Re: [PATCH v3 1/5] virtio: rtc: Add virtio RTC driver

[Kuan-Wei Chiu <visitorckw@gmail.com>]

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 <alan.tseng.cs@gmail.com>
> ---
> 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 <alan.tseng.cs@gmail.com>
> + *
> + * 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
>