[PATCH] s390/cio: introduce driver_override on the css bus

Cornelia Huck
2019-06-13T11:08:21Z

Sometimes, we want to control which of the matching drivers
binds to a subchannel device (e.g. for subchannels we want to
handle via vfio-ccw).

For pci devices, a mechanism to do so has been introduced in
782a985d7af2 ("PCI: Introduce new device binding path using
pci_dev.driver_override"). It makes sense to introduce the
driver_override attribute for subchannel devices as well, so
that we can easily extend the 'driverctl' tool (which makes
use of the driver_override attribute for pci).

Note that unlike pci we still require a driver override to
match the subchannel type; matching more than one subchannel
type is probably not useful anyway.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
---

Lightly tested; did not yet attempt to adapt driverctl to actually
make use of it.

For some background, refer to the thread around
https://lore.kernel.org/kvm/20190612091439.3a33f17b.cohuck@redhat.com/

---
Documentation/ABI/testing/sysfs-bus-css | 23 +++++++++++
drivers/s390/cio/cio.h | 1 +
drivers/s390/cio/css.c | 53 +++++++++++++++++++++++++
3 files changed, 77 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-bus-css b/Documentation/ABI/testing/sysfs-bus-css
index 2979c40c10e9..966f8504bd7b 100644
--- a/Documentation/ABI/testing/sysfs-bus-css
+++ b/Documentation/ABI/testing/sysfs-bus-css
@@ -33,3 +33,26 @@ Description: Contains the PIM/PAM/POM values, as reported by the
in sync with the values current in the channel subsystem).
Note: This is an I/O-subchannel specific attribute.
Users: s390-tools, HAL
+
+What: /sys/bus/css/devices/.../driver_override
+Date: June 2019
+Contact: Cornelia Huck <cohuck@redhat.com>
+ linux-s390@vger.kernel.org
+Description: This file allows the driver for a device to be specified. When
+ specified, only a driver with a name matching the value written
+ to driver_override will have an opportunity to bind to the
+ device. The override is specified by writing a string to the
+ driver_override file (echo vfio-ccw > driver_override) and
+ may be cleared with an empty string (echo > driver_override).
+ This returns the device to standard matching rules binding.
+ Writing to driver_override does not automatically unbind the
+ device from its current driver or make any attempt to
+ automatically load the specified driver. If no driver with a
+ matching name is currently loaded in the kernel, the device
+ will not bind to any driver. This also allows devices to
+ opt-out of driver binding using a driver_override name such as
+ "none". Only a single driver may be specified in the override,
+ there is no support for parsing delimiters.
+ Note that unlike the mechanism of the same name for pci, this
+ file does not allow to override basic matching rules. I.e.,
+ the driver must still match the subchannel type of the device.
diff --git a/drivers/s390/cio/cio.h b/drivers/s390/cio/cio.h
index 06a91743335a..8c4af88f1ac3 100644
--- a/drivers/s390/cio/cio.h
+++ b/drivers/s390/cio/cio.h
@@ -113,6 +113,7 @@ struct subchannel {
enum sch_todo todo;
struct work_struct todo_work;
struct schib_config config;
+ char *driver_override; /* Driver name to force a match */
} __attribute__ ((aligned(8)));

DECLARE_PER_CPU_ALIGNED(struct irb, cio_irb);
diff --git a/drivers/s390/cio/css.c b/drivers/s390/cio/css.c
index aea502922646..f3436a17e3b5 100644
--- a/drivers/s390/cio/css.c
+++ b/drivers/s390/cio/css.c
@@ -165,6 +165,7 @@ static void css_subchannel_release(struct device *dev)

sch->config.intparm = 0;
cio_commit_config(sch);
+ kfree(sch->driver_override);
kfree(sch->lock);
kfree(sch);
}
@@ -315,9 +316,57 @@ static ssize_t modalias_show(struct device *dev, struct device_attribute *attr,

static DEVICE_ATTR_RO(modalias);

+static ssize_t driver_override_store(struct device *dev,
+ struct device_attribute *attr,
+ const char *buf, size_t count)
+{
+ struct subchannel *sch = to_subchannel(dev);
+ char *driver_override, *old, *cp;
+
+ /* We need to keep extra room for a newline */
+ if (count >= (PAGE_SIZE - 1))
+ return -EINVAL;
+
+ driver_override = kstrndup(buf, count, GFP_KERNEL);
+ if (!driver_override)
+ return -ENOMEM;
+
+ cp = strchr(driver_override, '\n');
+ if (cp)
+ *cp = '\0';
+
+ device_lock(dev);
+ old = sch->driver_override;
+ if (strlen(driver_override)) {
+ sch->driver_override = driver_override;
+ } else {
+ kfree(driver_override);
+ sch->driver_override = NULL;
+ }
+ device_unlock(dev);
+
+ kfree(old);
+
+ return count;
+}
+
+static ssize_t driver_override_show(struct device *dev,
+ struct device_attribute *attr, char *buf)
+{
+ struct subchannel *sch = to_subchannel(dev);
+ ssize_t len;
+
+ device_lock(dev);
+ len = snprintf(buf, PAGE_SIZE, "%s\n", sch->driver_override);
+ device_unlock(dev);
+ return len;
+}
+static DEVICE_ATTR_RW(driver_override);
+
static struct attribute *subch_attrs[] = {
&dev_attr_type.attr,
&dev_attr_modalias.attr,
+ &dev_attr_driver_override.attr,
NULL,
};

@@ -1222,6 +1271,10 @@ static int css_bus_match(struct device *dev, struct device_driver *drv)
struct css_driver *driver = to_cssdriver(drv);
struct css_device_id *id;

+ /* When driver_override is set, only bind to the matching driver */
+ if (sch->driver_override && strcmp(sch->driver_override, drv->name))
+ return 0;
+
for (id = driver->subchannel_type; id->match_flags; id++) {
if (sch->st == id->type)
return 1;
--
2.20.1