udelay.c

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/* SPDX-License-Identifier: GPL-2.0-or-later */
 
/*
 * udelay() implementation for SMI handlers
 * This is neat in that it never writes to hardware registers, and thus does
 *  not modify the state of the hardware while servicing SMIs.
 */
 
#include <cpu/x86/msr.h>
#include <cpu/amd/msr.h>
#include <cpu/x86/tsc.h>
#include <delay.h>
#include <stdint.h>
 
void udelay(uint32_t us)
{
        uint8_t fid, did, pstate_idx;
        uint64_t tsc_clock, tsc_start, tsc_now, tsc_wait_ticks;
        msr_t msr;
        const uint64_t tsc_base = 100000000;
 
        /* Get initial timestamp before we do the math */
        tsc_start = rdtscll();
 
        /* Get the P-state. This determines which MSR to read */
        msr = rdmsr(PS_STS_REG);
        pstate_idx = msr.lo & 0x07;
 
        /* Get FID and VID for current P-State */
        msr = rdmsr(PSTATE_0_MSR + pstate_idx);
 
        /* Extract the FID and VID values */
        fid = msr.lo & 0x3f;
        did = (msr.lo >> 6) & 0x7;
 
        /* Calculate the CPU clock (from base freq of 100MHz) */
        tsc_clock = tsc_base * (fid + 0x10) / (1 << did);
 
        /* Now go on and wait */
        tsc_wait_ticks = (tsc_clock / 1000000) * us;
 
        do {
                tsc_now = rdtscll();
        } while (tsc_now - tsc_wait_ticks < tsc_start);
}