[PATCH] usb: common: Detect USB storage media with "miscellaneous" USB devices

[Christian Kohlschütter]

> On 30. Sep 2022, at 04:42, Marek Vasut <marex at denx.de> wrote:
> 
> On 9/26/22 23:55, Christian Kohlschütter wrote:
>> On 26. Sep 2022, at 01:45, Marek Vasut <marex at denx.de> wrote:
>>> 
>>> On 9/25/22 16:46, Christian Kohlschütter wrote:
>>>> When detecting USB storage devices, we currently skip everything that is
>>>> not marked as "undefined device class".
>>>> Composite devices such as tinyusb's CDC+MSC identify as "miscellaneous"
>>>> (class 0xEF).
>>>> Introduce a new constant, USB_CLASS_MISC (0xEF), and allow the detection
>>>> process to proceed for USB devices with this device class.
>>> 
>>> What does Linux do with such a device , is there a specific quirk or does it simply not check the bDeviceClass at all ?
>> I didn't see any  relevant checks in drivers/usb/storage, however changing the bDeviceClass from "MISC" (0xEF) to "UNSPECIFIED"/"PER_INTERFACE" (0x00) causes the device to be no longer detected by either Linux or macOS, but it would work with U-Boot (i.e., without the change).
>> macOS' USB prober would report the USB device as "unconfigured". On Linux, using usbutils but not busybox, lsusb would actually hang.
>> I think it's a good idea to keep the set of permitted device classes to a minimum (0x00 / 0xEF) and abort early in all other cases. This way, we can skip the more elaborate checks in usb_storage_probe for devices that are definitely not supported.
> 
> I think the check in the usb_storage.c is actually correct as-is and the device you have might be odd.
> 
> https://www.usb.org/sites/default/files/usbmassbulk_10.pdf
> 
> "
> 4.1 Device Descriptor
> Each USB device has one device descriptor (per USB Specification). The device shall specify the device class and subclass codes in the interface descriptor, and not in the device descriptor.
> ...
> 4 | bDeviceClass | Byte | 00h | Class is specified in the interface descriptor.
> ...
> NOTE: Information in this table is from the USB Specification version 1.1 table 9-7. Bold text has been added for clarifications when using these descriptors with this specification.
> "
> 
> And then
> 
> "
> 4.3 Interface Descriptors
> The device shall support at least one interface, known herein as the Bulk-Only Data Interface. The Bulk-Only Data Interface uses three endpoints.
> Composite mass storage devices may support additional interfaces, to provide other features such as audio or video capabilities. This specification does not define such interfaces.
> The interface may have multiple alternate settings. The host shall examine each of the alternate settings to look for the bInterfaceProtocol and bInterfaceSubClass it supports optimally.
> "
> 
> If I read this right, then such a composite device like you have should have bDeviceClass=0x00 and two Interface descriptors, one for the mass storage device and one for CDC ?

See https://github.com/hathach/tinyusb/blob/master/examples/device/cdc_msc/src/usb_descriptors.c for an example of such a device. It has one mass storage device and one CDC.

Changing the device class to 0x00 breaks support on Linux and macOS (I didn't try Windows).