CODEWITHSUNDEEP
macosusbiokitscsi
Thomas Tempelmann
Thomas Tempelmann

Reputation: 12043

How do I pass SCSI (CDB) commands through a USB connection

I am trying to talk to a MSC USB device (interface class 8, subclass 6, protocol 0x50) via a plain USB API with endpoints (all set up for me).

Provided I have a valid CDB, such as for "Test Unit Ready", how to I send that over the USB interface?

Where can I find examples or docs for how this is done?

Background: The actual platform is macOS, which doesn't provide SCSI-passthrough for block devices, and the native SCSI API is also not available in this case.

I have, however, been able to initiate communication on the USB level with the device, and am now trying to circumvent the blocked SCSI device level access by talking thru USB directly.

Upvotes: 0

Views: 1034

Answers (1)

pmdj
pmdj

Reputation: 23428

Most such devices implement the so-called “Bulk Only” protocol, which is specified here: https://usb.org/document-library/mass-storage-bulk-only-10

Essentially, you send a 31-byte “Command Block Wrapper”, which includes the CDB and is specified in section 5.1 of the spec, to the device via the bulk out endpoint. You then read or write the data to be transferred from the input bulk endpoint or to the output bulk endpoint and finally read the 13-byte command status wrapper from the bulk in pipe.

However, note that you’ll need to make sure the OS hasn’t already loaded a driver for the device - from user space, the system won’t give you access to the endpoints when a kernel driver has claimed them anyway, but if you were to attempt to use the same pipes as the default driver from a kext, you’d get unpredictable results.

Upvotes: 1

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