Re: [PATCH v2] usb: typec: Add a sysfs node to manage port type

[Guenter Roeck <linux@roeck-us.net>]

On 05/24/2017 08:10 PM, Badhri Jagan Sridharan wrote:
> Thanks comments inline.
> 
> On Tue, May 23, 2017 at 7:38 PM, Guenter Roeck <linux@roeck-us.net> wrote:
>> On 05/23/2017 06:28 PM, Badhri Jagan Sridharan wrote:
>>>
>>> User space applications in some cases have the need to enforce a
>>> specific port type(DFP/UFP/DRP). This change allows userspace to
>>> attempt setting the desired port type. Low level drivers can
>>> however reject the request if the specific port type is not supported.
>>>
>>> Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
>>> ---
>>> Changelog since v1:
>>> - introduced a new variable port_type in struct typec_port to track
>>>     the current port type instead of changing type member in
>>>     typec_capability to address Heikki Krogerus comments.
>>> - changed the output format and strings that would be displayed as
>>>     suggested by Heikki Krogerus.
>>>>    Documentation/ABI/testing/sysfs-class-typec | 13 ++++++
>>>    drivers/usb/typec/typec.c                   | 66
>>> +++++++++++++++++++++++++++++
>>>    include/linux/usb/typec.h                   |  4 ++
>>>    3 files changed, 83 insertions(+)
>>>
>>> diff --git a/Documentation/ABI/testing/sysfs-class-typec
>>> b/Documentation/ABI/testing/sysfs-class-typec
>>> index d4a3d23eb09c..1f224c2e391f 100644
>>> --- a/Documentation/ABI/testing/sysfs-class-typec
>>> +++ b/Documentation/ABI/testing/sysfs-class-typec
>>> @@ -73,6 +73,19 @@ Description:
>>>                  Valid values: source, sink, none (to remove preference)
>>>    +What:           /sys/class/typec/<port>/port_type
>>> +Date:           May 2017
>>> +Description:
>>> +               Indicates the type of the port. This attribute can be used
>>> for
>>> +               requesting a change in the port type. Port type change is
>>> +               supported as a synchronous operation, so write(2) to the
>>> +               attribute will not return until the operation has
>>> finished.
>>> +
>>> +               Valid values:
>>> +               - source
>>> +               - sink
>>> +               - dual
>>> +
>>>    What:         /sys/class/typec/<port>/supported_accessory_modes
>>>    Date:         April 2017
>>>    Contact:      Heikki Krogerus <heikki.krogerus@linux.intel.com>
>>> diff --git a/drivers/usb/typec/typec.c b/drivers/usb/typec/typec.c
>>> index 89e540bb7ff3..5063d6e0f8c7 100644
>>> --- a/drivers/usb/typec/typec.c
>>> +++ b/drivers/usb/typec/typec.c
>>> @@ -69,6 +69,7 @@ struct typec_port {
>>>          enum typec_role                 pwr_role;
>>>          enum typec_role                 vconn_role;
>>>          enum typec_pwr_opmode           pwr_opmode;
>>> +       enum typec_port_type            port_type;
>>
>>
>> I am missing where this variable is initialized (when the port is registered
>> ?).
> 
> Yes.. I missed it while cleaning up the patch. Will add it to my next patch.
> 
>>
>>>          const struct typec_capability   *cap;
>>>    };
>>> @@ -789,6 +790,12 @@ static const char * const typec_data_roles[] = {
>>>          [TYPEC_HOST]    = "host",
>>>    };
>>>    +static const char * const typec_port_types[] = {
>>> +       [TYPEC_PORT_DFP] = "source",
>>> +       [TYPEC_PORT_UFP] = "sink",
>>> +       [TYPEC_PORT_DRP] = "dual",
>>> +};
>>> +
>>>    static ssize_t
>>>    preferred_role_store(struct device *dev, struct device_attribute *attr,
>>>                       const char *buf, size_t size)
>>> @@ -856,6 +863,12 @@ static ssize_t data_role_store(struct device *dev,
>>>                  return -EOPNOTSUPP;
>>>          }
>>>    +     if (port->port_type != TYPEC_PORT_DRP) {
>>> +               dev_dbg(dev, "port type fixed at \"%s\"",
>>> +                            typec_port_types[port->port_type]);
>>> +               return -EIO;
>>
>>
>> ?? This is already there, or am I missing something ?
> 
> I am checking against the current value of port_type variable.
> Dont we want to reject role swaps if the port_type is not
> TYPEC_PORT_DRP ? My understanding is that if the port type
> is fixed at say PORT_TYPE_DFP by userspace, then unless
> the userspace sets port_type back to PORT_TYPE_DRP,
> role swap requests have to rejected. Is my understanding not
> correct ?
> 

Ah yes, the existing check is for port->cap->type. But why not just
replace that check with port->port_type ? Checking both seems overkill.

>>
>>> +       }
>>> +
>>>          ret = sysfs_match_string(typec_data_roles, buf);
>>>          if (ret < 0)
>>>                  return ret;
>>> @@ -897,6 +910,12 @@ static ssize_t power_role_store(struct device *dev,
>>>                  return -EOPNOTSUPP;
>>>          }
>>>    +     if (port->port_type != TYPEC_PORT_DRP) {
>>> +               dev_dbg(dev, "port type fixed at \"%s\"",
>>> +                            typec_port_types[port->port_type]);
>>> +               return -EIO;
>>
>>
>> Unrelated change; should be in a separate patch. Also, it should
>> probably return -EOPNOTSUPP.
> 
> similar to what I am doing for data_role_store function.
> 

Not sure here. This is currently treated differently. A host-only
or device-only port was still considered supportable if it supports
power delivery.

