linux-kernel - Re: [PATCH] usb: roles: Switch on role-switch uevent reporting

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Date:   Sat, 9 May 2020 17:37:47 +0100
From:   Bryan O'Donoghue <bryan.odonoghue@...aro.org>
To:     Wen Yang <wenyang@...ux.alibaba.com>,
        heikki.krogerus@...ux.intel.com, gregkh@...uxfoundation.org,
        linux-usb@...r.kernel.org
Cc:     Chunfeng Yun <chunfeng.yun@...iatek.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        chenqiwu <chenqiwu@...omi.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: roles: Switch on role-switch uevent reporting

On 09/05/2020 04:24, Wen Yang wrote:
> 
> 
> 在 2020/5/9 上午12:29, Bryan O'Donoghue 写道:
>> Right now we don't report to user-space a role switch when doing a
>> usb_role_switch_set_role() despite having registered the uevent 
>> callbacks.
>>
>> This patch switches on the notifications allowing user-space to see
>> role-switch change notifications and subsequently determine the current
>> controller data-role.
>>
>> example:
>> PFX=/devices/platform/soc/78d9000.usb/ci_hdrc.0
>>
>> root@...ebox# udevadm monitor -p
>>
>> KERNEL[49.894994] change $PFX/usb_role/ci_hdrc.0-role-switch (usb_role)
>> ACTION=change
>> DEVPATH=$PFX/usb_role/ci_hdrc.0-role-switch
>> SUBSYSTEM=usb_role
>> DEVTYPE=usb_role_switch
>> USB_ROLE_SWITCH=ci_hdrc.0-role-switch
>> SEQNUM=2432
>>
>> Cc: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> Cc: Chunfeng Yun <chunfeng.yun@...iatek.com>
>> Cc: Suzuki K Poulose <suzuki.poulose@....com>
>> Cc: Alexandre Belloni <alexandre.belloni@...tlin.com>
>> Cc: Wen Yang <wenyang@...ux.alibaba.com>
>> Cc: chenqiwu <chenqiwu@...omi.com>
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@...aro.org>
>> ---
>>   drivers/usb/roles/class.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/usb/roles/class.c b/drivers/usb/roles/class.c
>> index 5b17709821df..27d92af29635 100644
>> --- a/drivers/usb/roles/class.c
>> +++ b/drivers/usb/roles/class.c
>> @@ -49,8 +49,10 @@ int usb_role_switch_set_role(struct usb_role_switch 
>> *sw, enum usb_role role)
>>       mutex_lock(&sw->lock);
>>       ret = sw->set(sw, role);
>> -    if (!ret)
>> +    if (!ret) {
>>           sw->role = role;
>> +        kobject_uevent(&sw->dev.kobj, KOBJ_CHANGE);
>> +    }
>>       mutex_unlock(&sw->lock);
>>
> 
> Hi, we may also need to deal with the return value of kobject_uevent(). 

For an KOBJ_ADD you'd return an error.

grep -r "= kobject_uevent(" *
drivers/misc/cxl/sysfs.c:	rc = kobject_uevent(&cr->kobj, KOBJ_ADD);
drivers/uio/uio.c:		ret = kobject_uevent(&map->kobj, KOBJ_ADD);
drivers/uio/uio.c:		ret = kobject_uevent(&portio->kobj, KOBJ_ADD);
drivers/visorbus/visorchipset.c:	res = 
kobject_uevent(&chipset_dev->acpi_device->dev.kobj, KOBJ_ONLINE);
drivers/visorbus/visorchipset.c:	int res = 
kobject_uevent(&chipset_dev->acpi_device->dev.kobj,

For a KOBJ_CHANGE I guess we could print an error

if (kobject_uevent(&sw->dev.kobj, KOBJ_CHANGE)
     dev_err(&sw->dev, "failed to signal USB role-switch uevent\n");

Nobody else seems that bothered about it.

grep -r "if (kobject_uevent(" *

> Should we move it under the line mutex_unlock(&sw->lock)?

I think probably not. the mutex serializes the notification. In theory 
outside the mutex you could get an out-of-order notification.

The main reason I put it where it is, is we already test ret and should 
only notify the change, when the role-switch has suceeded.

As I say, in theory anyway, the mutex enforces the signalling, whatever 
about the reception, of the role-switch change, so IMO inside the bounds 
of the mutex is the right place to put it.

---
bod