powerpc: Improve resolution of VDSO clock_gettime
Currently the clock_gettime implementation in the VDSO produces a result with microsecond resolution for the cases that are handled without a system call, i.e. CLOCK_REALTIME and CLOCK_MONOTONIC. The nanoseconds field of the result is obtained by computing a microseconds value and multiplying by 1000. This changes the code in the VDSO to do the computation for clock_gettime with nanosecond resolution. That means that the resolution of the result will ultimately depend on the timebase frequency. Because the timestamp in the VDSO datapage (stamp_xsec, the real time corresponding to the timebase count in tb_orig_stamp) is in units of 2^-20 seconds, it doesn't have sufficient resolution for computing a result with nanosecond resolution. Therefore this adds a copy of xtime to the VDSO datapage and updates it in update_gtod() along with the other time-related fields. Signed-off-by: Paul Mackerras <paulus@samba.org>
This commit is contained in:
Paul Mackerras
@@ -75,90 +75,49 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime)
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mflr r12 /* r12 saves lr */
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.cfi_register lr,r12
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mr r10,r3 /* r10 saves id */
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mr r11,r4 /* r11 saves tp */
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bl V_LOCAL_FUNC(__get_datapage) /* get data page */
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beq cr1,50f /* if monotonic -> jump there */
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/*
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* CLOCK_REALTIME
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*/
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bl V_LOCAL_FUNC(__do_get_xsec) /* get xsec from tb & kernel */
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lis r7,15 /* r7 = 1000000 = USEC_PER_SEC */
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ori r7,r7,16960
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rldicl r5,r4,44,20 /* r5 = sec = xsec / XSEC_PER_SEC */
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rldicr r6,r5,20,43 /* r6 = sec * XSEC_PER_SEC */
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std r5,TSPC64_TV_SEC(r11) /* store sec in tv */
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subf r0,r6,r4 /* r0 = xsec = (xsec - r6) */
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mulld r0,r0,r7 /* usec = (xsec * USEC_PER_SEC) /
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* XSEC_PER_SEC
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*/
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rldicl r0,r0,44,20
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mulli r0,r0,1000 /* nsec = usec * 1000 */
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std r0,TSPC64_TV_NSEC(r11) /* store nsec in tp */
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mtlr r12
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crclr cr0*4+so
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li r3,0
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blr
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50: bl V_LOCAL_FUNC(__do_get_tspec) /* get time from tb & kernel */
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bne cr1,80f /* if not monotonic, all done */
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/*
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* CLOCK_MONOTONIC
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*/
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50: bl V_LOCAL_FUNC(__do_get_xsec) /* get xsec from tb & kernel */
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lis r7,15 /* r7 = 1000000 = USEC_PER_SEC */
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ori r7,r7,16960
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rldicl r5,r4,44,20 /* r5 = sec = xsec / XSEC_PER_SEC */
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rldicr r6,r5,20,43 /* r6 = sec * XSEC_PER_SEC */
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subf r0,r6,r4 /* r0 = xsec = (xsec - r6) */
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mulld r0,r0,r7 /* usec = (xsec * USEC_PER_SEC) /
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* XSEC_PER_SEC
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*/
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rldicl r6,r0,44,20
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mulli r6,r6,1000 /* nsec = usec * 1000 */
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/* now we must fixup using wall to monotonic. We need to snapshot
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* that value and do the counter trick again. Fortunately, we still
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* have the counter value in r8 that was returned by __do_get_xsec.
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* At this point, r5,r6 contain our sec/nsec values.
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* can be used
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* have the counter value in r8 that was returned by __do_get_tspec.
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* At this point, r4,r5 contain our sec/nsec values.
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*/
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lwa r4,WTOM_CLOCK_SEC(r3)
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lwa r7,WTOM_CLOCK_NSEC(r3)
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lwa r6,WTOM_CLOCK_SEC(r3)
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lwa r9,WTOM_CLOCK_NSEC(r3)
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/* We now have our result in r4,r7. We create a fake dependency
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/* We now have our result in r6,r9. We create a fake dependency
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* on that result and re-check the counter
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*/
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or r9,r4,r7
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xor r0,r9,r9
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or r0,r6,r9
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xor r0,r0,r0
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add r3,r3,r0
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ld r0,CFG_TB_UPDATE_COUNT(r3)
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cmpld cr0,r0,r8 /* check if updated */
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bne- 50b
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/* Calculate and store result. Note that this mimmics the C code,
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* which may cause funny results if nsec goes negative... is that
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* possible at all ?
