On Sun, Oct 18, 2026 at 02:46:18AM +0800, Hung-Chun Tseng wrote: > Add a driver for the virtio RTC device (virtio device ID 17), which > QEMU provides as virtio-rtc-pci and virtio-rtc-device since v11.1.0. > > The device exposes its clocks through a request virtqueue. As the > virtqueue must not be used before the virtio uclass sets DRIVER_OK, > which happens after probe(), the first read looks for the clock: it > asks for the number of clocks with VIRTIO_RTC_REQ_CFG and queries the > type of each with VIRTIO_RTC_REQ_CLOCK_CAP. Reading the time sends a > VIRTIO_RTC_REQ_READ for the chosen clock and converts the nanoseconds > since the epoch with rtc_to_tm(). Each request gives up with > -ETIMEDOUT if the device does not answer within a second, as virtio_9p > does. The clocks are read-only, so there is no set operation, and the > optional alarm feature is not negotiated.
This is a bit subjective, but my feeling is that this has become somewhat too verbose. Ideally, a commit message should explain the "why" rather than just stating what was done. What was done can basically be seen directly from the code changes themselves. Regards, Kuan-Wei > > Like Linux, the driver prefers a VIRTIO_RTC_CLOCK_UTC_SMEARED clock. > Linux registers an RTC class device only for such a clock and skips > VIRTIO_RTC_CLOCK_UTC and VIRTIO_RTC_CLOCK_UTC_MAYBE_SMEARED, which may > step on leap seconds. QEMU, however, exposes a single clock of type > VIRTIO_RTC_CLOCK_UTC, so following Linux strictly would leave U-Boot > without an RTC there. If no clock smears leap seconds, the driver > therefore falls back to the first UTC or maybe-smeared UTC clock. A > leap second step matters little to U-Boot, which only reads the time. > > Like Linux, the driver stops looking for a clock as soon as a request > fails. After a timeout the request is still on the virtqueue, so its > late response could be taken for that of a later request. > > VIRTIO_RTC defaults to y when VIRTIO and DM_RTC are enabled, like > VIRTIO_RNG does for DM_RNG. > > If none of the clocks is UTC, the driver remembers it and fails later > reads with -ENODEV at once, as the set of clocks never changes. > > Tested with QEMU v11.1.2 on qemu-riscv64 over both PCI and MMIO. > Sandbox tests, which emulate the device, follow later in this series. > > Signed-off-by: Hung-Chun Tseng <[email protected]> > --- > Changes in v3: > - Give up on a request after one second instead of polling forever > (Kuan-Wei) > - Remember that no clock is UTC, so that later reads fail at once > (Simon) > - Expand the VIRTIO_RTC help text (Simon) > - Drop __packed from the message structs, which are naturally aligned > (Kuan-Wei) > - Prefer a VIRTIO_RTC_CLOCK_UTC_SMEARED clock as Linux does, fall back > to the first other UTC clock, and explain why in the commit message > (Kuan-Wei) > - Stop looking for a clock as soon as a request fails, as Linux does > (Kuan-Wei) > - Mention in the commit message that sandbox tests follow (Simon) > > Changes in v2: > - Look for the UTC clock on the first virtio_rtc_get() instead