HID report map data size limit?

[Matthew K]

Hi guys. May I ask some questions for ble_app_hids_keyboard_pca10056_s113 project from 17.1.0 nRF5 SDK:

Sadly a similar ticket was there but there were no clear answer:  Is there a limit to HID report map data?

My setup is NRF52840-QIAA with s113, 3 BLE services (DIS, BAS, HID similar to the original keyboard example).

Thanks to HID keyboard + Mouse combo answers  BLE HID Mouse and BLE HID keyboard integration in the same program with nrf52 dk. Need urgent help

I also tested this and the report map data was added successfully.

So now I'm trying to add consumer control as well.

https://github.com/Rallare/nrf51_ble_app_hids_kbd_consumercontrol/blob/eb9253685fe097f9b0c88212bb51c5e5ff3f7e08/main.c#L220

May I ask what's the size limit of report_map_data[] array? Will this be 251, similar to LE data length extension feature?

Or can this size be indefinite depending on how many BLE services and characteristics are supported?

[Kenneth]

Hello,

The report map is limited to 255bytes yes, see here for why:
RE: [nRF52833] GATT Read of HID Report Map Char exceeding ATT MTU

Kenneth

[Matthew K]

Thank you, Kenneth.

"We see a failure mode where the HID enumeration fails because the GATT Read fails (due to a malformed packet exceeding MTU error). "

Then how about

[1] can I set the ATT_MTU size higher, like 510 so that I can set the report_map_data[] array size to 500?

or

[2] Have two HID services?

HID service 1 has keyboard

HID service 2 has mouse + consumer control

[Kenneth]

Matthew K said:

Doesn't that mean the report map can go upto 2^16 - 1 = 65535 bytes in total?

A good point, it may be that it's a not a limit anymore.

Matthew K said:

I have another nRF52840-DK to work as the nRF Sniffer v4.1.1

but I don't know what filters to use to track my nRF52 device.

I assume you have performed sniffing of the connection you want first by using:
https://infocenter.nordicsemi.com/topic/ug_sniffer_ble/UG/sniffer_ble/action_connection.html 

After performing sniffing you can find display filters here:
https://infocenter.nordicsemi.com/topic/ug_sniffer_ble/UG/sniffer_ble/inspecting_data.html 

A filter I use a lot: btle.data_header.llid > 0 && btle.length > 0

Best regards,
Kenneth

[Matthew K]

Thank you Kenneth. Although this report_map_data[] array size of 266 > 255 is working,

let me use the nRF sniffer v4.1.1 to verify the MTU fragmentation. Back to my questions,

first of all, what's the max MTU size I can use while using NRF52840-QIAA with s113?

This should be 512 bytes, right?

Matthew K said:

[1] can I set the ATT_MTU size higher, like 510 so that I can set the report_map_data[] array size to 500?

So this means the max report_map_data[] array size is 509 for Bluetooth 5, right?

Matthew K said:

[2] Have two HID services?

HID service 1 has keyboard

HID service 2 has mouse + consumer control

If the report_map_data[] size exceeds 509, I should split this into several HID services, right?

[Kenneth]

Matthew K said:

If the report_map_data[] size exceeds 509, I should split this into several HID services, right?

That is my expectations yes.

Kenneth

[Matthew K]

Thank you Kenneth.

Matthew K said:

first of all, what's the max MTU size I can use while using NRF52840-QIAA with s113?

Just to double check, is this true? Thank you for your help

[Kenneth]

Up to 512 should be possible yes.

Kenneth

[Matthew K]

Hello Kenneth.

Kenneth said:

may be fragmented over several att mtu packet

When using the nRF sniffer v4.1.1, what Wireshark filter should I use to detect this?

Thank you for your help!

[Kenneth]

Possible btatt? Ref:
https://infocenter.nordicsemi.com/topic/ug_sniffer_ble/UG/sniffer_ble/inspecting_data.html 

Typically in wireshark if you click on a packet, it will automatically show using arrows to the left of the line other packets that are part of the procedure etc.

Kenneth