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Diffstat (limited to 'Documentation/sparc/sbus_drivers.txt')
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diff --git a/Documentation/sparc/sbus_drivers.txt b/Documentation/sparc/sbus_drivers.txt new file mode 100644 index 00000000000..876195dc2ae --- /dev/null +++ b/Documentation/sparc/sbus_drivers.txt @@ -0,0 +1,272 @@ + + Writing SBUS Drivers + + David S. Miller (davem@redhat.com) + + The SBUS driver interfaces of the Linux kernel have been +revamped completely for 2.4.x for several reasons. Foremost were +performance and complexity concerns. This document details these +new interfaces and how they are used to write an SBUS device driver. + + SBUS drivers need to include <asm/sbus.h> to get access +to functions and structures described here. + + Probing and Detection + + Each SBUS device inside the machine is described by a +structure called "struct sbus_dev". Likewise, each SBUS bus +found in the system is described by a "struct sbus_bus". For +each SBUS bus, the devices underneath are hung in a tree-like +fashion off of the bus structure. + + The SBUS device structure contains enough information +for you to implement your device probing algorithm and obtain +the bits necessary to run your device. The most commonly +used members of this structure, and their typical usage, +will be detailed below. + + Here is how probing is performed by an SBUS driver +under Linux: + + static void init_one_mydevice(struct sbus_dev *sdev) + { + ... + } + + static int mydevice_match(struct sbus_dev *sdev) + { + if (some_criteria(sdev)) + return 1; + return 0; + } + + static void mydevice_probe(void) + { + struct sbus_bus *sbus; + struct sbus_dev *sdev; + + for_each_sbus(sbus) { + for_each_sbusdev(sdev, sbus) { + if (mydevice_match(sdev)) + init_one_mydevice(sdev); + } + } + } + + All this does is walk through all SBUS devices in the +system, checks each to see if it is of the type which +your driver is written for, and if so it calls the init +routine to attach the device and prepare to drive it. + + "init_one_mydevice" might do things like allocate software +state structures, map in I/O registers, place the hardware +into an initialized state, etc. + + Mapping and Accessing I/O Registers + + Each SBUS device structure contains an array of descriptors +which describe each register set. We abuse struct resource for that. +They each correspond to the "reg" properties provided by the OBP firmware. + + Before you can access your device's registers you must map +them. And later if you wish to shutdown your driver (for module +unload or similar) you must unmap them. You must treat them as +a resource, which you allocate (map) before using and free up +(unmap) when you are done with it. + + The mapping information is stored in an opaque value +typed as an "unsigned long". This is the type of the return value +of the mapping interface, and the arguments to the unmapping +interface. Let's say you want to map the first set of registers. +Perhaps part of your driver software state structure looks like: + + struct mydevice { + unsigned long control_regs; + ... + struct sbus_dev *sdev; + ... + }; + + At initialization time you then use the sbus_ioremap +interface to map in your registers, like so: + + static void init_one_mydevice(struct sbus_dev *sdev) + { + struct mydevice *mp; + ... + + mp->control_regs = sbus_ioremap(&sdev->resource[0], 0, + CONTROL_REGS_SIZE, "mydevice regs"); + if (!mp->control_regs) { + /* Failure, cleanup and return. */ + } + } + + Second argument to sbus_ioremap is an offset for +cranky devices with broken OBP PROM. The sbus_ioremap uses only +a start address and flags from the resource structure. +Therefore it is possible to use the same resource to map +several sets of registers or even to fabricate a resource +structure if driver gets physical address from some private place. +This practice is discouraged though. Use whatever OBP PROM +provided to you. + + And here is how you might unmap these registers later at +driver shutdown or module unload time, using the sbus_iounmap +interface: + + static void mydevice_unmap_regs(struct mydevice *mp) + { + sbus_iounmap(mp->control_regs, CONTROL_REGS_SIZE); + } + + Finally, to actually access your registers there are 6 +interface routines at your disposal. Accesses are byte (8 bit), +word (16 bit), or longword (32 bit) sized. Here they are: + + u8 sbus_readb(unsigned long reg) /* read byte */ + u16 sbus_readw(unsigned long reg) /* read word */ + u32 sbus_readl(unsigned long reg) /* read longword */ + void sbus_writeb(u8 value, unsigned long reg) /* write byte */ + void sbus_writew(u16 value, unsigned long reg) /* write word */ + void sbus_writel(u32 value, unsigned long reg) /* write longword */ + + So, let's say your device has a control register of some