of in > probe(), as the virtqueue must not be used before DRIVER_OK (Bin) > > drivers/virtio/Kconfig | 11 ++ > drivers/virtio/Makefile | 1 + > drivers/virtio/virtio-uclass.c | 1 + > drivers/virtio/virtio_rtc.c | 249 +++++++++++++++++++++++++++++++++ > include/virtio.h | 4 +- > 5 files changed, 265 insertions(+), 1 deletion(-) > create mode 100644 drivers/virtio/virtio_rtc.c > > diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig > index 55b444280..48ab669f3 100644 > --- a/drivers/virtio/Kconfig > +++ b/drivers/virtio/Kconfig > @@ -84,4 +84,15 @@ config VIRTIO_9P > help > This enables the 9P transport driver over virtio. > It can be used with QEMU based targets. > + > +config VIRTIO_RTC > + bool "virtio RTC driver" > + depends on DM_RTC > + depends on VIRTIO > + default y > + help > + This enables the virtio RTC driver, which reads the UTC clock > + exposed by a virtio RTC device. QEMU v11.1.0 or later provides > + it as virtio-rtc-pci and virtio-rtc-device. The clocks are > + read-only, so the time cannot be set. > endmenu > diff --git a/drivers/virtio/Makefile b/drivers/virtio/Makefile > index 6e7bcfc8e..ddb5ce4eb 100644 > --- a/drivers/virtio/Makefile > +++ b/drivers/virtio/Makefile > @@ -12,3 +12,4 @@ obj-$(CONFIG_VIRTIO_NET) += virtio_net.o > obj-$(CONFIG_VIRTIO_BLK) += virtio_blk.o > obj-$(CONFIG_VIRTIO_RNG) += virtio_rng.o > obj-$(CONFIG_VIRTIO_9P) += virtio_9p.o > +obj-$(CONFIG_VIRTIO_RTC) += virtio_rtc.o > diff --git a/drivers/virtio/virtio-uclass.c b/drivers/virtio/virtio-uclass.c > index c163b2fd7..c6cf55922 100644 > --- a/drivers/virtio/virtio-uclass.c > +++ b/drivers/virtio/virtio-uclass.c > @@ -31,6 +31,7 @@ static const char *const virtio_drv_name[VIRTIO_ID_MAX_NUM] > = { > [VIRTIO_ID_BLOCK] = VIRTIO_BLK_DRV_NAME, > [VIRTIO_ID_RNG] = VIRTIO_RNG_DRV_NAME, > [VIRTIO_ID_9P] = VIRTIO_9P_DRV_NAME, > + [VIRTIO_ID_CLOCK] = VIRTIO_RTC_DRV_NAME, > }; > > int virtio_get_config(struct udevice *vdev, unsigned int offset, > diff --git a/drivers/virtio/virtio_rtc.c b/drivers/virtio/virtio_rtc.c > new file mode 100644 > index 000000000..806abe237 > --- /dev/null > +++ b/drivers/virtio/virtio_rtc.c > @@ -0,0 +1,249 @@ > +// SPDX-License-Identifier: GPL-2.0+ > +/* > + * Copyright (C) 2026, Hung-Chun Tseng <[email protected]> > + * > + * Driver for the virtio RTC device (virtio device ID 17). The device > + * exposes one or more read-only clocks through a request virtqueue; this > + * driver reads a UTC clock, preferring one which smears leap seconds, and > + * reports it through the RTC uclass. > + */ > + > +#include <div64.h> > +#include <dm.h> > +#include <log.h> > +#include <rtc.h> > +#include <time.h> > +#include <virtio_types.h> > +#include <virtio.h> > +#include <virtio_ring.h> > +#include <linux/types.h> > + > +#define VIRTIO_RTC_TIMEOUT_MS 1000 > + > +#define VIRTIO_RTC_REQ_READ 0x0001 > +#define VIRTIO_RTC_REQ_CFG 0x1000 > +#define VIRTIO_RTC_REQ_CLOCK_CAP 0x1001 > + > +#define VIRTIO_RTC_S_OK 0 > + > +#define VIRTIO_RTC_CLOCK_UTC 0 > +#define VIRTIO_RTC_CLOCK_UTC_SMEARED 3 > +#define VIRTIO_RTC_CLOCK_UTC_MAYBE_SMEARED 