Yann/feature/ble/allow larger packet reception #1356
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ladislas
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May 10, 2024
| @@ -15,6 +15,7 @@ | |||
| "bluenrg_ms.SPI_nCS": "BLE_SPI_NSS", | |||
| "bluenrg_ms.SPI_SCK": "BLE_SPI_SCK", | |||
| "cordio.desired-att-mtu": 251, | |||
| "cordio.max-prepared-writes": 1, | |||
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@YannLocatelli can you explain why the default value of 4 was not working and why the value of 1 is better?
my understanding is that a lower value might impact the efficiency of operations requiring multiple attribute writes.
is 1 used to avoid more than one thing happening at once? and to make sure each command are processed one at a time?
can you explain the issue you were encountering?
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I talked about it few months ago, with value of 4 it works but we have to handle it differently.
You receive 4 packets at the same time in the robot and since you write directly in the file the data you received, you have to handle writing + ordering at the same time
I tried some solutions but never succeed, if you have any idea, let me know.
config/mbed_app.json
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| @@ -15,6 +15,7 @@ | |||
| "bluenrg_ms.SPI_nCS": "BLE_SPI_NSS", | |||
| "bluenrg_ms.SPI_SCK": "BLE_SPI_SCK", | |||
| "cordio.desired-att-mtu": 251, | |||
| "cordio.rx-acl-buffer-size": 189, | |||
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same here, it's not clear to me why the default value 70 was not working and why the value 189 is the one chosen.
my understanding (but I might be wrong) is that the value must be bigger than the desired-att-mtu size.
what would happen with a value of 256?
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Never said it does not work
It is a size for a "packet" and it was downed to 189 to fit with iPad max buffer size (modulo some header or wrapper related to BLE protocol probably)
As you suggest with ChatGPT, value of 259 works
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This setting determines how many prepare write requests can be queued before they are executed. Each prepare write request can contain a segment of the data to be written to an attribute. When cordio.max-prepared-writes is set to 1, it means your system can only hold one prepare write request at a time. This setting minimizes memory usage but can slow down the process if the device needs to wait for each segment to be confirmed before sending the next. Default value is 4 With the default value, we were often receiving 4 packets at the same time. For DFU, this meant adding more logic to handle the 4 packets correctly which we deemed too much work for now. Setting the value to 1 prevents mutliple packets from being received at once and breaking the DFU process.
This parameter configures the size of the buffer used for receiving ACL (Asynchronous Connection-Less) data packets. The size of this buffer should be large enough to accommodate the largest expected incoming data payload. Default is 70 In our case we wanted the maximum size of 251 bytes for the MTU. Here’s how to calculate an appropriate ACL buffer size: MTU Consideration: The ATT MTU size of 251 bytes means that the maximum size of data that can be sent in one ATT packet is 251 bytes. L2CAP Header: The L2CAP protocol uses a 4-byte header on each packet. HCI Header: When data is transmitted over the HCI interface, there is also an HCI header involved, typically adding an additional 4 bytes (for ACL-U packets). Given these, the total size for each ACL packet when transmitting maximum sized ATT packets would be: Total ACL packet size = ATT MTU + L2CAP Header + HCI Header So in our case: Total ACL packet size = 251 bytes (ATT MTU) + 4 bytes (L2CAP Header) + 4 bytes (HCI Header) = 259
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