On Mon, Jun 19, 2023 at 2:09 PM Xiaofan Chen <xiaofanc@xxxxxxxxx> wrote: > > I know that thurrent documentation has been there since it was created by > Alan Ott many years ago. And he started the HIDAPI project around that > time as well. However, I am starting to doubt whether it is correct or not > based on the testing using HIDAPI. > > Please help to clarify. Thanks. > > https://docs.kernel.org/hid/hidraw.html > +++++++++++++++++++++++++++++++++++++++++++++++++++++ > HIDIOCGFEATURE(len): > > Get a Feature Report > > This ioctl will request a feature report from the device using the > control endpoint. The first byte of the supplied buffer should be > set to the report number of the requested report. For devices > which do not use numbered reports, set the first byte to 0. The > returned report buffer will contain the report number in the first > byte, followed by the report data read from the device. For devices > which do not use numbered reports, the report data will begin at the > first byte of the returned buffer. > ++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > I have doubts about the last sentence. It seems to me that for > devices which do not use numbered reports, the first byte will > be 0 and the report data begins in the second byte. > > This is based on testing results using hidapi and hidapitester, which > use the ioctl. > int HID_API_EXPORT hid_get_feature_report(hid_device *dev, unsigned > char *data, size_t length) > { > int res; > > register_device_error(dev, NULL); > > res = ioctl(dev->device_handle, HIDIOCGFEATURE(length), data); > if (res < 0) > register_device_error_format(dev, "ioctl (GFEATURE): %s", > strerror(errno)); > > return res; > } > > Reference discussion: > https://github.com/libusb/hidapi/issues/589 > > Test device is Jan Axelson's generic HID example which I have tested using > libusb and hidapi across platforms as well as using Windows HID API. > So I believe the FW is good. > http://janaxelson.com/hidpage.htm#MyExampleCode (USB PIC MCU) > Modified testing code from the following Linux kernel samples/hidraw/hid-example.c https://github.com/libusb/hidapi/issues/589#issuecomment-1597356054 Results are shown here. We can clearly see that the "Fake Report ID" 0 is in the first byte of the returned buffer, matching the output from hidapi/hidapitester, mcuee@UbuntuSwift3:~/build/hid/hidraw$ gcc -o myhid-example myhid-example.c mcuee@UbuntuSwift3:~/build/hid/hidraw$ sudo ./myhid-example Report Descriptor Size: 47 Report Descriptor: 6 a0 ff 9 1 a1 1 9 3 15 0 26 ff 0 75 8 95 3f 81 2 9 4 15 0 26 ff 0 75 8 95 3f 91 2 9 5 15 0 26 ff 0 75 8 95 3f b1 2 c0 Raw Name: Microchip Technology Inc. Generic HID Raw Phys: usb-0000:00:14.0-1/input0 Raw Info: bustype: 3 (USB) vendor: 0x0925 product: 0x7001 ioctl HIDIOCSFEATURE returned: 64 ioctl HIDIOCGFEATURE returned: 64 Report data: 0 41 42 43 44 45 46 47 48 14 4 18 4 4d 72 31 50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f 60 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 78 79 7a 7b 7c 7d 7e 7f write() wrote 64 bytes read() read 63 bytes: 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33 34 35 36 37 38 39 3a 3b 3c 3d 3e 3f 40 41 42 43 44 45 46 47 48 49 4a 4b 4c 4d 4e 4f 50 51 52 53 54 55 56 57 58 59 5a 5b 5c 5d 5e 5f -- Xiaofan
