Dear Arnd, Josh Boyer suggested you might provide some insight...
I'm currently looking for a solution how to provide architecture independent I/O accessor functions to U-Boot. In the past, lots of code used direct pointer accesses, relying on the idea that "volatile" would be sufficient to convince the compiler and the hardware to do what was expected; some architectures (like ARM and others) used readl() / writel(), while others (like PPC) used in_8, in_le16, in_be16, in_le32, in_be32, in_le64, in_be64 etc. As we like to borrow code from Linux, I'm trying to find out what the big plan for Linux is. My understanding is that in Linux the ioreadX() / iowriteX() / ioreadXbe() / iowriteXbe() functions are supposed to provide architecture independent I/O accessors, and that the plain ioreadX() / iowriteX() functions (without the "be") are always guaranteed to be little-endian on all architectures, while the "be" functions are, well, big-endian. Is this understanding correct? If yes, does that mean that in the future we will see more Linux code using ioreadX[be]() / iowriteX[be]()? So far I did not find much hints that support this aproach - only memory-barriers.txt has only a short sentence about these functions, with basicly no explanation. What I liked from the in_[le]X() / out_[le]X() accessors on PPC was that they allowed for type checking - the compiler would raise a warning when you used in_[le]16() to read from a 32 bit wide register. However, ioreadX[be]() / iowriteX[be]() use a void * "iomem cookie", so no type checking can be done. Basicly I have two questions: 1) Can you make a statement which direction Linux is heading to? Will more (new) code use ioreadX() / iowriteX()? 2) What would be your recommendation what we should do in U-Boot? Provide for all architectures in_8, in_le16, in_be16, in_le32, in_be32, in_le64, in_be64 etc. similar to what we have for the Power architecture, well knowing that Linux will not follow that route, or use ioreadX[be]() / iowriteX[be]() which does not provide type checking, and which eventually does not find wider use in Linux either? Or even something else - like ioreadX[be]() / iowriteX[be]() with type checking added? Thanks in advance. Best regards, Wolfgang Denk -- DENX Software Engineering GmbH, MD: Wolfgang Denk & Detlev Zundel HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: w...@denx.de The ideal situation is to have massive computing power right at home. Something that dims the streetlights and shrinks the picture on the neighbours' TVs when you boot it up. -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
