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+This is the first kernel that contains a major shake up of some of the
+major architecture-specific subsystems.
+Firstly, it contains some pretty major changes to the way we handle the
+MMU TLB. Each MMU TLB variant is now handled completely separately -
+we have TLB v3, TLB v4 (without write buffer), TLB v4 (with write buffer),
+and finally TLB v4 (with write buffer, with I TLB invalidate entry).
+There is more assembly code inside each of these functions, mainly to
+allow more flexible TLB handling for the future.
+Secondly, the IRQ subsystem.
+The 2.5 kernels will be having major changes to the way IRQs are handled.
+Unfortunately, this means that machine types that touch the irq_desc[]
+array (basically all machine types) will break, and this means every
+machine type that we currently have.
+Lets take an example. On the Assabet with Neponset, we have:
+ GPIO25 IRR:2
+ SA1100 ------------> Neponset -----------> SA1111
+ IIR:1
+ -----------> USAR
+ IIR:0
+ -----------> SMC9196
+The way stuff currently works, all SA1111 interrupts are mutually
+exclusive of each other - if you're processing one interrupt from the
+SA1111 and another comes in, you have to wait for that interrupt to
+finish processing before you can service the new interrupt. Eg, an
+IDE PIO-based interrupt on the SA1111 excludes all other SA1111 and
+SMC9196 interrupts until it has finished transferring its multi-sector
+data, which can be a long time. Note also that since we loop in the
+SA1111 IRQ handler, SA1111 IRQs can hold off SMC9196 IRQs indefinitely.
+The new approach brings several new ideas...
+We introduce the concept of a "parent" and a "child". For example,
+to the Neponset handler, the "parent" is GPIO25, and the "children"d
+are SA1111, SMC9196 and USAR.
+We also bring the idea of an IRQ "chip" (mainly to reduce the size of
+the irqdesc array). This doesn't have to be a real "IC"; indeed the
+SA11x0 IRQs are handled by two separate "chip" structures, one for
+GPIO0-10, and another for all the rest. It is just a container for
+the various operations (maybe this'll change to a better name).
+This structure has the following operations:
+struct irqchip {
+ /*
+ * Acknowledge the IRQ.
+ * If this is a level-based IRQ, then it is expected to mask the IRQ
+ * as well.
+ */
+ void (*ack)(unsigned int irq);
+ /*
+ * Mask the IRQ in hardware.
+ */
+ void (*mask)(unsigned int irq);
+ /*
+ * Unmask the IRQ in hardware.
+ */
+ void (*unmask)(unsigned int irq);
+ /*
+ * Re-run the IRQ
+ */
+ void (*rerun)(unsigned int irq);
+ /*
+ * Set the type of the IRQ.
+ */
+ int (*type)(unsigned int irq, unsigned int, type);
+ack - required. May be the same function as mask for IRQs
+ handled by do_level_IRQ.
+mask - required.
+unmask - required.
+rerun - optional. Not required if you're using do_level_IRQ for all
+ IRQs that use this 'irqchip'. Generally expected to re-trigger
+ the hardware IRQ if possible. If not, may call the handler
+ directly.
+type - optional. If you don't support changing the type of an IRQ,
+ it should be null so people can detect if they are unable to
+ set the IRQ type.
+For each IRQ, we keep the following information:
+ - "disable" depth (number of disable_irq()s without enable_irq()s)
+ - flags indicating what we can do with this IRQ (valid, probe,
+ noautounmask) as before
+ - status of the IRQ (probing, enable, etc)
+ - chip
+ - per-IRQ handler
+ - irqaction structure list
+The handler can be one of the 3 standard handlers - "level", "edge" and
+"simple", or your own specific handler if you need to do something special.
+The "level" handler is what we currently have - its pretty simple.
+"edge" knows about the brokenness of such IRQ implementations - that you
+need to leave the hardware IRQ enabled while processing it, and queueing
+further IRQ events should the IRQ happen again while processing. The
+"simple" handler is very basic, and does not perform any hardware
+manipulation, nor state tracking. This is useful for things like the
+SMC9196 and USAR above.
+So, what's changed?
+1. Machine implementations must not write to the irqdesc array.
+2. New functions to manipulate the irqdesc array. The first 4 are expected
+ to be useful only to machine specific code. The last is recommended to
+ only be used by machine specific code, but may be used in drivers if
+ absolutely necessary.
+ set_irq_chip(irq,chip)
+ Set the mask/unmask methods for handling this IRQ
+ set_irq_handler(irq,handler)
+ Set the handler for this IRQ (level, edge, simple)
+ set_irq_chained_handler(irq,handler)
+ Set a "chained" handler for this IRQ - automatically
+ enables this IRQ (eg, Neponset and SA1111 handlers).
+ set_irq_flags(irq,flags)
+ Set the valid/probe/noautoenable flags.
+ set_irq_type(irq,type)
+ Set active the IRQ edge(s)/level. This replaces the
+ SA1111 INTPOL manipulation, and the set_GPIO_IRQ_edge()
+ function. Type should be one of the following:
+ #define IRQT_NOEDGE (0)
+ #define IRQT_LOW (__IRQT_LOWLVL)
+3. set_GPIO_IRQ_edge() is obsolete, and should be replaced by set_irq_type.
+4. Direct access to SA1111 INTPOL is depreciated. Use set_irq_type instead.
+5. A handler is expected to perform any necessary acknowledgement of the
+ parent IRQ via the correct chip specific function. For instance, if
+ the SA1111 is directly connected to a SA1110 GPIO, then you should
+ acknowledge the SA1110 IRQ each time you re-read the SA1111 IRQ status.
+6. For any child which doesn't have its own IRQ enable/disable controls
+ (eg, SMC9196), the handler must mask or acknowledge the parent IRQ
+ while the child handler is called, and the child handler should be the
+ "simple" handler (not "edge" nor "level"). After the handler completes,
+ the parent IRQ should be unmasked, and the status of all children must
+ be re-checked for pending events. (see the Neponset IRQ handler for
+ details).
+7. fixup_irq() is gone, as is include/asm-arm/arch-*/irq.h
+Please note that this will not solve all problems - some of them are
+hardware based. Mixing level-based and edge-based IRQs on the same
+parent signal (eg neponset) is one such area where a software based
+solution can't provide the full answer to low IRQ latency.