sort +at offset zero. The following might implement resetting your device: + + #define CONTROL 0x00UL + + #define CONTROL_RESET 0x00000001 /* Reset hardware */ + + static void mydevice_reset(struct mydevice *mp) + { + sbus_writel(CONTROL_RESET, mp->regs + CONTROL); + } + + Or perhaps there is a data port register at an offset of +16 bytes which allows you to read bytes from a fifo in the device: + + #define DATA 0x10UL + + static u8 mydevice_get_byte(struct mydevice *mp) + { + return sbus_readb(mp->regs + DATA); + } + + It's pretty straightforward, and clueful readers may have +noticed that these interfaces mimick the PCI interfaces of the +Linux kernel. This was not by accident. + + WARNING: + + DO NOT try to treat these opaque register mapping + values as a memory mapped pointer to some structure + which you can dereference. + + It may be memory mapped, it may not be. In fact it + could be a physical address, or it could be the time + of day xor'd with 0xdeadbeef. :-) + + Whatever it is, it's an implementation detail. The + interface was done this way to shield the driver + author from such complexities. + + Doing DVMA + + SBUS devices can perform DMA transactions in a way similar +to PCI but dissimilar to ISA, e.g. DMA masters supply address. +In contrast to PCI, however, that address (a bus address) is +translated by IOMMU before a memory access is performed and therefore +it is virtual. Sun calls this procedure DVMA. + + Linux supports two styles of using SBUS DVMA: "consistent memory" +and "streaming DVMA". CPU view of consistent memory chunk is, well, +consistent with a view of a device. Think of it as an uncached memory. +Typically this way of doing DVMA is not very fast and drivers use it +mostly for control blocks or queues. On some CPUs we cannot flush or +invalidate individual pages or cache lines and doing explicit flushing +over ever little byte in every control block would be wasteful. + +Streaming DVMA is a preferred way to transfer large amounts of data. +This process works in the following way: +1. a CPU stops accessing a certain part of memory, + flushes its caches covering that memory; +2. a device does DVMA accesses, then posts an interrupt; +3. CPU invalidates its caches and starts to access the memory. + +A single streaming DVMA operation can touch several discontiguous +regions of a virtual bus address space. This is called a scatter-gather +DVMA. + +[TBD: Why do not we neither Solaris attempt to map disjoint pages +into a single virtual chunk with the help of IOMMU, so that non SG +DVMA masters would do SG? It'd be very helpful for RAID.] + + In order to perform a consistent DVMA a driver does something +like the following: + + char *mem; /* Address in the CPU space */ + u32 busa; /* Address in the SBus space */ + + mem = (char *) sbus_alloc_consistent(sdev, MYMEMSIZE, &busa); + + Then mem is used when CPU accesses this memory and u32 +is fed to the device so that it can do DVMA. This is typically +done with an sbus_writel() into some device register. + + Do not forget to free the DVMA resources once you are done: + + sbus_free_consistent(sdev, MYMEMSIZE, mem, busa); + + Streaming DVMA is more interesting. First you allocate some +memory suitable for it or pin down some user pages. Then it all works +like this: + + char *mem = argumen1; + unsigned int size = argument2; + u32 busa; /* Address in the SBus space */ + + *mem = 1; /* CPU can access */ + busa = sbus_map_single(sdev, mem, size); + if (busa == 0) ....... + + /* Tell the device to use busa here */ + /* CPU cannot access the memory without sbus_dma_sync_single() */ + + sbus_unmap_single(sdev, busa, size); + if (*mem == 0) .... /* CPU can access again */ + + It is possible to retain mappings and ask the device to +access data again and again without calling sbus_unmap_single. +However, CPU caches must be invalidated with sbus_dma_sync_single +before such access. + +[TBD but what about writeback caches here... do we have any?] + + There is an equivalent set of functions doing the same thing +only with several memory segments at once for devices capable of +scatter-gather transfers. Use the Source, Luke. + + Examples + + drivers/net/sunhme.c + This is a complicated driver which illustrates many concepts +discussed above and plus it handles both PCI and SBUS boards. + + drivers/scsi/esp.c + Check it out for scatter-gather DVMA. + + drivers/sbus/char/bpp.c + A non-DVMA device. + + drivers/net/sunlance.c + Lance driver abuses consistent mappings for data transfer. +It is a nifty trick which we do not particularly recommend... +Just check it out and know that it's legal. + + Bad examples, do NOT use + + drivers/video/cgsix.c + This one uses result of sbus_ioremap as if it is an address. +This does NOT work on sparc64 and therefore is broken. We will +convert it at a later date. |