On Wed, Jul 15, 2015 at 10:44:30AM +1200, Chris Packham wrote: > Update __read_32bit_c0_register() and __read_32bit_c0_ctrl_register() to > use "unsigned int res;" instead of "int res;". There is little reason to > treat these register values as signed. They are either counters (which > by definition are unsigned) or are made up of various bit fields to be > interpreted as per the CPU datasheet. > > Signed-off-by: Chris Packham <judge.packham@xxxxxxxxx> > > --- > This has come up via u-boot[1] which sync's asm/mipsregs.h with the > kernel. In u-boots case the value read from read_c0_count() is assigned > to an unsigned long [2] which triggers a sign extension and causes a > bug. > > U-boot should probably be more explicit about the types used for the > timer_read_counter() API but that aside is there any reason to treat > these values as signed integers? A quick grep around the arch/mips makes > me thing that there may be some bugs lurking when read_c0_count() starts > to yield a negative value but I haven't really explored any of them. Known issue but I've always been concerned about math with cycle values like: unsigned int now, timeout = read_c0_counter() + a_bit_of_time; waste_some_time(); if (timeout - read_c0_counter() < 0) do_timeout_stuff(); Which now with both variables being unsigned would yield a positive value thus the if would never be taken. This particular construction GCC would warn about but there are other, constructs that wouldn't trigger a warning. I don't even want to think about what C type propagation rules say about mixing signed and unsigned types. Whenever such knowledge is required to figure out what a piece of code is doing it probably should be considered broken anyway - but the mess resulting from unwanted sign is no better! Anyway, I've queued your patch for 4.3. Thanks! > I also notice that read_32bit_cp1_register has a similar issue. I > haven't touched it at this stage but it probably makes sense to do so > for consistency if the CP0 macros are changed. Looking at the users of > read_32bit_cp1_register() it's probably less of an issue. I've cooked up a patch for read_32bit_cp1_register and queued it for 4.3. Ralf
