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Old 06-10-2010, 09:52 AM
"Henrik Rydberg"
 
Default UBUNTU: (evdev) Use multi-reader buffer to save space (rev4)

Preparing for larger buffer needs, convert the current per-client
circular buffer to a single buffer with multiple clients. Ideally, there
should be a mechanism where clients wait during buffer collision only.
Meanwhile, let clients take the dev->event_lock, which is already held
during buffer writes.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
---
drivers/input/evdev.c | 46 ++++++++++++++++++++++++++--------------------
1 files changed, 26 insertions(+), 20 deletions(-)

diff --git a/drivers/input/evdev.c b/drivers/input/evdev.c
index 2ee6c7a..7117589 100644
--- a/drivers/input/evdev.c
+++ b/drivers/input/evdev.c
@@ -33,13 +33,13 @@ struct evdev {
spinlock_t client_lock; /* protects client_list */
struct mutex mutex;
struct device dev;
+ int head;
+ struct input_event buffer[EVDEV_BUFFER_SIZE];
};

struct evdev_client {
- struct input_event buffer[EVDEV_BUFFER_SIZE];
int head;
int tail;
- spinlock_t buffer_lock; /* protects access to buffer, head and tail */
struct fasync_struct *fasync;
struct evdev *evdev;
struct list_head node;
@@ -48,18 +48,13 @@ struct evdev_client {
static struct evdev *evdev_table[EVDEV_MINORS];
static DEFINE_MUTEX(evdev_table_mutex);

-static void evdev_pass_event(struct evdev_client *client,
- struct input_event *event)
+static inline void evdev_sync_event(struct evdev_client *client,
+ struct evdev *evdev, int type)
{
- /*
- * Interrupts are disabled, just acquire the lock
- */
- spin_lock(&client->buffer_lock);
- client->buffer[client->head++] = *event;
- client->head &= EVDEV_BUFFER_SIZE - 1;
- spin_unlock(&client->buffer_lock);
-
- if (event->type == EV_SYN)
+ /* sync the reader such that it never becomes empty */
+ if (client->tail != evdev->head)
+ client->head = evdev->head;
+ if (type == EV_SYN)
kill_fasync(&client->fasync, SIGIO, POLL_IN);
}

@@ -78,14 +73,18 @@ static void evdev_event(struct input_handle *handle,
event.code = code;
event.value = value;

+ /* dev->event_lock held */
+ evdev->buffer[evdev->head] = event;
+ evdev->head = (evdev->head + 1) & (EVDEV_BUFFER_SIZE - 1);
+
rcu_read_lock();

client = rcu_dereference(evdev->grab);
if (client)
- evdev_pass_event(client, &event);
+ evdev_sync_event(client, evdev, type);
else
list_for_each_entry_rcu(client, &evdev->client_list, node)
- evdev_pass_event(client, &event);
+ evdev_sync_event(client, evdev, type);

rcu_read_unlock();

@@ -269,7 +268,6 @@ static int evdev_open(struct inode *inode, struct file *file)
goto err_put_evdev;
}

- spin_lock_init(&client->buffer_lock);
client->evdev = evdev;
evdev_attach_client(evdev, client);

@@ -325,19 +323,27 @@ static ssize_t evdev_write(struct file *file, const char __user *buffer,
}

static int evdev_fetch_next_event(struct evdev_client *client,
+ struct evdev *evdev,
struct input_event *event)
{
+ struct input_dev *dev = evdev->handle.dev;
int have_event;

- spin_lock_irq(&client->buffer_lock);
+ /*
+ * FIXME: taking event_lock protects against reentrant fops
+ * reads and provides sufficient buffer locking. However,
+ * clients should not block writes, and having multiple clients
+ * waiting for each other is suboptimal.
+ */
+ spin_lock_irq(&dev->event_lock);

have_event = client->head != client->tail;
if (have_event) {
- *event = client->buffer[client->tail++];
+ *event = evdev->buffer[client->tail++];
client->tail &= EVDEV_BUFFER_SIZE - 1;
}

- spin_unlock_irq(&client->buffer_lock);
+ spin_unlock_irq(&dev->event_lock);

return have_event;
}
@@ -366,7 +372,7 @@ static ssize_t evdev_read(struct file *file, char __user *buffer,
return -ENODEV;

while (retval + input_event_size() <= count &&
- evdev_fetch_next_event(client, &event)) {
+ evdev_fetch_next_event(client, evdev, &event)) {

if (input_event_to_user(buffer + retval, &event))
return -EFAULT;
--
1.6.3.3


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