[riscv] Add support for RDTIME as a timer source

The Zicntr extension defines an unprivileged wall-clock time CSR that
roughly matches the behaviour of an invariant TSC on x86.  The nominal
frequency of this timer may be read from the "timebase-frequency"
property of the CPU node in the device tree.

Add a timer source using RDTIME to provide implementations of udelay()
and currticks(), modelled on the existing RDTSC-based timer for x86.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/riscv/core/zicntr.c

@@ -0,0 +1,193 @@
+/*
+ * Copyright (C) 2024 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/** @file
+ *
+ * Base counters and timers extension (Zicntr)
+ *
+ */
+
+#include <string.h>
+#include <errno.h>
+#include <ipxe/fdt.h>
+#include <ipxe/hart.h>
+#include <ipxe/timer.h>
+
+/** Timer increment per microsecond */
+static unsigned long zicntr_mhz;
+
+/** Minimum resolution for scaled timer */
+#define ZICNTR_SCALED_HZ 32
+
+/**
+ * Timer scale (expressed as a bit shift)
+ *
+ * We use this to avoid the need for 64-bit divsion on 32-bit systems.
+ */
+static unsigned int zicntr_scale;
+
+/** Number of timer ticks per scaled timer increment */
+static unsigned long zicntr_ticks;
+
+/** Colour for debug messages */
+#define colour &zicntr_mhz
+
+/**
+ * Get low XLEN bits of current time
+ *
+ * @ret time		Current time
+ */
+static inline __attribute__ (( always_inline )) unsigned long
+rdtime_low ( void ) {
+	unsigned long time;
+
+	/* Read low XLEN bits of current time */
+	__asm__ ( "rdtime %0" : "=r" ( time ) );
+	return time;
+}
+
+/**
+ * Get current time, scaled to avoid rollover within a realistic timescale
+ *
+ * @ret time		Scaled current time
+ */
+static inline __attribute__ (( always_inline )) unsigned long
+rdtime_scaled ( void ) {
+	union {
+		uint64_t time;
+		struct {
+			uint32_t low;
+			uint32_t high;
+		};
+	} u;
+	unsigned long tmp __attribute__ (( unused ));
+
+	/* Read full current time */
+#if __riscv_xlen >= 64
+	__asm__ ( "rdtime %0" : "=r" ( u.time ) );
+#else
+	__asm__ ( "1:\n\t"
+		  "rdtimeh %1\n\t"
+		  "rdtime %0\n\t"
+		  "rdtimeh %2\n\t"
+		  "bne %1, %2, 1b\n\t"
+		  : "=r" ( u.low ), "=r" ( u.high ), "=r" ( tmp ) );
+#endif
+
+	/* Scale time to avoid XLEN-bit rollover */
+	return ( u.time >> zicntr_scale );
+}
+
+/**
+ * Get current system time in ticks
+ *
+ * @ret ticks		Current time, in ticks
+ */
+static unsigned long zicntr_currticks ( void ) {
+	unsigned long scaled;
+
+	/* Get scaled time and convert to ticks */
+	scaled = rdtime_scaled();
+	return ( scaled * zicntr_ticks );
+}
+
+/**
+ * Delay for a fixed number of microseconds
+ *
+ * @v usecs		Number of microseconds for which to delay
+ */
+static void zicntr_udelay ( unsigned long usecs ) {
+	unsigned long start;
+	unsigned long elapsed;
+	unsigned long threshold;
+
+	/* Delay until sufficient time has elapsed */
+	start = rdtime_low();
+	threshold = ( usecs * zicntr_mhz );
+	do {
+		elapsed = ( rdtime_low() - start );
+	} while ( elapsed < threshold );
+}
+
+/**
+ * Probe timer
+ *
+ * @ret rc		Return status code
+ */
+static int zicntr_probe ( void ) {
+	unsigned int offset;
+	union {
+		uint64_t freq;
+		int64_t sfreq;
+	} u;
+	int rc;
+
+	/* Check if Zicntr extension is supported */
+	if ( ( rc = hart_supported ( "_zicntr" ) ) != 0 ) {
+		DBGC ( colour, "ZICNTR not supported: %s\n", strerror ( rc ) );
+		return rc;
+	}
+
+	/* Get timer frequency */
+	if ( ( ( rc = fdt_path ( "/cpus", &offset ) ) != 0 ) ||
+	     ( ( rc = fdt_u64 ( offset, "timebase-frequency",
+				&u.freq ) ) != 0 ) ) {
+		DBGC ( colour, "ZICNTR could not determine frequency: %s\n",
+		       strerror ( rc ) );
+		return rc;
+	}
+
+	/* Convert to MHz (without 64-bit division) */
+	do {
+		zicntr_mhz++;
+		u.sfreq -= 1000000;
+	} while ( u.sfreq > 0 );
+
+	/* Calibrate currticks() scaling factor */
+	zicntr_scale = 31;
+	zicntr_ticks = ( ( 1UL << zicntr_scale ) /
+			 ( zicntr_mhz * ( 1000000 / TICKS_PER_SEC ) ) );
+	while ( zicntr_ticks > ( TICKS_PER_SEC / ZICNTR_SCALED_HZ ) ) {
+		zicntr_scale--;
+		zicntr_ticks >>= 1;
+	}
+	DBGC ( colour, "ZICNTR at %ld MHz, %ld ticks per 2^%d increments\n",
+	       zicntr_mhz, zicntr_ticks, zicntr_scale );
+	if ( ! zicntr_ticks ) {
+		DBGC ( colour, "ZICNTR has zero ticks per 2^%d increments\n",
+		       zicntr_scale );
+		return -EIO;
+	}
+
+	return 0;
+}
+
+/** Zicntr timer */
+struct timer zicntr_timer __timer ( TIMER_PREFERRED ) = {
+	.name = "zicntr",
+	.probe = zicntr_probe,
+	.currticks = zicntr_currticks,
+	.udelay = zicntr_udelay,
+};

src/arch/riscv/include/bits/errfile.h

@@ -16,6 +16,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #define ERRFILE_sbi_reboot	( ERRFILE_ARCH | ERRFILE_CORE | 0x00000000 )
 #define ERRFILE_hart		( ERRFILE_ARCH | ERRFILE_CORE | 0x00010000 )
+#define ERRFILE_zicntr		( ERRFILE_ARCH | ERRFILE_CORE | 0x00020000 )
 
 /** @} */
 

src/config/config_timer.c

@@ -49,3 +49,6 @@ REQUIRE_OBJECT ( linux_timer );
 #ifdef TIMER_ACPI
 REQUIRE_OBJECT ( acpi_timer );
 #endif
+#ifdef TIMER_ZICNTR
+REQUIRE_OBJECT ( zicntr );
+#endif