Ralf Baechle (ralf@xxxxxxxxxxxxxx) writes: > > 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()"... -- Dominic Sweetman MIPS Technologies
