct lguest_device*dev = container_of(_dev,struct lguest_device,dev);
+        struct lguest_driver*drv = container_of(dev->dev.driver,
                                                 struct lguest_driver, drv);
 
         lguest_devices[dev->index].status |= LGUEST_DEVICE_S_DRIVER;
@@ -85,6 +103,10 @@ static int lguest_dev_probe(struct device *_dev)
         return ret;
 }
 
+/* The last part of the bus infrastructure is the function lguest drivers use
+ * to register themselves.  Firstly, we do nothing if there's no lguest bus
+ * (ie. this is not a Guest), otherwise we fill in the embedded generic "struct
+ * driver" fields and call the generic driver_register(). */
 int register_lguest_driver(struct lguest_driver *drv)
 {
         if (!lguest_devices)
@@ -97,12 +119,36 @@ int register_lguest_driver(struct lguest_driver *drv)
 
         return driver_register(&drv->drv);
 }
+
+/* At the moment we build all the drivers into the kernel because they're so
+ * simple: 8144 bytes for all three of them as I type this.  And as the console
+ * really needs to be built in, it's actually only 3527 bytes for the network
+ * and block drivers.
+ *
+ * If they get complex it will make sense for them to be modularized, so we
+ * need to explicitly export the symbol.
+ *
+ * I don't think non-GPL modules make sense, so it's a GPL-only export.
+ */
 EXPORT_SYMBOL_GPL(register_lguest_driver);
 
+/*D:120 This is the core of the lguest bus: actually adding a new device.
+ * It's a separate function because it's neater that way, and because an
+ * earlier version of the code supported hotplug and unplug.  They were removed
+ * early on because they were never used.
+ *
+ * As Andrew Tridgell says, "Untested code is buggy code".
+ *
+ * It's worth reading this carefully: we start with an index into the array of
+ * "struct lguest_device_desc"s indicating the device which is new: */
 static void add_lguest_device(unsigned int index)
 {
         struct lguest_device *new;
 
+        /* Each "struct lguest_device_desc" has a "status" field, which the
+         * Guest updates as the device is probed.  In the worst case, the Host
+         * can look at these bits to tell what part of device setup failed,
+         * even if the console isn't available. */
         lguest_devices[index].status |= LGUEST_DEVICE_S_ACKNOWLEDGE;
         new = kmalloc(sizeof(struct lguest_device), GFP_KERNEL);
         if (!new) {
@@ -111,12 +157,17 @@ static void add_lguest_device(unsigned int index)
                 return;
         }
 
+        /* The "struct lguest_device" setup is pretty straight-forward example
+         * code. */
         new->index = index;
         new->private = NULL;
         memset(&new->dev, 0, sizeof(new->dev));
         new->dev.parent = &lguest_bus.dev;
         new->dev.bus = &lguest_bus.bus;
         sprintf(new->dev.bus_id, "%u", index);
+
+        /* device_register() causes the bus infrastructure to look for a
+         * matching driver. */
         if (device_register(&new->dev) != 0) {
                 printk(KERN_EMERG "Cannot register lguest device %u\n", index);
                 lguest_devices[index].status |= LGUEST_DEVICE_S_FAILED;
@@ -124,6 +175,9 @@ static void add_lguest_device(unsigned int index)
         }
 }
 
+/*D:110 scan_devices() simply iterates through the device array.  The type 0
+ * is reserved to mean "no device", and anything else means we have found a
+ * device: add it. */
 static void scan_devices(void)
 {
         unsigned int i;
@@ -133,12 +187,23 @@ static void scan_devices(void)
                         add_lguest_device(i);
 }
 
+/*D:100 Fairly early in boot, lguest_bus_init() is called to set up the lguest
+ * bus.  We check that we are a Guest by checking paravirt_ops.name: there are
+ * other ways of checking, but this seems most obvious to me.
+ *
+ * So we can access the array of "struct lguest_device_desc"s easily, we map
+ * that memory and store the pointer in the global "lguest_devices".  Then we
+ * register the bus with the core.  Doing two registrations seems clunky to me,
+ * but it seems to be the correct sysfs incantation.
+ *
+ * Finally we call scan_devices() which adds all the devices found in the
+ * "struct lguest_device_desc" array. */
 static int __init lguest_bus_init(void)
 {
         if (strcmp(paravirt_ops.name, "lguest") != 0)
                 return 0;
 
-        /* Devices are in page above top of "normal" mem. */
+        /* Devices are in a single page above top of "normal" mem */
         lguest_devices = lguest_map(max_pfn<<PAGE_SHIFT, 1);
 
         if (bus_register(&lguest_bus.bus) != 0
@@ -148,4 +213,5 @@ static int __init lguest_bus_init(void)
         scan_devices();
         return 0;
 }
+/* Do this after core stuff, before devices. */
 postcore_initcall(lguest_bus_init);