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.. SPDX-License-Identifier: GPL-2.0+ | ||
=========== | ||
Folio Queue | ||
=========== | ||
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:Author: David Howells <[email protected]> | ||
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.. Contents: | ||
* Overview | ||
* Initialisation | ||
* Adding and removing folios | ||
* Querying information about a folio | ||
* Querying information about a folio_queue | ||
* Folio queue iteration | ||
* Folio marks | ||
* Lockless simultaneous production/consumption issues | ||
Overview | ||
======== | ||
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The folio_queue struct forms a single segment in a segmented list of folios | ||
that can be used to form an I/O buffer. As such, the list can be iterated over | ||
using the ITER_FOLIOQ iov_iter type. | ||
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The publicly accessible members of the structure are:: | ||
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struct folio_queue { | ||
struct folio_queue *next; | ||
struct folio_queue *prev; | ||
... | ||
}; | ||
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A pair of pointers are provided, ``next`` and ``prev``, that point to the | ||
segments on either side of the segment being accessed. Whilst this is a | ||
doubly-linked list, it is intentionally not a circular list; the outward | ||
sibling pointers in terminal segments should be NULL. | ||
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Each segment in the list also stores: | ||
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* an ordered sequence of folio pointers, | ||
* the size of each folio and | ||
* three 1-bit marks per folio, | ||
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but hese should not be accessed directly as the underlying data structure may | ||
change, but rather the access functions outlined below should be used. | ||
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The facility can be made accessible by:: | ||
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#include <linux/folio_queue.h> | ||
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and to use the iterator:: | ||
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#include <linux/uio.h> | ||
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Initialisation | ||
============== | ||
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A segment should be initialised by calling:: | ||
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void folioq_init(struct folio_queue *folioq); | ||
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with a pointer to the segment to be initialised. Note that this will not | ||
necessarily initialise all the folio pointers, so care must be taken to check | ||
the number of folios added. | ||
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Adding and removing folios | ||
========================== | ||
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Folios can be set in the next unused slot in a segment struct by calling one | ||
of:: | ||
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unsigned int folioq_append(struct folio_queue *folioq, | ||
struct folio *folio); | ||
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unsigned int folioq_append_mark(struct folio_queue *folioq, | ||
struct folio *folio); | ||
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Both functions update the stored folio count, store the folio and note its | ||
size. The second function also sets the first mark for the folio added. Both | ||
functions return the number of the slot used. [!] Note that no attempt is made | ||
to check that the capacity wasn't overrun and the list will not be extended | ||
automatically. | ||
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A folio can be excised by calling:: | ||
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void folioq_clear(struct folio_queue *folioq, unsigned int slot); | ||
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This clears the slot in the array and also clears all the marks for that folio, | ||
but doesn't change the folio count - so future accesses of that slot must check | ||
if the slot is occupied. | ||
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Querying information about a folio | ||
================================== | ||
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Information about the folio in a particular slot may be queried by the | ||
following function:: | ||
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struct folio *folioq_folio(const struct folio_queue *folioq, | ||
unsigned int slot); | ||
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If a folio has not yet been set in that slot, this may yield an undefined | ||
pointer. The size of the folio in a slot may be queried with either of:: | ||
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unsigned int folioq_folio_order(const struct folio_queue *folioq, | ||
unsigned int slot); | ||
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size_t folioq_folio_size(const struct folio_queue *folioq, | ||
unsigned int slot); | ||
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The first function returns the size as an order and the second as a number of | ||
bytes. | ||
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Querying information about a folio_queue | ||
======================================== | ||
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Information may be retrieved about a particular segment with the following | ||
functions:: | ||
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unsigned int folioq_nr_slots(const struct folio_queue *folioq); | ||
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unsigned int folioq_count(struct folio_queue *folioq); | ||
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bool folioq_full(struct folio_queue *folioq); | ||
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The first function returns the maximum capacity of a segment. It must not be | ||
assumed that this won't vary between segments. The second returns the number | ||
of folios added to a segments and the third is a shorthand to indicate if the | ||
segment has been filled to capacity. | ||
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Not that the count and fullness are not affected by clearing folios from the | ||
segment. These are more about indicating how many slots in the array have been | ||
initialised, and it assumed that slots won't get reused, but rather the segment | ||
will get discarded as the queue is consumed. | ||
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Folio marks | ||
=========== | ||
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Folios within a queue can also have marks assigned to them. These marks can be | ||
used to note information such as if a folio needs folio_put() calling upon it. | ||
There are three marks available to be set for each folio. | ||
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The marks can be set by:: | ||
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void folioq_mark(struct folio_queue *folioq, unsigned int slot); | ||
void folioq_mark2(struct folio_queue *folioq, unsigned int slot); | ||
void folioq_mark3(struct folio_queue *folioq, unsigned int slot); | ||
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Cleared by:: | ||
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void folioq_unmark(struct folio_queue *folioq, unsigned int slot); | ||
void folioq_unmark2(struct folio_queue *folioq, unsigned int slot); | ||
void folioq_unmark3(struct folio_queue *folioq, unsigned int slot); | ||
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And the marks can be queried by:: | ||
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bool folioq_is_marked(const struct folio_queue *folioq, unsigned int slot); | ||
bool folioq_is_marked2(const struct folio_queue *folioq, unsigned int slot); | ||
bool folioq_is_marked3(const struct folio_queue *folioq, unsigned int slot); | ||
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The marks can be used for any purpose and are not interpreted by this API. | ||
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Folio queue iteration | ||
===================== | ||
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A list of segments may be iterated over using the I/O iterator facility using | ||
an ``iov_iter`` iterator of ``ITER_FOLIOQ`` type. The iterator may be | ||
initialised with:: | ||
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void iov_iter_folio_queue(struct iov_iter *i, unsigned int direction, | ||
const struct folio_queue *folioq, | ||
unsigned int first_slot, unsigned int offset, | ||
size_t count); | ||
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This may be told to start at a particular segment, slot and offset within a | ||
queue. The iov iterator functions will follow the next pointers when advancing | ||
and prev pointers when reverting when needed. | ||
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Lockless simultaneous production/consumption issues | ||
=================================================== | ||
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If properly managed, the list can be extended by the producer at the head end | ||
and shortened by the consumer at the tail end simultaneously without the need | ||
to take locks. The ITER_FOLIOQ iterator inserts appropriate barriers to aid | ||
with this. | ||
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Care must be taken when simultaneously producing and consuming a list. If the | ||
last segment is reached and the folios it refers to are entirely consumed by | ||
the IOV iterators, an iov_iter struct will be left pointing to the last segment | ||
with a slot number equal to the capacity of that segment. The iterator will | ||
try to continue on from this if there's another segment available when it is | ||
used again, but care must be taken lest the segment got removed and freed by | ||
the consumer before the iterator was advanced. | ||
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It is recommended that the queue always contain at least one segment, even if | ||
that segment has never been filled or is entirely spent. This prevents the | ||
head and tail pointers from collapsing. | ||
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API Function Reference | ||
====================== | ||
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.. kernel-doc:: include/linux/folio_queue.h |
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Original file line number | Diff line number | Diff line change |
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@@ -102,7 +102,7 @@ properties: | |
default: 2 | ||
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interrupts: | ||
anyOf: | ||
oneOf: | ||
- minItems: 1 | ||
items: | ||
- description: TX interrupt | ||
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Original file line number | Diff line number | Diff line change |
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@@ -302,7 +302,7 @@ allOf: | |
reg-names: | ||
items: | ||
enum: | ||
- scu | ||
- sru | ||
- ssi | ||
- adg | ||
# for Gen2/Gen3 | ||
|
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