+++ /dev/null
-
- 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 a piece of skeleton code for performing a device
-probe in an SBUS driver under Linux:
-
- static int __devinit mydevice_probe_one(struct sbus_dev *sdev)
- {
- struct mysdevice *mp = kzalloc(sizeof(*mp), GFP_KERNEL);
-
- if (!mp)
- return -ENODEV;
-
- ...
- dev_set_drvdata(&sdev->ofdev.dev, mp);
- return 0;
- ...
- }
-
- static int __devinit mydevice_probe(struct of_device *dev,
- const struct of_device_id *match)
- {
- struct sbus_dev *sdev = to_sbus_device(&dev->dev);
-
- return mydevice_probe_one(sdev);
- }
-
- static int __devexit mydevice_remove(struct of_device *dev)
- {
- struct sbus_dev *sdev = to_sbus_device(&dev->dev);
- struct mydevice *mp = dev_get_drvdata(&dev->dev);
-
- return mydevice_remove_one(sdev, mp);
- }
-
- static struct of_device_id mydevice_match[] = {
- {
- .name = "mydevice",
- },
- {},
- };
-
- MODULE_DEVICE_TABLE(of, mydevice_match);
-
- static struct of_platform_driver mydevice_driver = {
- .match_table = mydevice_match,
- .probe = mydevice_probe,
- .remove = __devexit_p(mydevice_remove),
- .driver = {
- .name = "mydevice",
- },
- };
-
- static int __init mydevice_init(void)
- {
- return of_register_driver(&mydevice_driver, &sbus_bus_type);
- }
-
- static void __exit mydevice_exit(void)
- {
- of_unregister_driver(&mydevice_driver);
- }
-
- module_init(mydevice_init);
- module_exit(mydevice_exit);
-
- The mydevice_match table is a series of entries which
-describes what SBUS devices your driver is meant for. In the
-simplest case you specify a string for the 'name' field. Every
-SBUS device with a 'name' property matching your string will
-be passed one-by-one to your .probe method.
-
- You should store away your device private state structure
-pointer in the drvdata area so that you can retrieve it later on
-in your .remove method.
-
- Any memory allocated, registers mapped, IRQs registered,
-etc. must be undone by your .remove method so that all resources
-of your device are released by the time it returns.
-
- You should _NOT_ use the for_each_sbus(), for_each_sbusdev(),
-and for_all_sbusdev() interfaces. They are deprecated, will be
-removed, and no new driver should reference them ever.
-
- 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.
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