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gerrit-public.fairphone.software / kernel / msm / 0e77575346c2ed371e06aa3b409c74221be09c38 / . / Documentation / video4linux / pxa_camera.txt
blob: 4f6d0ca019564f86b4dfed75fff7b9597dbd838b [file] [log] [blame]
Robert Jarzmik256b0232009-03-31 03:44:21 -0300[diff] [blame]1 PXA-Camera Host Driver
2 ======================
3
4Constraints
5-----------
6 a) Image size for YUV422P format
7 All YUV422P images are enforced to have width x height % 16 = 0.
8 This is due to DMA constraints, which transfers only planes of 8 byte
9 multiples.
10
11
12Global video workflow
13---------------------
14 a) QCI stopped
15 Initialy, the QCI interface is stopped.
16 When a buffer is queued (pxa_videobuf_ops->buf_queue), the QCI starts.
17
18 b) QCI started
19 More buffers can be queued while the QCI is started without halting the
20 capture. The new buffers are "appended" at the tail of the DMA chain, and
21 smoothly captured one frame after the other.
22
23 Once a buffer is filled in the QCI interface, it is marked as "DONE" and
24 removed from the active buffers list. It can be then requeud or dequeued by
25 userland application.
26
27 Once the last buffer is filled in, the QCI interface stops.
28
Robert Jarzmik56600092009-04-24 12:58:35 -0300[diff] [blame]29 c) Capture global finite state machine schema
30
31 +----+ +---+ +----+
32 | DQ | | Q | | DQ |
33 | v | v | v
34 +-----------+ +------------------------+
35 | STOP | | Wait for capture start |
36 +-----------+ Q +------------------------+
37+-> | QCI: stop | ------------------> | QCI: run | <------------+
38| | DMA: stop | | DMA: stop | |
39| +-----------+ +-----> +------------------------+ |
40| / | |
41| / +---+ +----+ | |
42|capture list empty / | Q | | DQ | | QCI Irq EOF |
43| / | v | v v |
44| +--------------------+ +----------------------+ |
45| | DMA hotlink missed | | Capture running | |
46| +--------------------+ +----------------------+ |
47| | QCI: run | +-----> | QCI: run | <-+ |
48| | DMA: stop | / | DMA: run | | |
49| +--------------------+ / +----------------------+ | Other |
50| ^ /DMA still | | channels |
51| | capture list / running | DMA Irq End | not |
52| | not empty / | | finished |
53| | / v | yet |
54| +----------------------+ +----------------------+ | |
55| | Videobuf released | | Channel completed | | |
56| +----------------------+ +----------------------+ | |
57+-- | QCI: run | | QCI: run | --+ |
58 | DMA: run | | DMA: run | |
59 +----------------------+ +----------------------+ |
60 ^ / | |
61 | no overrun / | overrun |
62 | / v |
63 +--------------------+ / +----------------------+ |
64 | Frame completed | / | Frame overran | |
65 +--------------------+ <-----+ +----------------------+ restart frame |
66 | QCI: run | | QCI: stop | --------------+
67 | DMA: run | | DMA: stop |
68 +--------------------+ +----------------------+
69
70 Legend: - each box is a FSM state
71 - each arrow is the condition to transition to another state
72 - an arrow with a comment is a mandatory transition (no condition)
73 - arrow "Q" means : a buffer was enqueued
74 - arrow "DQ" means : a buffer was dequeued
75 - "QCI: stop" means the QCI interface is not enabled
76 - "DMA: stop" means all 3 DMA channels are stopped
77 - "DMA: run" means at least 1 DMA channel is still running
Robert Jarzmik256b0232009-03-31 03:44:21 -0300[diff] [blame]78
79DMA usage
80---------
81 a) DMA flow
82 - first buffer queued for capture
83 Once a first buffer is queued for capture, the QCI is started, but data
84 transfer is not started. On "End Of Frame" interrupt, the irq handler
85 starts the DMA chain.
