| # USB mass storage driver |
| |
| Caution: This page may contain information that is specific to the legacy |
| version of the driver framework (DFv1). |
| |
| The USB mass storage driver is used to communicate with mass storage devices |
| such as flash drives, external hard drives, and other types of removable media |
| connected through USB. The USB mass storage driver is split into two parts: |
| |
| * [SCSI block device][scsi-block-device] uses the [block][block] protocol. |
| * [Core][core] device interfaces with the USB stack. |
| |
| ## SCSI block device |
| |
| The block device implements [`BlockImplQuery`][blockimplquery] and |
| [`BlockImplQueue`][blockimplqueue]. It supports read, write, and flush |
| operations. If power is lost between a write operation and a flush operation, |
| changes written to a USB mass storage device may not be persisted to the device. |
| The driver has no mechanism to inform drivers higher up in the stack of when |
| a write has actually been written to physical media. For the purposes of USB |
| mass storage, a write is considered complete when the device acknowledges the |
| write. |
| |
| ## Core device |
| |
| The core device serves as the interface between the block device and the USB |
| stack. The core accepts requests from the block device, and converts them into |
| USB requests, which are eventually sent to hardware through the USB stack. For |
| each request, the following steps are performed: |
| |
| * Request is added to a queue. |
| * Request is picked up by the worker thread. |
| * SCSI command stored in the request is sent to the device. |
| * Request status is read back from the device. |
| * Completion callback is invoked, informing the block device layer that the |
| request has been completed. |
| |
| Some USB mass storage devices may have multiple block devices such as an array |
| of disks. In this case, the core driver creates one block device per disk. |
| |
| <!-- Reference links --> |
| |
| [scsi-block-device]: /src/devices/block/lib/scsi/block-device.cc |
| [block]: /sdk/fidl/fuchsia.hardware.block.driver/block.fidl |
| [core]: /src/devices/block/drivers/usb-mass-storage/usb-mass-storage.cc |
| [blockimplquery]: /sdk/fidl/fuchsia.hardware.block.driver/block.fidl#95 |
| [blockimplqueue]: /sdk/fidl/fuchsia.hardware.block.driver/block.fidl#102 |
