Re: [PATCH v2 16/21] ipmi: kcs_bmc: Add a "raw" character device interface

[Zev Weiss]

On Fri, Mar 19, 2021 at 01:27:47AM CDT, Andrew Jeffery wrote:
>The existing IPMI chardev encodes IPMI behaviours as the name suggests.
>However, KCS devices are useful beyond IPMI (or keyboards), as they
>provide a means to generate IRQs and exchange arbitrary data between a
>BMC and its host system.
>
>Implement a "raw" KCS character device that exposes the IDR, ODR and STR
>registers to userspace via read() and write() implemented on a character
>device:
>
>+--------+--------+---------+
>| Offset | read() | write() |
>+--------+--------+---------+
>|   0    |   IDR  |   ODR   |
>+--------+--------+---------+
>|   1    |   STR  |   STR   |
>+--------+--------+---------+
>
>This interface allows userspace to implement arbitrary (though somewhat
>inefficient) protocols for exchanging information between a BMC and host
>firmware. Conceptually the KCS interface can be used as an out-of-band
>machanism for interrupt-signaled control messages while bulk data

Typo ("mechanism")

>transfers occur over more appropriate interfaces between the BMC and the
>host (which may lack their own interrupt mechanism, e.g. LPC FW cycles).
>
>poll() is provided, which will wait for IBF or OBE conditions for data
>reads and writes respectively. Reads of STR on its own never blocks,
>though accessing both offsets in the one system call may block if the
>data registers are not ready.
>
>Signed-off-by: Andrew Jeffery <andrew@xxxxxxxx>
>---
> Documentation/ABI/testing/dev-raw-kcs |  25 ++
> drivers/char/ipmi/Kconfig             |  17 +
> drivers/char/ipmi/Makefile            |   1 +
> drivers/char/ipmi/kcs_bmc_cdev_raw.c  | 443 ++++++++++++++++++++++++++
> 4 files changed, 486 insertions(+)
> create mode 100644 Documentation/ABI/testing/dev-raw-kcs
> create mode 100644 drivers/char/ipmi/kcs_bmc_cdev_raw.c
>
>diff --git a/Documentation/ABI/testing/dev-raw-kcs b/Documentation/ABI/testing/dev-raw-kcs
>new file mode 100644
>index 000000000000..06e7e2071562
>--- /dev/null
>+++ b/Documentation/ABI/testing/dev-raw-kcs
>@@ -0,0 +1,25 @@
>+What:		/dev/raw-kcs*
>+Date:		2021-02-15
>+KernelVersion:	5.13
>+Contact:	openbmc@xxxxxxxxxxxxxxxx
>+Contact:	openipmi-developer@xxxxxxxxxxxxxxxxxxxxx
>+Contact:	Andrew Jeffery <andrew@xxxxxxxx>
>+Description:	``/dev/raw-kcs*`` exposes to userspace the data and
>+		status registers of Keyboard-Controller-Style (KCS) IPMI
>+		interfaces via read() and write() syscalls. Direct
>+		exposure of the data and status registers enables
>+		inefficient but arbitrary protocols to be implemented
>+		over the device. A typical approach is to use KCS
>+		devices for out-of-band signalling for bulk data
>+		transfers over other interfaces between a Baseboard
>+		Management Controller and its host.
>+
>+		+--------+--------+---------+
>+		| Offset | read() | write() |
>+		+--------+--------+---------+
>+		|   0    |   IDR  |   ODR   |
>+		+--------+--------+---------+
>+		|   1    |   STR  |   STR   |
>+		+--------+--------+---------+
>+
>+Users:		libmctp: https://github.com/openbmc/libmctp
>diff --git a/drivers/char/ipmi/Kconfig b/drivers/char/ipmi/Kconfig
>index bc5f81899b62..273ac1a1f870 100644
>--- a/drivers/char/ipmi/Kconfig
>+++ b/drivers/char/ipmi/Kconfig
>@@ -137,6 +137,23 @@ config IPMI_KCS_BMC_CDEV_IPMI
> 	  This support is also available as a module. The module will be
> 	  called kcs_bmc_cdev_ipmi.
