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Based on kernel version 4.3. Page generated on 2015-11-02 12:43 EST.

1	2.5.2-rmk5
2	----------
4	This is the first kernel that contains a major shake up of some of the
5	major architecture-specific subsystems.
7	Firstly, it contains some pretty major changes to the way we handle the
8	MMU TLB.  Each MMU TLB variant is now handled completely separately -
9	we have TLB v3, TLB v4 (without write buffer), TLB v4 (with write buffer),
10	and finally TLB v4 (with write buffer, with I TLB invalidate entry).
11	There is more assembly code inside each of these functions, mainly to
12	allow more flexible TLB handling for the future.
14	Secondly, the IRQ subsystem.
16	The 2.5 kernels will be having major changes to the way IRQs are handled.
17	Unfortunately, this means that machine types that touch the irq_desc[]
18	array (basically all machine types) will break, and this means every
19	machine type that we currently have.
21	Lets take an example.  On the Assabet with Neponset, we have:
23	                  GPIO25                 IRR:2
24	        SA1100 ------------> Neponset -----------> SA1111
25	                                         IIR:1
26	                                      -----------> USAR
27	                                         IIR:0
28	                                      -----------> SMC9196
30	The way stuff currently works, all SA1111 interrupts are mutually
31	exclusive of each other - if you're processing one interrupt from the
32	SA1111 and another comes in, you have to wait for that interrupt to
33	finish processing before you can service the new interrupt.  Eg, an
34	IDE PIO-based interrupt on the SA1111 excludes all other SA1111 and
35	SMC9196 interrupts until it has finished transferring its multi-sector
36	data, which can be a long time.  Note also that since we loop in the
37	SA1111 IRQ handler, SA1111 IRQs can hold off SMC9196 IRQs indefinitely.
40	The new approach brings several new ideas...
42	We introduce the concept of a "parent" and a "child".  For example,
43	to the Neponset handler, the "parent" is GPIO25, and the "children"d
44	are SA1111, SMC9196 and USAR.
46	We also bring the idea of an IRQ "chip" (mainly to reduce the size of
47	the irqdesc array).  This doesn't have to be a real "IC"; indeed the
48	SA11x0 IRQs are handled by two separate "chip" structures, one for
49	GPIO0-10, and another for all the rest.  It is just a container for
50	the various operations (maybe this'll change to a better name).
51	This structure has the following operations:
53	struct irqchip {
54	        /*
55	         * Acknowledge the IRQ.
56	         * If this is a level-based IRQ, then it is expected to mask the IRQ
57	         * as well.
58	         */
59	        void (*ack)(unsigned int irq);
60	        /*
61	         * Mask the IRQ in hardware.
62	         */
63	        void (*mask)(unsigned int irq);
64	        /*
65	         * Unmask the IRQ in hardware.
66	         */
67	        void (*unmask)(unsigned int irq);
68	        /*
69	         * Re-run the IRQ
70	         */
71	        void (*rerun)(unsigned int irq);
72	        /*
73	         * Set the type of the IRQ.
74	         */
75	        int (*type)(unsigned int irq, unsigned int, type);
76	};
78	ack    - required.  May be the same function as mask for IRQs
79	         handled by do_level_IRQ.
80	mask   - required.
81	unmask - required.
82	rerun  - optional.  Not required if you're using do_level_IRQ for all
83	         IRQs that use this 'irqchip'.  Generally expected to re-trigger
84	         the hardware IRQ if possible.  If not, may call the handler
85		 directly.
86	type   - optional.  If you don't support changing the type of an IRQ,
87	         it should be null so people can detect if they are unable to
88	         set the IRQ type.
90	For each IRQ, we keep the following information:
92	        - "disable" depth (number of disable_irq()s without enable_irq()s)
93	        - flags indicating what we can do with this IRQ (valid, probe,
94	          noautounmask) as before
95	        - status of the IRQ (probing, enable, etc)
96	        - chip
97	        - per-IRQ handler
98	        - irqaction structure list
100	The handler can be one of the 3 standard handlers - "level", "edge" and
101	"simple", or your own specific handler if you need to do something special.
103	The "level" handler is what we currently have - its pretty simple.
104	"edge" knows about the brokenness of such IRQ implementations - that you
105	need to leave the hardware IRQ enabled while processing it, and queueing
106	further IRQ events should the IRQ happen again while processing.  The
107	"simple" handler is very basic, and does not perform any hardware
108	manipulation, nor state tracking.  This is useful for things like the
109	SMC9196 and USAR above.
111	So, what's changed?
113	1. Machine implementations must not write to the irqdesc array.
115	2. New functions to manipulate the irqdesc array.  The first 4 are expected
116	   to be useful only to machine specific code.  The last is recommended to
117	   only be used by machine specific code, but may be used in drivers if
118	   absolutely necessary.
120	        set_irq_chip(irq,chip)
122	                Set the mask/unmask methods for handling this IRQ
124	        set_irq_handler(irq,handler)
126	                Set the handler for this IRQ (level, edge, simple)
128	        set_irq_chained_handler(irq,handler)
130	                Set a "chained" handler for this IRQ - automatically
131	                enables this IRQ (eg, Neponset and SA1111 handlers).
133	        set_irq_flags(irq,flags)
135	                Set the valid/probe/noautoenable flags.
137	        set_irq_type(irq,type)
139	                Set active the IRQ edge(s)/level.  This replaces the
140	                SA1111 INTPOL manipulation, and the set_GPIO_IRQ_edge()
141	                function.  Type should be one of IRQ_TYPE_xxx defined in
142			<linux/irq.h>
144	3. set_GPIO_IRQ_edge() is obsolete, and should be replaced by set_irq_type.
146	4. Direct access to SA1111 INTPOL is deprecated.  Use set_irq_type instead.
148	5. A handler is expected to perform any necessary acknowledgement of the
149	   parent IRQ via the correct chip specific function.  For instance, if
150	   the SA1111 is directly connected to a SA1110 GPIO, then you should
151	   acknowledge the SA1110 IRQ each time you re-read the SA1111 IRQ status.
153	6. For any child which doesn't have its own IRQ enable/disable controls
154	   (eg, SMC9196), the handler must mask or acknowledge the parent IRQ
155	   while the child handler is called, and the child handler should be the
156	   "simple" handler (not "edge" nor "level").  After the handler completes,
157	   the parent IRQ should be unmasked, and the status of all children must
158	   be re-checked for pending events.  (see the Neponset IRQ handler for
159	   details).
161	7. fixup_irq() is gone, as is arch/arm/mach-*/include/mach/irq.h
163	Please note that this will not solve all problems - some of them are
164	hardware based.  Mixing level-based and edge-based IRQs on the same
165	parent signal (eg neponset) is one such area where a software based
166	solution can't provide the full answer to low IRQ latency.
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