[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>
diff --git a/src/arch/riscv/core/zicntr.c b/src/arch/riscv/core/zicntr.c
new file mode 100644
index 0000000..0ba453c
--- /dev/null
+++ b/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,
+};
diff --git a/src/arch/riscv/include/bits/errfile.h b/src/arch/riscv/include/bits/errfile.h
index 03f98c2..62288ad 100644
--- a/src/arch/riscv/include/bits/errfile.h
+++ b/src/arch/riscv/include/bits/errfile.h
@@ -16,6 +16,7 @@
#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 )
/** @} */
diff --git a/src/config/config_timer.c b/src/config/config_timer.c
index d53c399..a4fe69b 100644
--- a/src/config/config_timer.c
+++ b/src/config/config_timer.c
@@ -49,3 +49,6 @@
#ifdef TIMER_ACPI
REQUIRE_OBJECT ( acpi_timer );
#endif
+#ifdef TIMER_ZICNTR
+REQUIRE_OBJECT ( zicntr );
+#endif
