Android USB OTG阅读Usb Drive

Question

I am currently working on a project which requires Usb Host mode in Android (using 4.2.2). I need to be able to do standard file operations like listing files and reading from a usb drive. Using the host mode documentation I have gotten to the point of being able to detect the drive is connected, is USB mass storage, and find the various endpoints.

Where I get stuck is what to do after that. I guess I don't really understand the difference between control and bulk transfers or in the case of control transfers what is really even happening. Unfortunately my knowledge of USB itself is somewhat limited and I hear the documentation is rather daunting.

Can anyone help guide me through, or point me towards a working example for, what I would need to do to essentially "mount" the usb drive? As I said previously I just need those file operations of listing files and reading them, not writing. I should also mention that the target phone is the Galaxy Nexus by Samsung.

Thanks

Answer 1

a control endpoint is (usually) used to send commands to usb device.
a bulk endpoint is (usually) used to transfer data to usb device.

in bulk the structure of the data is not defined.
but in control transfer the structure of data is defined.
the host send a 8byte packet containing the information about the command (like request ID, command parameters, length of data to transfer in data stage, etc...). and then optionally some data is received/send and finally both the host/device agrees on success of command by moving to status stage. thats it!
but, devils is in the details, see usb specification. :)

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all usb memories (pendrives, memory card, hard drives) work on the usb mass storage class.

using the mass storage class, you can perform memory read-write but the filesystem (ex: EXT3, FAT...) on top of it need another driver.

so, essentially to read/write a file. you need to connect to the device:

1. connect to the device (you have done it)
2. identify and take control over the mass storage interface
3. write a mass storage class driver
4. write filesystem driver that will read/write files using mass storage class driver
5. use the filesystem driver api in your project.

to write a mass storage class driver, you have to read the USB Mass storage clas specification http://www.usb.org/developers/docs/devclass_docs/ and probebly you are looking for the Bulk only

and then, refer the filesystem(s) you are targetting for there design and specification. (if available)

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AFAIK, Android has filesystem mounting facility connected to USB, so you are probebly reinventing the wheel.

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extra-note:
some filesystem are not openly documented (though reverse engg information probely exists) (ex: NTFS)
and some filesystem are (partly) patented. (ex: FAT)