>
>+config IPMI_KCS_BMC_CDEV_RAW
>+	depends on IPMI_KCS_BMC
>+	tristate "Raw character device interface for BMC KCS devices"
>+	help
>+	  Provides a BMC-side character device directly exposing the
>+	  data and status registers of a KCS device to userspace. While
>+	  KCS devices are commonly used to implement IPMI message
>+	  passing, they provide a general interface for exchange of
>+	  interrupts, data and status information between the BMC and
>+	  its host.
>+
>+	  Say YES if you wish to use the KCS devices to implement
>+	  protocols that are not IPMI.
>+
>+	  This support is also available as a module. The module will be
>+	  called kcs_bmc_cdev_raw.
>+
> config ASPEED_BT_IPMI_BMC
> 	depends on ARCH_ASPEED || COMPILE_TEST
> 	depends on REGMAP && REGMAP_MMIO && MFD_SYSCON
>diff --git a/drivers/char/ipmi/Makefile b/drivers/char/ipmi/Makefile
>index fcfa676afddb..c8cc248ddd90 100644
>--- a/drivers/char/ipmi/Makefile
>+++ b/drivers/char/ipmi/Makefile
>@@ -24,6 +24,7 @@ obj-$(CONFIG_IPMI_WATCHDOG) += ipmi_watchdog.o
> obj-$(CONFIG_IPMI_POWEROFF) += ipmi_poweroff.o
> obj-$(CONFIG_IPMI_KCS_BMC) += kcs_bmc.o
> obj-$(CONFIG_IPMI_KCS_BMC_CDEV_IPMI) += kcs_bmc_cdev_ipmi.o
>+obj-$(CONFIG_IPMI_KCS_BMC_CDEV_RAW) += kcs_bmc_cdev_raw.o
> obj-$(CONFIG_ASPEED_BT_IPMI_BMC) += bt-bmc.o
> obj-$(CONFIG_ASPEED_KCS_IPMI_BMC) += kcs_bmc_aspeed.o
> obj-$(CONFIG_NPCM7XX_KCS_IPMI_BMC) += kcs_bmc_npcm7xx.o
>diff --git a/drivers/char/ipmi/kcs_bmc_cdev_raw.c b/drivers/char/ipmi/kcs_bmc_cdev_raw.c
>new file mode 100644
>index 000000000000..bdd258648c8e
>--- /dev/null
>+++ b/drivers/char/ipmi/kcs_bmc_cdev_raw.c
>@@ -0,0 +1,443 @@
>+// SPDX-License-Identifier: GPL-2.0-or-later
>+/* Copyright (c) 2021 IBM Corp. */
>+
>+#include <linux/delay.h>
>+#include <linux/device.h>
>+#include <linux/errno.h>
>+#include <linux/fs.h>
>+#include <linux/list.h>
>+#include <linux/miscdevice.h>
>+#include <linux/module.h>
>+#include <linux/poll.h>
>+
>+#include "kcs_bmc_client.h"
>+
>+#define DEVICE_NAME "raw-kcs"
>+
>+struct kcs_bmc_raw {
>+	struct list_head entry;
>+
>+	struct kcs_bmc_client client;
>+
>+	wait_queue_head_t queue;
>+	u8 events;
>+	bool writable;
>+	bool readable;
>+	u8 idr;
>+
>+	struct miscdevice miscdev;
>+};
>+
>+static inline struct kcs_bmc_raw *client_to_kcs_bmc_raw(struct kcs_bmc_client *client)
>+{
>+	return container_of(client, struct kcs_bmc_raw, client);
>+}
>+
>+/* Call under priv->queue.lock */
>+static void kcs_bmc_raw_update_event_mask(struct kcs_bmc_raw *priv, u8 mask, u8 state)
>+{
>+	kcs_bmc_update_event_mask(priv->client.dev, mask, state);
>+	priv->events &= ~mask;
>+	priv->events |= state & mask;
>+}
>+
>+static int kcs_bmc_raw_event(struct kcs_bmc_client *client)
>+{
>+	struct kcs_bmc_raw *priv;
>+	struct device *dev;
>+	u8 status, handled;
>+
>+	priv = client_to_kcs_bmc_raw(client);
>+	dev = priv->miscdev.this_device;
>+
>+	spin_lock(&priv->queue.lock);
>+
>+	status = kcs_bmc_read_status(client->dev);
>+	handled = 0;
>+
>+	if ((priv->events & KCS_BMC_EVENT_TYPE_IBF) && (status & KCS_BMC_STR_IBF)) {
>+		if (priv->readable)
>+			dev_err(dev, "Storm brewing!");

That seems a *touch* cryptic...