4 > + > +struct virtio_rtc_req_head { > + __le16 msg_type; > + u8 reserved[6]; > +}; > + > +struct virtio_rtc_resp_head { > + u8 status; > + u8 reserved[7]; > +}; > + > +/* Request for VIRTIO_RTC_REQ_READ and VIRTIO_RTC_REQ_CLOCK_CAP */ > +struct virtio_rtc_req_clock { > + struct virtio_rtc_req_head head; > + __le16 clock_id; > + u8 reserved[6]; > +}; > + > +struct virtio_rtc_resp_read { > + struct virtio_rtc_resp_head head; > + __le64 clock_reading; > +}; > + > +struct virtio_rtc_resp_cfg { > + struct virtio_rtc_resp_head head; > + __le16 num_clocks; > + u8 reserved[6]; > +}; > + > +struct virtio_rtc_resp_clock_cap { > + struct virtio_rtc_resp_head head; > + u8 type; > + u8 leap_second_smearing; > + u8 flags; > + u8 reserved[5]; > +}; > + > +struct virtio_rtc_priv { > + struct virtqueue *vq; > + u16 clock_id; > + bool clock_found; > + bool no_utc_clock; > +}; > + > +static int virtio_rtc_xfer(struct udevice *dev, void *req, size_t req_len, > + void *resp, size_t resp_len) > +{ > + struct virtio_rtc_priv *priv = dev_get_priv(dev); > + struct virtio_rtc_resp_head *head = resp; > + struct virtio_sg req_sg = { req, req_len }; > + struct virtio_sg resp_sg = { resp, resp_len }; > + struct virtio_sg *sgs[] = { &req_sg, &resp_sg }; > + unsigned int len; > + ulong start; > + int ret; > + > + ret = virtqueue_add(priv->vq, sgs, 1, 1); > + if (ret) > + return ret; > + > + virtqueue_kick(priv->vq); > + > + start = get_timer(0); > + while (!virtqueue_get_buf(priv->vq, &len)) { > + if (get_timer(start) > VIRTIO_RTC_TIMEOUT_MS) > + return -ETIMEDOUT; > + } > + > + if (len < sizeof(*head) || head->status != VIRTIO_RTC_S_OK) > + return -EIO; > + > + if (len < resp_len) > + return -EPROTO; > + > + return 0; > +} > + > +static bool virtio_rtc_is_utc(u8 type) > +{ > + return type == VIRTIO_RTC_CLOCK_UTC || > + type == VIRTIO_RTC_CLOCK_UTC_SMEARED || > + type == VIRTIO_RTC_CLOCK_UTC_MAYBE_SMEARED; > +} > + > +static int virtio_rtc_find_utc_clock(struct udevice *dev) > +{ > + struct virtio_rtc_priv *priv = dev_get_priv(dev); > + struct virtio_rtc_req_head cfg_req = { > + .msg_type = cpu_to_le16(VIRTIO_RTC_REQ_CFG), > + }; > + struct virtio_rtc_resp_cfg cfg = {}; > + u16 num_clocks, id, utc_id = 0; > + bool utc_found = false; > + int ret; > + > + ret = virtio_rtc_xfer(dev, &cfg_req, sizeof(cfg_req), &cfg, > sizeof(cfg)); > + if (ret) > + return ret; > + > + num_clocks = le16_to_cpu(cfg.num_clocks); > + for (id = 0; id < num_clocks; id++) { > + struct virtio_rtc_req_clock req = { > + .head.msg_type = cpu_to_le16(VIRTIO_RTC_REQ_CLOCK_CAP), > + .clock_id = cpu_to_le16(id), > + }; > + struct virtio_rtc_resp_clock_cap cap = {}; > + > + /* > + * Give up on any error: after a timeout the request is still > + * on the virtqueue, so a later response could be mismatched. > + */ > + ret = virtio_rtc_xfer(dev, &req, sizeof(req), &cap, > sizeof(cap)); > + if (ret) > + return ret; > + > + /* > + * Like Linux, prefer a clock which smears leap seconds. Unlike > + * Linux, fall back to the first other UTC clock, since that is > + * the only