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…davem/net-2.6 Conflicts: drivers/net/stmmac/stmmac_main.c drivers/net/wireless/wl12xx/wl1271_cmd.c drivers/net/wireless/wl12xx/wl1271_main.c drivers/net/wireless/wl12xx/wl1271_spi.c net/core/ethtool.c net/mac80211/scan.c
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@@ -16,6 +16,15 @@ | |
</address> | ||
</affiliation> | ||
</author> | ||
<author> | ||
<firstname>William</firstname> | ||
<surname>Cohen</surname> | ||
<affiliation> | ||
<address> | ||
<email>[email protected]</email> | ||
</address> | ||
</affiliation> | ||
</author> | ||
</authorgroup> | ||
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<legalnotice> | ||
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@@ -91,4 +100,8 @@ | |
!Iinclude/trace/events/signal.h | ||
</chapter> | ||
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<chapter id="block"> | ||
<title>Block IO</title> | ||
!Iinclude/trace/events/block.h | ||
</chapter> | ||
</book> |
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================ | ||
CIRCULAR BUFFERS | ||
================ | ||
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||
By: David Howells <[email protected]> | ||
Paul E. McKenney <[email protected]> | ||
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||
Linux provides a number of features that can be used to implement circular | ||
buffering. There are two sets of such features: | ||
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(1) Convenience functions for determining information about power-of-2 sized | ||
buffers. | ||
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(2) Memory barriers for when the producer and the consumer of objects in the | ||
buffer don't want to share a lock. | ||
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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. | ||
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Contents: | ||
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(*) What is a circular buffer? | ||
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(*) Measuring power-of-2 buffers. | ||
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(*) Using memory barriers with circular buffers. | ||
- The producer. | ||
- The consumer. | ||
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========================== | ||
WHAT IS A CIRCULAR BUFFER? | ||
========================== | ||
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First of all, what is a circular buffer? A circular buffer is a buffer of | ||
fixed, finite size into which there are two indices: | ||
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(1) A 'head' index - the point at which the producer inserts items into the | ||
buffer. | ||
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(2) A 'tail' index - the point at which the consumer finds the next item in | ||
the buffer. | ||
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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. | ||
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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. | ||
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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. | ||
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============================ | ||
MEASURING POWER-OF-2 BUFFERS | ||
============================ | ||
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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. | ||
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Linux provides a set of macros for handling power-of-2 circular buffers. These | ||
can be made use of by: | ||
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#include <linux/circ_buf.h> | ||
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The macros are: | ||
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(*) Measure the remaining capacity of a buffer: | ||
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CIRC_SPACE(head_index, tail_index, buffer_size); | ||
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This returns the amount of space left in the buffer[1] into which items | ||
can be inserted. | ||
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(*) Measure the maximum consecutive immediate space in a buffer: | ||
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CIRC_SPACE_TO_END(head_index, tail_index, buffer_size); | ||
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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. | ||
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(*) Measure the occupancy of a buffer: | ||
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CIRC_CNT(head_index, tail_index, buffer_size); | ||
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This returns the number of items currently occupying a buffer[2]. | ||
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(*) Measure the non-wrapping occupancy of a buffer: | ||
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CIRC_CNT_TO_END(head_index, tail_index, buffer_size); | ||
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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. | ||
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Each of these macros will nominally return a value between 0 and buffer_size-1, | ||
however: | ||
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[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. | ||
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To the consumer it will show an upper bound as the producer may be busy | ||
depleting the space. | ||
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[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. | ||
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To the producer it will show an upper bound as the consumer may be busy | ||
emptying the buffer. | ||
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[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. | ||
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=========================================== | ||
USING MEMORY BARRIERS WITH CIRCULAR BUFFERS | ||
=========================================== | ||
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By using memory barriers in conjunction with circular buffers, you can avoid | ||
the need to: | ||
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(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 | ||
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(2) use atomic counter operations. | ||
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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. | ||
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THE PRODUCER | ||
------------ | ||
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The producer will look something like this: | ||
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spin_lock(&producer_lock); | ||
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unsigned long head = buffer->head; | ||
unsigned long tail = ACCESS_ONCE(buffer->tail); | ||
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if (CIRC_SPACE(head, tail, buffer->size) >= 1) { | ||
/* insert one item into the buffer */ | ||
struct item *item = buffer[head]; | ||
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produce_item(item); | ||
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smp_wmb(); /* commit the item before incrementing the head */ | ||
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buffer->head = (head + 1) & (buffer->size - 1); | ||
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/* wake_up() will make sure that the head is committed before | ||
* waking anyone up */ | ||
wake_up(consumer); | ||
} | ||
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spin_unlock(&producer_lock); | ||
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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. | ||
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Note that wake_up() doesn't have to be the exact mechanism used, but whatever | ||
is used must guarantee a (write) memory barrier between the update of the head | ||
index and the change of state of the consumer, if a change of state occurs. | ||
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THE CONSUMER | ||
------------ | ||
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The consumer will look something like this: | ||
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spin_lock(&consumer_lock); | ||
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unsigned long head = ACCESS_ONCE(buffer->head); | ||
unsigned long tail = buffer->tail; | ||
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if (CIRC_CNT(head, tail, buffer->size) >= 1) { | ||
/* read index before reading contents at that index */ | ||
smp_read_barrier_depends(); | ||
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/* extract one item from the buffer */ | ||
struct item *item = buffer[tail]; | ||
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consume_item(item); | ||
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smp_mb(); /* finish reading descriptor before incrementing tail */ | ||
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buffer->tail = (tail + 1) & (buffer->size - 1); | ||
} | ||
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spin_unlock(&consumer_lock); | ||
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This will instruct the CPU to make sure the index is up to date before reading | ||
the new item, and then it shall make sure the CPU has finished reading the item | ||
before it writes the new tail pointer, which will erase the item. | ||
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Note the use of ACCESS_ONCE() in both algorithms to read the opposition index. | ||
This prevents the compiler from discarding and reloading its cached value - | ||
which some compilers will do across smp_read_barrier_depends(). This isn't | ||
strictly needed if you can be sure that the opposition index will _only_ be | ||
used the once. | ||
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=============== | ||
FURTHER READING | ||
=============== | ||
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See also Documentation/memory-barriers.txt for a description of Linux's memory | ||
barrier facilities. |
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@@ -1,9 +1,9 @@ | ||
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What is imacfb? | ||
What is efifb? | ||
=============== | ||
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This is a generic EFI platform driver for Intel based Apple computers. | ||
Imacfb is only for EFI booted Intel Macs. | ||
efifb is only for EFI booted Intel Macs. | ||
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Supported Hardware | ||
================== | ||
|
@@ -16,16 +16,16 @@ MacMini | |
How to use it? | ||
============== | ||
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Imacfb does not have any kind of autodetection of your machine. | ||
efifb does not have any kind of autodetection of your machine. | ||
You have to add the following kernel parameters in your elilo.conf: | ||
Macbook : | ||
video=imacfb:macbook | ||
video=efifb:macbook | ||
MacMini : | ||
video=imacfb:mini | ||
video=efifb:mini | ||
Macbook Pro 15", iMac 17" : | ||
video=imacfb:i17 | ||
video=efifb:i17 | ||
Macbook Pro 17", iMac 20" : | ||
video=imacfb:i20 | ||
video=efifb:i20 | ||
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-- | ||
Edgar Hucek <[email protected]> |
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