Libdivecomputer Android API discussion
Venkatesh Shukla
venkatesh.shukla.eee11 at itbhu.ac.in
Sun Jun 29 15:40:34 PDT 2014
On 06/27/2014 04:34 PM, Jef Driesen wrote:
> On 2014-06-25 14:22, Venkatesh Shukla wrote:
>> I have been working on bringing libdivecomputer on android.
>> I have started with support for ftdi-chipset based divecomputers. For
>> talking with ftdi based devices on android, I have implemented
>> serial_ftdi.c. . I have tried to implement all the the necessary
>> functions.
>> Though some of them are left out due to their unavailability in libftdi.
>>
>> These left out functions are
>>
>> 1. serial_set_break - This is used only in reefnet_sensus_pro
>> hanshaking. I do not know if it uses ftdi chipset. If it does, it
>> would
>> become important to implement this function. In the present
>> implementation,
>> it remains unusable.
>
> Reefnet uses the Prolific pl2303 chipset, so it's not really an issue.
> The Sensus Pro is a pretty outdated model too, so there will be very
> few people who still have one.
>
>> 2. serial_send_break - I could not find any usage of this function in
>> libdivecomputer.
>
> Correct. It's not used anywhere.
>
>> 3. serial_get_transmitted - Even though the received bytes in the
>> buffer
>> can be accessed, I couldn't find any way to get the transmitted
>> bytes. This
>> function is used internally for logging. Returns -1 as it is not
>> implemented.
>
> It doesn't return the number of transmitted bytes, but the number of
> bytes that are currently present in the serial port buffer. Nowadays,
> we always call tcdrain when writing data, so this value should always
> be zero. This used to be different. Anyway, it's not an important
> function and it might even get removed in the future. So an
> implementation that always return 0 would be fine.
>
>> Changes made for building libdivecomputer for android are
>>
>> 1. Check for presence of libftdi1
>> 2. serial_ftdi.c is compiled and used instead of serial_posix.c
>> 3. libftdi1 is included in Libs.private in libdivecomputer.pc
>>
>> All these changes can be viewed here
>> <https://github.com/venkateshshukla/libdivecomputer>. I have been
>> testing
>> it with universal script and subsurface-android. Results have been
>> encouraging.
>
> I have tested your code on Linux, with an OSTC3 (or was it an OSTC2,
> don't remember), and it worked fine. I didn't had time to review it
> properly. asa
Thank you. I hope to clean out the bugs as soon as possible and
integrate the android code to libdivecomputer
>
>> Now, toward providing a workable API for android, I have following
>> points
>> in mind. These were mentioned by Jef in an earlier mail.
>>
>> 1. Support for only ftdi devices are implemented for now. But support
>> for other chipsets need to be included later on. Hence,
>> serial_ftdi.c alone
>> is not enough. We need a serial_android.c which will redirect
>> calls to
>> respective chipset specific scripts.
>> 2. I have come to the conclusion that for android, it will be best
>> if we
>> pass the file descriptor of the USB device after acquiring the
>> necessary
>> permissions. This fd can be used to create a libusb_device and
>> subsequently
>> all USB related features can be accessed. For making a
>> libusb_device from
>> file descriptor obtained from android, I had tweaked libusb. This
>> tweaked
>> version is present here (
>> https://github.com/venkateshshukla/libusb ).
>> This libusb_device could be used to obtain VID and PID and then
>> control can
>> then be branched accordingly to ftdi, pl2303, cp210x and other
>> chipset
>> specific scripts on android.
>> 3. We will require a generalized mechanism for passing parameters.
>> This
>> is especially needed now as we have several branches - Normal TTY
>> usage (
>> posix/win32 , path to the device), android (file descriptor) and
>> Bluetooth
>> address.
>>
>> Please provide your thoughts on the above points. Also any other
>> thing that
>> I should keep in mind for android API?
