On Tue, 2008-06-03 at 12:43 -0700, Trent Piepho wrote: > > Byte-swapping vs not byte-swapping is not usually what the programmer wants. > Usually your device's registers are defined as being big-endian or > little-endian and you want whatever is needed to give you that. Yes, which is why I (and some other archs) have writel_be/readl_be. The standard writel/readl being LE. However, the "raw" variants are defined to be native endian, which is of some use to -some- archs apparently where they have SoC device whose endianness follow the core. > I believe that on some archs that can be either byte order, some built-in > devices will change their registers to match, and so you want "native endian" > or no swapping for these. Though that's definitely in the minority. > > An accessors that always byte-swaps regardless of the endianness of the host > is never something I've seen a driver want. > > IOW, there are four ways one can defined endianness/swapping: > 1) Little-endian > 2) Big-endian > 3) Native-endian aka non-byte-swapping > 4) Foreign-endian aka byte-swapping > > 1 and 2 are by far the most used. Some code wants 3. No one wants 4. Yet > our API is providing 3 & 4, the two which are the least useful. No, we don't provide 4, it was something unclear with nick. We provide 1. (writel/readl and __variants), some archs provide 2 (writel_be/readl_be, tho I don't have __variants, I suppose I could), and everybody provides 3. though in some cases (like us) only in the form of __variants (ie, non ordered, like __raw_readl/__raw_writel). Nick's proposal is to plug those gaps, though it's, I believe, missing the _be variants. > Is it enough to provide only "all or none" for ordering strictness? For > instance on powerpc, one can get a speedup by dropping strict ordering for IO > vs cacheable memory, but still keeping ordering for IO vs IO and IO vs locks. > This is much easier to program for than no ordering at all. In fact, if one > doesn't use coherent DMA, it's basically the same as fully strict ordering. Ben. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