86 - capture of one videobuffer
87 The DMA chain starts transfering data into videobuffer RAM pages.
88 When all pages are transfered, the DMA irq is raised on "ENDINTR" status
89 - finishing one videobuffer
90 The DMA irq handler marks the videobuffer as "done", and removes it from
91 the active running queue
92 Meanwhile, the next videobuffer (if there is one), is transfered by DMA
93 - finishing the last videobuffer
94 On the DMA irq of the last videobuffer, the QCI is stopped.
95
96 b) DMA prepared buffer will have this structure
97
98 +------------+-----+---------------+-----------------+
99 | desc-sg[0] | ... | desc-sg[last] | finisher/linker |
100 +------------+-----+---------------+-----------------+
101
102 This structure is pointed by dma->sg_cpu.
103 The descriptors are used as follows :
104 - desc-sg[i]: i-th descriptor, transfering the i-th sg
105 element to the video buffer scatter gather
106 - finisher: has ddadr=DADDR_STOP, dcmd=ENDIRQEN
107 - linker: has ddadr= desc-sg[0] of next video buffer, dcmd=0
108
109 For the next schema, let's assume d0=desc-sg[0] .. dN=desc-sg[N],
110 "f" stands for finisher and "l" for linker.
111 A typical running chain is :
112
113 Videobuffer 1 Videobuffer 2
114 +---------+----+---+ +----+----+----+---+
115 | d0 | .. | dN | l | | d0 | .. | dN | f |
116 +---------+----+-|-+ ^----+----+----+---+
117 | |
118 +----+
119
120 After the chaining is finished, the chain looks like :
121
122 Videobuffer 1 Videobuffer 2 Videobuffer 3
123 +---------+----+---+ +----+----+----+---+ +----+----+----+---+
124 | d0 | .. | dN | l | | d0 | .. | dN | l | | d0 | .. | dN | f |
125 +---------+----+-|-+ ^----+----+----+-|-+ ^----+----+----+---+
126 | | | |
127 +----+ +----+
128 new_link
129
130 c) DMA hot chaining timeslice issue
131
132 As DMA chaining is done while DMA _is_ running, the linking may be done
133 while the DMA jumps from one Videobuffer to another. On the schema, that
134 would be a problem if the following sequence is encountered :
135
136 - DMA chain is Videobuffer1 + Videobuffer2
137 - pxa_videobuf_queue() is called to queue Videobuffer3
138 - DMA controller finishes Videobuffer2, and DMA stops
139 =>
140 Videobuffer 1 Videobuffer 2
141 +---------+----+---+ +----+----+----+---+
142 | d0 | .. | dN | l | | d0 | .. | dN | f |
143 +---------+----+-|-+ ^----+----+----+-^-+
144 | | |
145 +----+ +-- DMA DDADR loads DDADR_STOP
146
147 - pxa_dma_add_tail_buf() is called, the Videobuffer2 "finisher" is
148 replaced by a "linker" to Videobuffer3 (creation of new_link)
149 - pxa_videobuf_queue() finishes
150 - the DMA irq handler is called, which terminates Videobuffer2
151 - Videobuffer3 capture is not scheduled on DMA chain (as it stopped !!!)
152
153 Videobuffer 1 Videobuffer 2 Videobuffer 3
154 +---------+----+---+ +----+----+----+---+ +----+----+----+---+
155 | d0 | .. | dN | l | | d0 | .. | dN | l | | d0 | .. | dN | f |
156 +---------+----+-|-+ ^----+----+----+-|-+ ^----+----+----+---+
157 | | | |
158 +----+ +----+
159 new_link
160 DMA DDADR still is DDADR_STOP
161
162 - pxa_camera_check_link_miss() is called
163 This checks if the DMA is finished and a buffer is still on the
164 pcdev->capture list. If that's the case, the capture will be restarted,
165 and Videobuffer3 is scheduled on DMA chain.
166 - the DMA irq handler finishes
167
168 Note: if DMA stops just after pxa_camera_check_link_miss() reads DDADR()
169 value, we have the guarantee that the DMA irq handler will be called back
170 when the DMA will finish the buffer, and pxa_camera_check_link_miss() will
171 be called again, to reschedule Videobuffer3.
172
173--
174Author: Robert Jarzmik <robert.jarzmik@free.fr>