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Without the volatile attribute, compilers are entitled to optimize out the same asm(). In the case of __udelay() in syscounter.c, it calls `get_ticks()` twice, one for the starting time and the second in the loop to check the current time. When compilers inline `get_ticks()` they see the same `mrrc` instructions and optimize out the second one. This leads to infinite loop since we don't get updated value from the system counter. Here is a portion of the disassembly of __udelay: 88: 428b cmp r3, r1 8a: f8ce 20a4 str.w r2, [lr, #164] ; 0xa4 8e: bf08 it eq 90: 4282 cmpeq r2, r0 92: f8ce 30a0 str.w r3, [lr, #160] ; 0xa0 96: d3f7 bcc.n 88 <__udelay+0x88> 98: e8bd 8cf0 ldmia.w sp!, {r4, r5, r6, r7, sl, fp, pc} Note that final jump / loop at 96 to 88, we don't have any `mrrc`. With a volatile attribute, the above changes to this: 8a: ec53 2f0e mrrc 15, 0, r2, r3, cr14 8e: 42ab cmp r3, r5 90: f8c1 20a4 str.w r2, [r1, #164] ; 0xa4 94: bf08 it eq 96: 42a2 cmpeq r2, r4 98: f8c1 30a0 str.w r3, [r1, #160] ; 0xa0 9c: d3f5 bcc.n 8a <__udelay+0x8a> 9e: e8bd 8cf0 ldmia.w sp!, {r4, r5, r6, r7, sl, fp, pc} a2: bf00 nop I'm advised[1] to put volatile on all asm(), so this commit also adds it to the asm() in timer_init(). [1]: https://lists.denx.de/pipermail/u-boot/2018-March/322062.html Signed-off-by: Yasushi SHOJI <yasushi.shoji@gmail.com> Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com> |
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