kind of clock that QEMU provides. > + */ > + if (cap.type == VIRTIO_RTC_CLOCK_UTC_SMEARED) { > + utc_id = id; > + utc_found = true; > + break; > + } > + > + if (!utc_found && virtio_rtc_is_utc(cap.type)) { > + utc_id = id; > + utc_found = true; > + } > + } > + > + /* The set of clocks is fixed, so remember that none of them is UTC */ > + if (!utc_found) { > + priv->no_utc_clock = true; > + return -ENODEV; > + } > + > + priv->clock_id = utc_id; > + priv->clock_found = true; > + > + return 0; > +} > + > +static int virtio_rtc_get(struct udevice *dev, struct rtc_time *time) > +{ > + struct virtio_rtc_priv *priv = dev_get_priv(dev); > + struct virtio_rtc_req_clock req = { > + .head.msg_type = cpu_to_le16(VIRTIO_RTC_REQ_READ), > + }; > + struct virtio_rtc_resp_read resp = {}; > + u64 now; > + int ret; > + > + /* > + * The virtqueue must not be used before DRIVER_OK, which the virtio > + * uclass sets only after probe(), so look for the clock on first use. > + */ > + if (priv->no_utc_clock) > + return -ENODEV; > + > + if (!priv->clock_found) { > + ret = virtio_rtc_find_utc_clock(dev); > + if (ret) > + return ret; > + } > + > + req.clock_id = cpu_to_le16(priv->clock_id); > + > + ret = virtio_rtc_xfer(dev, &req, sizeof(req), &resp, sizeof(resp)); > + if (ret) > + return ret; > + > + now = le64_to_cpu(resp.clock_reading); > + do_div(now, 1000000000U); > + rtc_to_tm(now, time); > + > + return 0; > +} > + > +static int virtio_rtc_bind(struct udevice *dev) > +{ > + struct virtio_dev_priv *uc_priv = dev_get_uclass_priv(dev->parent); > + > + /* The alarm feature is not supported */ > + virtio_driver_features_init(uc_priv, NULL, 0, NULL, 0); > + > + return 0; > +} > + > +static int virtio_rtc_probe(struct udevice *dev) > +{ > + struct virtio_rtc_priv *priv = dev_get_priv(dev); > + int ret; > + > + ret = virtio_find_vqs(dev, 1, &priv->vq); > + if (ret < 0) { > + debug("%s: virtio_find_vqs failed\n", __func__); > + return ret; > + } > + > + return 0; > +} > + > +static const struct rtc_ops virtio_rtc_ops = { > + .get = virtio_rtc_get, > +}; > + > +U_BOOT_DRIVER(virtio_rtc) = { > + .name = VIRTIO_RTC_DRV_NAME, > + .id = UCLASS_RTC, > + .bind = virtio_rtc_bind, > + .probe = virtio_rtc_probe, > + .remove = virtio_reset, > + .ops = &virtio_rtc_ops, > + .priv_auto = sizeof(struct virtio_rtc_priv), > + .flags = DM_FLAG_ACTIVE_DMA, > +}; > diff --git a/include/virtio.h b/include/virtio.h > index b981eead5..95069a2de 100644 > --- a/include/virtio.h > +++ b/include/virtio.h > @@ -32,12 +32,14 @@ > #define VIRTIO_ID_BLOCK 2 /* virtio block */ > #define VIRTIO_ID_RNG 4 /* virtio rng */ > #define VIRTIO_ID_9P 9 /* virtio 9p */ > -#define VIRTIO_ID_MAX_NUM 10 > +#define VIRTIO_ID_CLOCK 17 /* virtio clock/timer (rtc) */ > +#define VIRTIO_ID_MAX_NUM 18 > > #define VIRTIO_NET_DRV_NAME "virtio-net" > #define VIRTIO_BLK_DRV_NAME "virtio-blk" > #define VIRTIO_RNG_DRV_NAME "virtio-rng" > #define VIRTIO_9P_DRV_NAME "virtio-9p" > +#define VIRTIO_RTC_DRV_NAME "virtio-rtc" > > /* Feature bits for virtio 9P */ > #define VIRTIO_9P_MOUNT_TAG 0 > -- > 2.43.0 >
