# USB Host MSC (Mass Storage Class) Driver
This directory contains an implementation of a USB Mass Storage Class Driver implemented on top of the [USB Host Library](https://docs.espressif.com/projects/esp-idf/en/latest/esp32s2/api-reference/peripherals/usb_host.html).
MSC driver allows access to USB flash drivers using the BOT (Bulk-Only Transport) protocol and the Transparent SCSI command set.
- First, usb host library has to be initialized by calling `usb_host_install`
- USB Host Library events have to be handled by invoking `usb_host_lib_handle_events` periodically.
In general, an application should spawn a dedicated task handle USB Host Library events.
However, in order to save RAM, an already existing task can also be used to call `usb_host_lib_handle_events`.
- Mass Storage Class driver is installed by calling `usb_msc_install` function along side with configuration.
- Supplied configuration contains user provided callback function invoked whenever MSC device is connected/disconnected
and optional parameters for creating background task handling MSC related events.
Alternatively, user can call `usb_msc_handle_events` function from already existing task.
- After receiving `MSC_DEVICE_CONNECTED` event, user has to install device with `usb_msc_install_device` function,
obtaining MSC device handle.
- USB descriptors can be printed out with `usb_msc_print_descriptors` and general information about MSC device retrieved
with `from usb_msc_get_device_info` function.
- Obtained device handle is then used in helper function `usb_msc_vfs_register` mounting USB Disk to Virtual filesystem.
- At this point, standard C functions for accessing storage (`fopen`, `fwrite`, `fread`, `mkdir` etc.) can be carried out.
- In order to uninstall the whole USB stack, deinitializing counterparts to functions above has to be called in reverse order.
## Performance tuning
The following performance tuning options have significant impact on data throughput in USB HighSpeed implementations.
For original FullSpeed implementations, the effects are negligible.
- By default, Newlib (the implementation of C Standard Library) creates cache for each opened file
- The greater the cache, the better performance for the cost of RAM
- Size of the cache can be set with C STD library function `setvbuf()`
- Sizes over 16kB do not improve the performance any more
## Known issues
- Driver only supports flash drives using the BOT (Bulk-Only Transport) protocol and the Transparent SCSI command set
- For an example, refer to [msc_host_example](https://github.com/espressif/esp-idf/tree/master/examples/peripherals/usb/host/msc) in ESP-IDF
- Fix `msc_host_get_device_info` for devices without Serial Number string descriptor https://github.com/espressif/esp-idf/issues/12163
- Fix regression from version 1.1.0 that files could not be opened in PSRAM https://github.com/espressif/idf-extra-components/issues/202
- Fix MSC driver event handling without background task
## 1.1.0 - yanked
- Significantly increase performance with Virtual File System by allowing longer transfers
- Optimize used heap memory by reusing the Virtual File System buffer
- Optimize CPU usage by putting the background MSC task to 'Blocked' state indefinitely when there is nothing to do
- Fix MSC commands for devices on interface numbers other than zero
- Replace unsafe debug functions for direct access of MSC sectors with private SCSI commands
- Claim support for USB composite devices
- Increase transfer timeout to 5 seconds to handle slower flash disks
- Fix compatibility with IDF v4.4
- Initial version