>
> Originally, the solution I had in mind for the different transport
> types (serial, irda, usb and bluetooth), was to introduce a specific
> data structure for each transport type:
>
> typedef dc_params_serial_t {
> const char *devname;
> } dc_params_serial_t;
>
> typedef dc_params_usb_t {
> unsigned int vid;
> unsigned int pid;
> } dc_params_usb_t;
>
> typedef dc_params_bluetooth_t {
> unsigned char address[6];
> } dc_params_bluetooth_t;
>
> dc_status_t
> dc_device_open (..., dc_descriptor_t *descriptor, const void *params);
>
> The application would then have to pass the appropriate data structure
> to the dc_device_open function, depending on the transport type
> indicated by the device descriptor:
>
> dc_params_serial_t serial;
> dc_params_usb_t usb;
> dc_params_bluetooth_t bluetooth;
>
> switch (dc_descriptor_get_transport(descriptor)) {
> case DC_TRANSPORT_SERIAL:
> serial.devname = ...;
> dc_device_open(..., descriptor, &serial);
> break;
> case DC_TRANSPORT_USB:
> usb.vid = ...;
> usb.pid = ...;
> dc_device_open(..., descriptor, &usb);
> break;
> case DC_TRANSPORT_BLUETOOTH:
> bluetooth.address = ...;
> dc_device_open(..., descriptor, &bluetooth);
> break;
> }
>
I was thinking something like
typedef struct dc_addr_t {
dc_transport_t transport;
dc_param_t param;
} dc_addr_t;
dc_status_t
dc_device_open (dc_device_t **out, dc_context_t *context,
dc_descriptor_t *descriptor, const dc_addr_t *addr);
dc_addr_t *dev_addr = (dc_addr_t *) malloc (sizeof (dc_addr_t));
switch (dc_descriptor_get_transport(descriptor)) {
case DC_TRANSPORT_SERIAL:
dev_addr->transport = DC_TRANSPORT_SERIAL;
dev_addr->param.devname = "/path/to/tty";
break;
case DC_TRANSPORT_USB:
dev_addr->transport = DC_TRANSPORT_USB;
dev_addr->param.fd = XX; // File descriptor obtained from android
break;
case DC_TRANSPORT_BLUETOOTH:
dev_addr->transport = DC_TRANSPORT_USB
dev_addr->param.mac_addr = ...;
break;
}
dc_device_open(..., descriptor, dev_addr);
The case of USB above is for Android usage as explained below.
> Although this will certainly work, there are some drawbacks:
>
> Ideally libdivecomputer should provide some api to enumerate serial,
> usb and bluetooth devices. When the user chooses a device that uses
> serial communication, you want to be able to show a list with all
> serial ports. The same goes for usb and bluetooth communication. But
> for some transport types like irda sockets (and maybe also bluetooth
> sockets) you need to open a socket before you can enumerate the
> available devices. But as long as the backend code opens the socket
> internally, this functionality can't be exposed to the application.
> Currently the USB (Atomics Cobalt) and IrDA (Uwatec Smart) based
> backends do work around this problem by performing the discovery
> internally, and pick the device that looks like the dive computer (e.g
> based on its name, USB VID/PID, etc). But this heuristics isn't
> perfect and can certainly fail. For example when connecting two
> identical dive computers, or when some other usb-serial gadget with
> the same USB VID/PID (not unlikely with simple usb-serial cables using
> the stock ftdi/pl2303 VID/PID) is connected. The heuristics will have
> to decide which device to pick (e.g. the first one), but that might be
> the wrong choice. Yes, I'm aware this is a corner case, but not
> impossible.
>
> It's also not clear to me how this solution should deal with multiple
> implementations of a single transport. For Android, there'll be
> multiple implementations for serial communication, one for each
> chipset (ftdi, pl2303, cdc-acm, etc). But how do you decide which
> chipset to pick? The device backend doesn't always have this
> information. For example for Suunto devices,there are several cables
> on the market using different usb-serial chipsets. So there is no one
> to one mapping between devices and usb-serial chipsets.
>
For usage of usb divecomputers on Android, the question of enumeration
wouldn't necessarily arise. It is provided by android and would be used
in the pre-libdivecomputer-usage stage (see step 0 below) in order to
get the file descriptor which would then be passed on to
libdivecomputer. Also connecting multiple USB devices on Android would
not be a common phenomenon.
If still enumeration is needed, one could use libusb to get a list of
all usb devices ( libusb_get_device_list ). This is a compulsory initial
steps in any libusb related operations and is shown below (step 4).
