On 04/29/2014 01:31 PM, Dave Martin wrote: > On Tue, Apr 29, 2014 at 05:36:30PM +0100, Joel Fernandes wrote: > > [...] > >>>> Sorry what I meant is, say its of Type function. What tells the firmware >>>> to switch to THUMB? >>>> >>>> What's typically done is a boot address register is written by the >>>> kernel, and the firmware jumps to it after WFE. >>> >>> Using ENTRY(x) ... ENDPROC(x) causes the symbol seen by the linker >>> for x to have the Thumb bit set if the code is Thumb. >>> >>> This means that any reference the linker fixes up for that symbol >>> will have the Thumb bit set appropriately. This applies to any kind >>> of reference, so code in another file that takes the address of the >>> symbol and then passes that address to the firmware should result in the >>> firmware getting an address with the Thumb bit. >>> >>> From the firmware's point of view it just gets a raw address, but >>> the Thumb bit will now be set. The firmware still needs to handle >>> this correctly when jumping, but from the look of the code this may >>> already work on omap3/4. It would be interesting to know whether it >>> works on omap5. >> >> Thanks a lot for the explanation. That makes perfect sense. I will try it and let you know if it works on OMAP5. > > ARM/thumb interworking making perfect sense? I'll have to frame that > and put it on the wall :) > > objdump and nm conveniently mask off the Thumb bit from all function > addresses they print out, but if you show the symbols using readelf > instead you'll see addresses with bit 0 set for Thumb functions. > > It's possible that the firmware still doesn't handle branching to Thumb > correctly, but if it does it would be nice to remove the requirement to > build an odd piece of a Thumb-2 kernel in ARM. Sure, it works great! thanks, I sent out a patch that applies on top of the first one. Thanks for spotting it. Regards, -Joel -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
