On Mar 14, 2011, at 15:41, York Sun wrote: > Kyle, > > On Mon, 2011-03-14 at 14:04 -0500, Moffett, Kyle D wrote: >> On 64-bit this change is basically a no-op, because do_div() is implemented >> as a literal 64-bit divide operation and the instruction scheduling works >> out almost the same. >> >> On 32-bit PowerPC a fully accurate 64/64 divide (__udivdi3 in libgcc) is >> 1.1kb of code and hundreds or thousands of dependent cycles to compute, all >> of which is linked in from libgcc. Another 1.2kb of code comes in for >> __umoddi3. >> >> The original reason I wrote this patch is that the native "libgcc" on my >> boards is hard-float and therefore generates warnings when linking to it >> from soft-float code. The toolchain is the native Debian powerpc (or >> powerpcspe), and most other native PowerPC distributions are also >> hard-float-only. >> >> When I combine this patch with the other patch I posted to create a minimal >> internal libgcc with a few 64-bit shift functions from the linux kernel, I >> can successfully build U-Boot on those native PowerPC systems. >> > > Points taken. Can you explain your algorithm? I see you want to do > clks/5^12/2^13, but I don't see when the clks is divided by 5^12. Did I > miss some lines?
The "do_div()" macro is a bit confusing that way, it simultaneously performs an in-place division of the first argument and returns the remainder. Unfortunately that's a clumsy interface that's been around in the Linux kernel for ages. So this line here: clks_rem = do_div(clks, UL_5pow12); It divides clks by UL_5pow12 and saves the remainder as clks_rem. Then the 2^^13 divide and remainder is perform with shifts and masks. Actually, it occurs to me that the one comment isn't quite right, the remainder is a 64-bit value (not a 32-bit one). I feel relatively confident that this is the right technical direction to go, because almost all of the Linux kernel timekeeping and scheduling code uses 64-bit values and careful application of do_div() or shift+mask for efficiency. Cheers, Kyle Moffett _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
