Re: [Qemu-devel] [PATCH 03/13] usb-host-libusb: Detach kernel drivers earlier

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 10/09/2013 10:55 AM, Gerd Hoffmann wrote:
> On Di, 2013-10-08 at 21:58 +0200, Hans de Goede wrote:
>> If we detach the kernel drivers on the first set_config, then they will
>> be still attached when the device gets its initial reset. Causing the drivers
>> to re-initialize the device after the reset, dirtying the device state.
>
>> @@ -1051,7 +1055,6 @@ static void usb_host_set_config(USBHostDevice *s, int config, USBPacket *p)
>>       trace_usb_host_set_config(s->bus_num, s->addr, config);
>>
>>       usb_host_release_interfaces(s);
>> -    usb_host_detach_kernel(s);
>>       rc = libusb_set_configuration(s->dh, config);
>>       if (rc != 0) {
>>           usb_host_libusb_error("libusb_set_configuration", rc);
>
> Sure we can safely remove the detach_kernel here?

Yes.

> Assuming we have a device with multiple configurations, each
> configuration has a different set of interfaces, guest switches from one
> config to another.  Do we correctly unbind kernel / claim interfaces
> then?

Yes we still have a usb_host_detach_kernel() call in the beginning
of usb_host_claim_interfaces(), which gets run on the new interfaces
immediately after the libusb_set_configuration call.

Doing a detach_kernel after a set_config has always been necessary,
since the kernel will automatically bind drivers to the new interfaces
if the set_config succeeds.

Doing detach_kernel before set_config is necessary if kernel drivers
are attached, because the kernel will not allow a set_config if any
drivers are bound. But we already do a set_config for the current
config on usb_host_open() now, and if a new config gets installed we
immediately detach the kernel drivers again from usb_host_claim_interfaces()
so when we enter usb_host_set_config no kernel drivers, other then
usbfs (the userspace access driver) will be attached, and usbfs is
detached by releasing the interfaces (*).

Regards,

Hans


*) In recent libusb versions libusb will even disallow using
libusb_detach_kernel_driver to detach usbfs, so as to stop a user space
program from breaking anothers userspaces program claim on the interface
by completelty detaching usbfs from the interface.