>+
>+		dev_dbg(dev, "Disabling IDR events for back-pressure\n");
>+		kcs_bmc_raw_update_event_mask(priv, KCS_BMC_EVENT_TYPE_IBF, 0);
>+		priv->idr = kcs_bmc_read_data(client->dev);
>+		priv->readable = true;
>+
>+		dev_dbg(dev, "IDR read, waking waiters\n");
>+		wake_up_locked(&priv->queue);
>+
>+		handled |= KCS_BMC_EVENT_TYPE_IBF;
>+	}
>+
>+	if ((priv->events & KCS_BMC_EVENT_TYPE_OBE) && !(status & KCS_BMC_STR_OBF)) {
>+		kcs_bmc_raw_update_event_mask(priv, KCS_BMC_EVENT_TYPE_OBE, 0);
>+		priv->writable = true;
>+
>+		dev_dbg(dev, "ODR writable, waking waiters\n");
>+		wake_up_locked(&priv->queue);
>+
>+		handled |= KCS_BMC_EVENT_TYPE_OBE;
>+	}
>+
>+	spin_unlock(&priv->queue.lock);
>+
>+	return handled ? KCS_BMC_EVENT_HANDLED : KCS_BMC_EVENT_NONE;

Hm, if we're just treating it as a boolean here, is there any need to
muck around with setting specific bits of 'handled' in the if-blocks
above?

>+}
>+
>+static const struct kcs_bmc_client_ops kcs_bmc_raw_client_ops = {
>+	.event = kcs_bmc_raw_event,
>+};
>+
>+static inline struct kcs_bmc_raw *file_to_kcs_bmc_raw(struct file *filp)
>+{
>+	return container_of(filp->private_data, struct kcs_bmc_raw, miscdev);
>+}
>+
>+static int kcs_bmc_raw_open(struct inode *inode, struct file *filp)
>+{
>+	struct kcs_bmc_raw *priv = file_to_kcs_bmc_raw(filp);
>+
>+	return kcs_bmc_enable_device(priv->client.dev, &priv->client);
>+}
>+
>+static bool kcs_bmc_raw_prepare_obe(struct kcs_bmc_raw *priv)
>+{
>+	bool writable;
>+
>+	/* Enable the OBE event so we can catch the host clearing OBF */
>+	kcs_bmc_raw_update_event_mask(priv, KCS_BMC_EVENT_TYPE_OBE, KCS_BMC_EVENT_TYPE_OBE);
>+
>+	/* Now that we'll catch an OBE event, check if it's already occurred */
>+	writable = !(kcs_bmc_read_status(priv->client.dev) & KCS_BMC_STR_OBF);
>+
>+	/* If OBF is clear we've missed the OBE event, so disable it */
>+	if (writable)
>+		kcs_bmc_raw_update_event_mask(priv, KCS_BMC_EVENT_TYPE_OBE, 0);
>+
>+	return writable;
>+}
>+
>+static __poll_t kcs_bmc_raw_poll(struct file *filp, poll_table *wait)
>+{
>+	struct kcs_bmc_raw *priv;
>+	__poll_t events = 0;
>+
>+	priv = file_to_kcs_bmc_raw(filp);
>+
>+	poll_wait(filp, &priv->queue, wait);
>+
>+	spin_lock_irq(&priv->queue.lock);
>+	if (kcs_bmc_raw_prepare_obe(priv))
>+		events |= (EPOLLOUT | EPOLLWRNORM);
>+
