Commit cf0dbba5 authored by Takashi Iwai's avatar Takashi Iwai

Merge remote branch 'alsa/devel' into topic/misc

parents 1cff399e 0340c7dc

Too many changes to show.

To preserve performance only 1000 of 1000+ files are displayed.
......@@ -16,6 +16,15 @@
</address>
</affiliation>
</author>
<author>
<firstname>William</firstname>
<surname>Cohen</surname>
<affiliation>
<address>
<email>wcohen@redhat.com</email>
</address>
</affiliation>
</author>
</authorgroup>
<legalnotice>
......@@ -91,4 +100,8 @@
!Iinclude/trace/events/signal.h
</chapter>
<chapter id="block">
<title>Block IO</title>
!Iinclude/trace/events/block.h
</chapter>
</book>
......@@ -1162,8 +1162,8 @@ where a driver received a request ala this before:
As mentioned, there is no virtual mapping of a bio. For DMA, this is
not a problem as the driver probably never will need a virtual mapping.
Instead it needs a bus mapping (pci_map_page for a single segment or
use blk_rq_map_sg for scatter gather) to be able to ship it to the driver. For
Instead it needs a bus mapping (dma_map_page for a single segment or
use dma_map_sg for scatter gather) to be able to ship it to the driver. For
PIO drivers (or drivers that need to revert to PIO transfer once in a
while (IDE for example)), where the CPU is doing the actual data
transfer a virtual mapping is needed. If the driver supports highmem I/O,
......
......@@ -340,7 +340,7 @@ Note:
5.3 swappiness
Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
Following cgroups' swapiness can't be changed.
Following cgroups' swappiness can't be changed.
- root cgroup (uses /proc/sys/vm/swappiness).
- a cgroup which uses hierarchy and it has child cgroup.
- a cgroup which uses hierarchy and not the root of hierarchy.
......
================
CIRCULAR BUFFERS
================
By: David Howells <dhowells@redhat.com>
Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Linux provides a number of features that can be used to implement circular
buffering. There are two sets of such features:
(1) Convenience functions for determining information about power-of-2 sized
buffers.
(2) Memory barriers for when the producer and the consumer of objects in the
buffer don't want to share a lock.
To use these facilities, as discussed below, there needs to be just one
producer and just one consumer. It is possible to handle multiple producers by
serialising them, and to handle multiple consumers by serialising them.
Contents:
(*) What is a circular buffer?
(*) Measuring power-of-2 buffers.
(*) Using memory barriers with circular buffers.
- The producer.
- The consumer.
==========================
WHAT IS A CIRCULAR BUFFER?
==========================
First of all, what is a circular buffer? A circular buffer is a buffer of
fixed, finite size into which there are two indices:
(1) A 'head' index - the point at which the producer inserts items into the
buffer.
(2) A 'tail' index - the point at which the consumer finds the next item in
the buffer.
Typically when the tail pointer is equal to the head pointer, the buffer is
empty; and the buffer is full when the head pointer is one less than the tail
pointer.
The head index is incremented when items are added, and the tail index when
items are removed. The tail index should never jump the head index, and both
indices should be wrapped to 0 when they reach the end of the buffer, thus
allowing an infinite amount of data to flow through the buffer.
Typically, items will all be of the same unit size, but this isn't strictly
required to use the techniques below. The indices can be increased by more
than 1 if multiple items or variable-sized items are to be included in the
buffer, provided that neither index overtakes the other. The implementer must
be careful, however, as a region more than one unit in size may wrap the end of
the buffer and be broken into two segments.
============================
MEASURING POWER-OF-2 BUFFERS
============================
Calculation of the occupancy or the remaining capacity of an arbitrarily sized
circular buffer would normally be a slow operation, requiring the use of a
modulus (divide) instruction. However, if the buffer is of a power-of-2 size,
then a much quicker bitwise-AND instruction can be used instead.
Linux provides a set of macros for handling power-of-2 circular buffers. These
can be made use of by:
#include <linux/circ_buf.h>
The macros are:
(*) Measure the remaining capacity of a buffer:
CIRC_SPACE(head_index, tail_index, buffer_size);
This returns the amount of space left in the buffer[1] into which items
can be inserted.
(*) Measure the maximum consecutive immediate space in a buffer:
CIRC_SPACE_TO_END(head_index, tail_index, buffer_size);
This returns the amount of consecutive space left in the buffer[1] into
which items can be immediately inserted without having to wrap back to the
beginning of the buffer.
(*) Measure the occupancy of a buffer:
CIRC_CNT(head_index, tail_index, buffer_size);
This returns the number of items currently occupying a buffer[2].
(*) Measure the non-wrapping occupancy of a buffer:
CIRC_CNT_TO_END(head_index, tail_index, buffer_size);
This returns the number of consecutive items[2] that can be extracted from
the buffer without having to wrap back to the beginning of the buffer.
Each of these macros will nominally return a value between 0 and buffer_size-1,
however:
[1] CIRC_SPACE*() are intended to be used in the producer. To the producer
they will return a lower bound as the producer controls the head index,
but the consumer may still be depleting the buffer on another CPU and
moving the tail index.
To the consumer it will show an upper bound as the producer may be busy
depleting the space.
[2] CIRC_CNT*() are intended to be used in the consumer. To the consumer they
will return a lower bound as the consumer controls the tail index, but the
producer may still be filling the buffer on another CPU and moving the
head index.
To the producer it will show an upper bound as the consumer may be busy
emptying the buffer.
[3] To a third party, the order in which the writes to the indices by the
producer and consumer become visible cannot be guaranteed as they are
independent and may be made on different CPUs - so the result in such a
situation will merely be a guess, and may even be negative.
===========================================
USING MEMORY BARRIERS WITH CIRCULAR BUFFERS
===========================================
By using memory barriers in conjunction with circular buffers, you can avoid
the need to:
(1) use a single lock to govern access to both ends of the buffer, thus
allowing the buffer to be filled and emptied at the same time; and
(2) use atomic counter operations.
There are two sides to this: the producer that fills the buffer, and the
consumer that empties it. Only one thing should be filling a buffer at any one
time, and only one thing should be emptying a buffer at any one time, but the
two sides can operate simultaneously.
THE PRODUCER
------------
The producer will look something like this:
spin_lock(&producer_lock);
unsigned long head = buffer->head;
unsigned long tail = ACCESS_ONCE(buffer->tail);
if (CIRC_SPACE(head, tail, buffer->size) >= 1) {
/* insert one item into the buffer */
struct item *item = buffer[head];
produce_item(item);
smp_wmb(); /* commit the item before incrementing the head */
buffer->head = (head + 1) & (buffer->size - 1);
/* wake_up() will make sure that the head is committed before
* waking anyone up */
wake_up(consumer);
}
spin_unlock(&producer_lock);
This will instruct the CPU that the contents of the new item must be written
before the head index makes it available to the consumer and then instructs the
CPU that the revised head index must be written before the consumer is woken.