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/* Add wall->monotonic offset and check for overflow or underflow.
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*/
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add r4,r4,r5
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add r7,r7,r6
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lis r9,NSEC_PER_SEC@h
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ori r9,r9,NSEC_PER_SEC@l
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cmpl cr0,r7,r9
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cmpli cr1,r7,0
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add r4,r4,r6
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add r5,r5,r9
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cmpd cr0,r5,r7
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cmpdi cr1,r5,0
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blt 1f
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subf r7,r9,r7
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subf r5,r7,r5
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addi r4,r4,1
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1: bge cr1,1f
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1: bge cr1,80f
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addi r4,r4,-1
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add r7,r7,r9
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1: std r4,TSPC64_TV_SEC(r11)
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std r7,TSPC64_TV_NSEC(r11)
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add r5,r5,r7
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80: std r4,TSPC64_TV_SEC(r11)
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std r5,TSPC64_TV_NSEC(r11)
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mtlr r12
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crclr cr0*4+so
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@@ -168,10 +127,6 @@ V_FUNCTION_BEGIN(__kernel_clock_gettime)
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/*
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* syscall fallback
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*/
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98:
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mtlr r12
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mr r3,r10
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mr r4,r11
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99:
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li r0,__NR_clock_gettime
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sc
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@@ -253,3 +208,59 @@ V_FUNCTION_BEGIN(__do_get_xsec)
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blr
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.cfi_endproc
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V_FUNCTION_END(__do_get_xsec)
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/*
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* This is the core of clock_gettime(), it returns the current
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* time in seconds and nanoseconds in r4 and r5.
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* It expects the datapage ptr in r3 and doesn't clobber it.
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* It clobbers r0 and r6 and returns NSEC_PER_SEC in r7.
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* On return, r8 contains the counter value that can be reused.
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* This clobbers cr0 but not any other cr field.
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*/
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V_FUNCTION_BEGIN(__do_get_tspec)
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.cfi_startproc
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/* check for update count & load values */
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1: ld r8,CFG_TB_UPDATE_COUNT(r3)
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andi. r0,r8,1 /* pending update ? loop */
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bne- 1b
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xor r0,r8,r8 /* create dependency */
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add r3,r3,r0
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/* Get TB & offset it. We use the MFTB macro which will generate
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* workaround code for Cell.
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*/
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MFTB(r7)
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ld r9,CFG_TB_ORIG_STAMP(r3)
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subf r7,r9,r7
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/* Scale result */
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ld r5,CFG_TB_TO_XS(r3)
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sldi r7,r7,12 /* compute time since stamp_xtime */
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mulhdu r6,r7,r5 /* in units of 2^-32 seconds */
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/* Add stamp since epoch */
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ld r4,STAMP_XTIME+TSPC64_TV_SEC(r3)
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ld r5,STAMP_XTIME+TSPC64_TV_NSEC(r3)
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or r0,r4,r5
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or r0,r0,r6
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xor r0,r0,r0
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add r3,r3,r0
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ld r0,CFG_TB_UPDATE_COUNT(r3)
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cmpld r0,r8 /* check if updated */
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bne- 1b /* reload if so */
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/* convert to seconds & nanoseconds and add to stamp */
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lis r7,NSEC_PER_SEC@h
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ori r7,r7,NSEC_PER_SEC@l
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mulhwu r0,r6,r7 /* compute nanoseconds and */
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srdi r6,r6,32 /* seconds since stamp_xtime */
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clrldi r0,r0,32
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add r5,r5,r0 /* add nanoseconds together */
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cmpd r5,r7 /* overflow? */
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add r4,r4,r6
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bltlr /* all done if no overflow */
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subf r5,r7,r5 /* if overflow, adjust */
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addi r4,r4,1
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blr
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.cfi_endproc
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V_FUNCTION_END(__do_get_tspec)
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