>+	if (priv->readable || (kcs_bmc_read_status(priv->client.dev) & KCS_BMC_STR_IBF))
>+		events |= (EPOLLIN | EPOLLRDNORM);
>+	spin_unlock_irq(&priv->queue.lock);
>+
>+	return events;
>+}
>+
>+static ssize_t kcs_bmc_raw_read(struct file *filp, char __user *buf,
>+			     size_t count, loff_t *ppos)
>+{
>+	struct kcs_bmc_device *kcs_bmc;
>+	struct kcs_bmc_raw *priv;
>+	bool read_idr, read_str;
>+	struct device *dev;
>+	u8 idr, str;
>+	ssize_t rc;
>+
>+	priv = file_to_kcs_bmc_raw(filp);
>+	kcs_bmc = priv->client.dev;
>+	dev = priv->miscdev.this_device;
>+
>+	if (!count)
>+		return 0;
>+
>+	if (count > 2 || *ppos > 1)
>+		return -EINVAL;
>+
>+	if (*ppos + count > 2)
>+		return -EINVAL;
>+
>+	read_idr = (*ppos == 0);
>+	read_str = (*ppos == 1) || (count == 2);
>+
>+	spin_lock_irq(&priv->queue.lock);
>+	if (read_idr) {
>+		dev_dbg(dev, "Waiting for IBF\n");
>+		str = kcs_bmc_read_status(kcs_bmc);
>+		if ((filp->f_flags & O_NONBLOCK) && (str & KCS_BMC_STR_IBF)) {
>+			rc = -EWOULDBLOCK;
>+			goto out;
>+		}
>+
>+		rc = wait_event_interruptible_locked(priv->queue,
>+						     priv->readable || (str & KCS_BMC_STR_IBF));
>+		if (rc < 0)
>+			goto out;
>+
>+		if (signal_pending(current)) {
>+			dev_dbg(dev, "Interrupted waiting for IBF\n");
>+			rc = -EINTR;
>+			goto out;
>+		}
>+
>+		/*
>+		 * Re-enable events prior to possible read of IDR (which clears
>+		 * IBF) to ensure we receive interrupts for subsequent writes
>+		 * to IDR. Writes to IDR by the host should not occur while IBF
>+		 * is set.
>+		 */
>+		dev_dbg(dev, "Woken by IBF, enabling IRQ\n");
>+		kcs_bmc_raw_update_event_mask(priv, KCS_BMC_EVENT_TYPE_IBF,
>+					      KCS_BMC_EVENT_TYPE_IBF);
>+
>+		/* Read data out of IDR into internal storage if necessary */
>+		if (!priv->readable) {
>+			WARN(!(str & KCS_BMC_STR_IBF), "Unknown reason for wakeup!");
>+
>+			priv->idr = kcs_bmc_read_data(kcs_bmc);
>+		}
>+
>+		/* Copy data from internal storage to userspace */
>+		idr = priv->idr;
>+
>+		/* We're done consuming the internally stored value */
>+		priv->readable = false;
>+	}
>+
>+	if (read_str) {
>+		str = kcs_bmc_read_status(kcs_bmc);
>+		if (*ppos == 0 || priv->readable)
>+			/*
>+			 * If we got this far with `*ppos == 0` then we've read
>+			 * data out of IDR, so set IBF when reporting back to
>+			 * userspace so userspace knows the IDR value is valid.