>>
>>> +       }
>>> +
>>>          if (port->pwr_opmode != TYPEC_PWR_MODE_PD) {
>>>                  dev_dbg(dev, "partner unable to swap power role\n");
>>>                  return -EIO;
>>> @@ -926,6 +945,52 @@ static ssize_t power_role_show(struct device *dev,
>>>    }
>>>    static DEVICE_ATTR_RW(power_role);
>>>    +static ssize_t
>>> +port_type_store(struct device *dev, struct device_attribute *attr,
>>> +                       const char *buf, size_t size)
>>> +{
>>> +       struct typec_port *port = to_typec_port(dev);
>>> +       int ret, type;
>>> +
>>
>>
>> I think 'type' should be 'enum typec_port_type'.
> 
> Will fix in my next patch.
> 
>>
>>> +       if (!port->cap->port_type_set || port->cap->type !=
>>> TYPEC_PORT_DRP) {
>>> +               dev_dbg(dev, "changing port type not supported\n");
>>> +               return -EOPNOTSUPP;
>>> +       }
>>> +
>>> +       ret = sysfs_match_string(typec_port_types, buf);
>>> +       if (ret < 0)
>>> +               return ret;
>>> +
>>
>> If the new type matches the current type, you could return immediately here.
> 
> Will fix in my next patch.
> 
>>
>>> +       type = ret;
>>> +
>>> +       ret = port->cap->port_type_set(port->cap, type);
>>> +       if (ret)
>>> +               return ret;
>>> +
>>> +       port->port_type = type;
>>
>>
>> We have no locking here, meaning a second request could be processed in
>> parallel.
>> There is no guarantee that the resulting value in port->port_type matches
>> the actual port type (for example, if the code flow is interrupted before
>> port_type is set).
>>
>> For other functions we have a callback from the driver, and the driver is
>> responsible for all locking. That doesn't work here, and a callback from
>> the driver to update the port type does not seem necessary (the port type,
>> unlike roles, does not change by itself). That means you'll need locking
>> to make sure that the port->port_type is in sync with the low level driver.
> 
> Going to introduce a mutex port_type_lock which will protect the port_type
> variable.
> 
>>
>>> +
>>> +       return size;
>>> +}
>>> +
>>> +static ssize_t
>>> +port_type_show(struct device *dev, struct device_attribute *attr,
>>> +               char *buf)
>>> +{
>>> +       struct typec_port *port = to_typec_port(dev);
>>> +
>>> +       if (port->cap->type == TYPEC_PORT_DRP) {
>>> +               if (port->port_type == TYPEC_PORT_DRP)
>>> +                       return sprintf(buf, "%s\n", "[dual] source sink");
>>> +               else if (port->port_type == TYPEC_PORT_DFP)
>>> +                       return sprintf(buf, "%s\n", "dual [source] sink");
>>
>>
>> Hmm.. I think this is another race condition. The port type could change
>> from
>> DFP to DRP after the variable was read the first time, and we would display
>> it as sink. You would need a mutex to protect against changes of
>> port->port_type,
>> or introduce an array with the three strings and use something like
>>
>> const char *something[] = {
>>          [TYPEC_PORT_DRP] = "[dual] source sink",
>>          ...
>> };
>>          ...
>>                  return sprintf(buf, ""%s\n", something[port->port_type]);
>>
>> which would be less code. After all, the strings are needed anyway.
> 
> Sounds good to me.
> 
>>
>>
>>> +               else
>>> +                       return sprintf(buf, "%s\n", "dual source [sink]");
>>> +       }
>>> +
>>> +       return sprintf(buf, "[%s]\n", typec_port_types[port->cap->type]);
>>> +}
>>> +static DEVICE_ATTR_RW(port_type);
>>> +
>>>    static const char * const typec_pwr_opmodes[] = {
>>>          [TYPEC_PWR_MODE_USB]    = "default",
>>>          [TYPEC_PWR_MODE_1_5A]   = "1.5A",
>>> @@ -1035,6 +1100,7 @@ static struct attribute *typec_attrs[] = {
>>>          &dev_attr_usb_power_delivery_revision.attr,
>>>          &dev_attr_usb_typec_revision.attr,
>>>          &dev_attr_vconn_source.attr,
>>> +       &dev_attr_port_type.attr,
>>>          NULL,
>>>    };
>>>    ATTRIBUTE_GROUPS(typec);
>>> diff --git a/include/linux/usb/typec.h b/include/linux/usb/typec.h
>>> index ec78204964ab..5badf6f66479 100644
>>> --- a/include/linux/usb/typec.h
>>> +++ b/include/linux/usb/typec.h
>>> @@ -190,6 +190,7 @@ struct typec_partner_desc {
>>>     * @pr_set: Set Power Role
>>>     * @vconn_set: Set VCONN Role
>>>     * @activate_mode: Enter/exit given Alternate Mode
>>> + * @port_type_set: Set port type
>>>     *
>>>     * Static capabilities of a single USB Type-C port.
>>>     */
>>> @@ -214,6 +215,9 @@ struct typec_capability {
>>>          int             (*activate_mode)(const struct typec_capability *,
>>>                                           int mode, int activate);
>>> +
>>> +       int             (*port_type_set)(const struct typec_capability *,
>>> +                                       enum typec_port_type);
>>>    };
>>>      /* Specific to try_role(). Indicates the user want's to clear the
>>> preference. */
>>>
>>
>