On Wed, Jul 27, 2005 at 08:28:38AM +0100, Dominic Sweetman wrote: > > > In tx4938, every register access is done by using "volatile" like below. > > > > Linus is right, volatile is a dangerous thing. If you want to write > > portable code there's a bunch of things that are not being taken care of > > by plain C - even though in my opinion foo->somereg = 42 is more > > readable than writel(somereg, 42). Among the things the pointer to > > volatile struct method doesn't catch are endianess conversion that might > > be necessary on some systems, write merging, dealing with write buffers > > or completly insane methods of attaching the bus such as the infamous > > ISA / EISA cage that's attached to the host system through a USB > > interface. > > Yes, this is far outside the compiler's reach. > > All of which suggests that it would make sense to define a standard function > which: > > o will produce just one fixed-width write cycle to the destination; > > o will deliver the data ordered so that the MSB of the C value is on > the "most significant" bit of the device's data bus, usually the > highest numbered bit (this doesn't solve all device endianess > issues, but it gives you a well-defined place to start solving them); > > o has a variant which returns only after some indication that the > data was delivered; > > The implementation of this function can then conceal the details of > the CPU and interconnect. > > Such a function should probably not be called "writel()" because that > sounds like "write long", and "long" is not a fixed-size data type, > which undermines the promises above... Tediously, you probably need > "writei32()", "writei16()", "writei8()"... Linux has a long tradition of grossly missnaming things, so readw reads 16-bit words, readl reads 32-bit words and readq 64-bit words, that is each of them operates on just half the quantity a MIPS programmer would expect. Same for writew, writel and writeq. Blame the Intel guys for it ;-) Ranting about grossly missnaming things, the DMA API calls coherent what MIPS calls non-coherent and vice versa. I'll stop now, birds are whistling way to nice behind The Fruit Farm for me to write a good rant today ;-) There are ioread8, ioread16, ioread32, iowrite8, iowrite16, iowrite32 already except they're primarily used with I/O busses such as PCI but that's not really an issue. Ralf