>+			 */
>+			str |= KCS_BMC_STR_IBF;
>+
>+		dev_dbg(dev, "Read status 0x%x\n", str);
>+
>+	}
>+
>+	rc = count;
>+out:
>+	spin_unlock_irq(&priv->queue.lock);
>+
>+	if (rc < 0)
>+		return rc;
>+
>+	/* Now copy the data in to the userspace buffer */
>+
>+	if (read_idr)
>+		if (copy_to_user(buf++, &idr, sizeof(idr)))
>+			return -EFAULT;
>+
>+	if (read_str)
>+		if (copy_to_user(buf, &str, sizeof(str)))
>+			return -EFAULT;
>+
>+	return count;
>+}
>+
>+static ssize_t kcs_bmc_raw_write(struct file *filp, const char __user *buf,
>+			      size_t count, loff_t *ppos)
>+{
>+	struct kcs_bmc_device *kcs_bmc;
>+	bool write_odr, write_str;
>+	struct kcs_bmc_raw *priv;
>+	struct device *dev;
>+	uint8_t data[2];
>+	ssize_t result;
>+	u8 str;
>+
>+	priv = file_to_kcs_bmc_raw(filp);
>+	kcs_bmc = priv->client.dev;
>+	dev = priv->miscdev.this_device;
>+
>+	if (!count)
>+		return count;
>+
>+	if (count > 2)
>+		return -EINVAL;
>+
>+	if (*ppos >= 2)
>+		return -EINVAL;
>+
>+	if (*ppos + count > 2)
>+		return -EINVAL;
>+
>+	if (copy_from_user(data, buf, count))
>+		return -EFAULT;
>+
>+	write_odr = (*ppos == 0);
>+	write_str = (*ppos == 1) || (count == 2);
>+
>+	spin_lock_irq(&priv->queue.lock);
>+
>+	/* Always write status before data, we generate the SerIRQ by writing ODR */
>+	if (write_str) {
>+		/* The index of STR in the userspace buffer depends on whether ODR is written */
>+		str = data[*ppos == 0];
>+		if (!(str & KCS_BMC_STR_OBF))
>+			dev_warn(dev, "Clearing OBF with status write: 0x%x\n", str);
>+		dev_dbg(dev, "Writing status 0x%x\n", str);
>+		kcs_bmc_write_status(kcs_bmc, str);
>+	}
>+
>+	if (write_odr) {
>+		/* If we're writing ODR it's always the first byte in the buffer */
>+		u8 odr = data[0];
>+
>+		str = kcs_bmc_read_status(kcs_bmc);
>+		if (str & KCS_BMC_STR_OBF) {
>+			if (filp->f_flags & O_NONBLOCK) {
>+				result = -EWOULDBLOCK;
>+				goto out;
>+			}
>+
>+			priv->writable = kcs_bmc_raw_prepare_obe(priv);
>+
>+			/* Now either OBF is already clear, or we'll get an OBE event to wake us */
>+			dev_dbg(dev, "Waiting for OBF to clear\n");
>+			wait_event_interruptible_locked(priv->queue, priv->writable);
>+
>+			if (signal_pending(current)) {
>+				kcs_bmc_raw_update_event_mask(priv, KCS_BMC_EVENT_TYPE_OBE, 0);
>+				result = -EINTR;
>+				goto out;
>+			}
>+
>+			WARN_ON(kcs_bmc_read_status(kcs_bmc) & KCS_BMC_STR_OBF);
>+		}
>+
>+		dev_dbg(dev, "Writing 0x%x to ODR\n", odr);
>+		kcs_bmc_write_data(kcs_bmc, odr);
>+	}
>+
>+	result = count;
>+out:
>+	spin_unlock_irq(&priv->queue.lock);
>+
>+	return result;
>+}
>+
>+static int kcs_bmc_raw_release(struct inode *inode, struct file *filp)
>+{
>+	struct kcs_bmc_raw *priv = file_to_kcs_bmc_raw(filp);
>+
>+	kcs_bmc_disable_device(priv->client.dev, &priv->client);
>+
>+	return 0;
>+}
>+
>+static const struct file_operations kcs_bmc_raw_fops = {
>+	.owner          = THIS_MODULE,
>+	.open		= kcs_bmc_raw_open,
>+	.llseek		= no_seek_end_llseek,