The steps for usage of libdivecomputer on android as I have thought of are:
0. Android application lists the devices. User chooses his
divecomputer. The application asks for permission to use it. On
getting permission, the application acquires the file descriptor of
the Usb Device attached. It is then passed to libdivecomputer for
opening the device.
1. The application makes an appropriate addr and adds data to it.
Then calls dc_device_open with this addr.
dc_addr_t *addr = (dc_addr_t *) malloc(sizeof(dc_addr_t));
addr->transport = DC_TRANSPORT_USB;
addr->param.fd = usb_fd;
dc_device_open(&device, context, descriptor, addr);
2. The dc_device_open looks for the device specific opening code
(ex. hw_ostc3_open) and finally reaches serial_open of android.
3. serial_open checks if bluetooth is to be used or usb.
int
serial_open (serial_t **out, dc_context_t *context, const dc_addr_t
*addr)
{
if (addr->transport == DC_TRANSPORT_USB)
serial_open_usb (out, context, addr->param.fd;
else if (addr->transport == DC_TRANSPORT_BLUETOOTH)
serial_open_bt (out, context, addr->param.mac_addr;
else
return -1;
}
4. If transport type is usb, make a libusb_device with the file
descriptor.
int
serial_open_usb (serial_t **out, dc_context_t *context, int fd)
{
int i, n, r;
libusb_init(NULL);
libusb_device *dev, **devs;
struct libusb_device_descriptor desc;
n = libusb_get_device_list (NULL, &devs);
for (i = 0; i < n; i++) {
dev = devs[i];
libusb_get_device_descriptor(dev, &desc);
if (is_dev_recognized( desc.idVendor, desc.idProduct) {
switch (get_dc_chipset(desc_idVendor, desc.idProduct) {
case FTDI:
return serial_open_ftdi(out, context, dev);
case PL2303:
return serial_open_pl2303 (out, context, dev);
case CP210X:
return serial_open_cp210x (out, context, dev);
default:
return -1; // Chipset not yet supported
}
}
}
return -2; // No recognized usb device found
}
Checks have been ignored for brevity.
The multiple-interfaces question is resolved because we can extract the
vid pid of the chip directly. And this is used for choosing the chipset
specific function. There is no manufacturer information needed.
I might be wrong or missing something. Does using different cables with
different usb-serial chipset change the vid pid of the usb device?
> The next thing that comes to my mind is a solution where the
> application opens the appropriate "transport "itself, and passes an
> instance of an abstract transport object to the dc_device_open function:
>
> dc_serial_t *serial = NULL;
> dc_usb_t *usb = NULL;
> dc_bluetooth_socket_t *bluetooth = NULL;
>
> switch (dc_descriptor_get_transport(descriptor)) {
> case DC_TRANSPORT_SERIAL:
> #ifdef ANDROID
> dc_serial_{ftdi,pl2303,...}_open(&serial, fd);
> #else
> dc_serial_open(&serial, name);
> #endif
> dc_device_open(..., descriptor, serial);
> break;
> case DC_TRANSPORT_USB:
> dc_usb_open(&usb, vid, pid);
> dc_device_open(..., descriptor, usb);
> break;
> case DC_TRANSPORT_BLUETOOTH:
> dc_bluetooth_socket_open(&bluetooth, ...);
> dc_bluetooth_socket_discovery(bluetooth, ...);
> dc_bluetooth_socket_connect(bluetooth, address);
> dc_device_open(..., descriptor, bluetooth);
> break;
> }
>
> This would allow multiple implementations of a transport, as long as
> each of them implements the same interface, but that's easy. It also
> solves the discovery problem, because the low-level transport
> interface is now available to the application.
>
> But unfortunately it also introduces a new problem. Take a look at the
> USB transport. Now the application suddenly needs to know which USB
> VID/PID to use. Previously the device backend took care of that
> internally (with the caveat mentioned earlier). For example the Atomic
> Cobalt uses the VID/PID 0x0471/0x0888. Right now the application
> doesn't need to know this, because the backend opens the usb device
> that matches these number. The same thing happens for Uwatec Smart,
> where the backend discovers the IrDA devices in range, and connects to
> first one that matches the known device names. How do we deal with
> this here?
>
> All ideas are welcome!
>
> Jef
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