>+	.read           = kcs_bmc_raw_read,
>+	.write          = kcs_bmc_raw_write,
>+	.poll		= kcs_bmc_raw_poll,
>+	.release	= kcs_bmc_raw_release,
>+};
>+
>+static DEFINE_SPINLOCK(kcs_bmc_raw_instances_lock);
>+static LIST_HEAD(kcs_bmc_raw_instances);
>+
>+static int kcs_bmc_raw_attach_cdev(struct kcs_bmc_device *kcs_bmc)
>+{
>+	struct kcs_bmc_raw *priv;
>+	int rc;
>+
>+	priv = devm_kzalloc(kcs_bmc->dev, sizeof(*priv), GFP_KERNEL);
>+	if (!priv)
>+		return -ENOMEM;
>+
>+	priv->client.dev = kcs_bmc;
>+	priv->client.ops = &kcs_bmc_raw_client_ops;
>+
>+	init_waitqueue_head(&priv->queue);
>+	priv->writable = false;
>+	priv->readable = false;
>+
>+	priv->miscdev.minor = MISC_DYNAMIC_MINOR;
>+	priv->miscdev.name = devm_kasprintf(kcs_bmc->dev, GFP_KERNEL, "%s%u", DEVICE_NAME,
>+					   kcs_bmc->channel);
>+	if (!priv->miscdev.name)
>+		return -EINVAL;
>+
>+	priv->miscdev.fops = &kcs_bmc_raw_fops;
>+
>+	/* Initialise our expected events. Listen for IBF but ignore OBE until necessary */
>+	kcs_bmc_raw_update_event_mask(priv, (KCS_BMC_EVENT_TYPE_IBF | KCS_BMC_EVENT_TYPE_OBE),
>+				      KCS_BMC_EVENT_TYPE_IBF);
>+
>+	rc = misc_register(&priv->miscdev);
>+	if (rc) {
>+		dev_err(kcs_bmc->dev, "Unable to register device\n");
>+		return rc;
>+	}
>+
>+	spin_lock_irq(&kcs_bmc_raw_instances_lock);
>+	list_add(&priv->entry, &kcs_bmc_raw_instances);
>+	spin_unlock_irq(&kcs_bmc_raw_instances_lock);
>+
>+	dev_info(kcs_bmc->dev, "Initialised raw client for channel %d", kcs_bmc->channel);
>+
>+	return 0;
>+}
>+
>+static int kcs_bmc_raw_detach_cdev(struct kcs_bmc_device *kcs_bmc)
>+{
>+	struct kcs_bmc_raw *priv = NULL, *pos;
>+
>+	spin_lock_irq(&kcs_bmc_raw_instances_lock);
>+	list_for_each_entry(pos, &kcs_bmc_raw_instances, entry) {
>+		if (pos->client.dev == kcs_bmc) {
>+			priv = pos;
>+			list_del(&pos->entry);
>+			break;
>+		}
>+	}
>+	spin_unlock_irq(&kcs_bmc_raw_instances_lock);
>+
>+	if (!priv)
>+		return 0;

Similarly to patch #12, might we want to indicate some sort of failure
here, or is this a normal/expected case?

>+
>+	misc_deregister(&priv->miscdev);
>+	kcs_bmc_disable_device(kcs_bmc, &priv->client);
>+	devm_kfree(priv->client.dev->dev, priv);
>+
>+	return 0;
>+}
>+
>+static const struct kcs_bmc_cdev_ops kcs_bmc_raw_cdev_ops = {
>+	.add_device = kcs_bmc_raw_attach_cdev,
>+	.remove_device = kcs_bmc_raw_detach_cdev,
>+};
>+
>+static struct kcs_bmc_cdev kcs_bmc_raw_cdev = {
>+	.ops = &kcs_bmc_raw_cdev_ops,
>+};
>+
>+static int kcs_bmc_raw_init(void)
>+{
>+	return kcs_bmc_register_cdev(&kcs_bmc_raw_cdev);
>+}
>+module_init(kcs_bmc_raw_init);
>+
>+static void kcs_bmc_raw_exit(void)
>+{
>+	int rc;
>+
>+	rc = kcs_bmc_unregister_cdev(&kcs_bmc_raw_cdev);
>+	if (rc)
>+		pr_warn("Failed to remove KCS BMC client: %d", rc);
>+}
>+module_exit(kcs_bmc_raw_exit);
>+
>+MODULE_LICENSE("GPL v2");
>+MODULE_AUTHOR("Andrew Jeffery <andrew@xxxxxxxx>");
>+MODULE_DESCRIPTION("Character device for raw access to a KCS device");
>-- 
>2.27.0
>