Long write only writes 4 packets

RE: Long write only writes 4 packets

nejcokle — Wed, 09 Mar 2016 15:47:35 GMT

There, now it works perfectly! I handle BLE_GATT_STATUS_ATTERR_PREPARE_QUEUE_FULL by sending execute like you said, and then continue sending packets. Thanks for your help. Best regards, Nejc

RE: Long write only writes 4 packets

Ulrich Myhre — Wed, 09 Mar 2016 14:45:48 GMT

Okay, then we're a bit closer at least. You need to verify that the buffer has enough space in it, and that you report this correctly in the memory request. Alternatively, you can send an execute when you get a BLE_GATT_STATUS_ATTERR_PREPARE_QUEUE_FULL, then continue sending more packets from where you left off.

RE: Long write only writes 4 packets

nejcokle — Wed, 09 Mar 2016 14:35:41 GMT

Ah yes, i was only watching of my event handle returned NRF_SUCCESS, not the status. I thought that since i get right write event, i don't have gatt_status errors. But yeah, you were right all along, sorry we had to go such a long way :).

After debugging i found out that i DO get "BLE_GATT_STATUS_ATTERR_PREPARE_QUEUE_FULL" after 4 packets.

RE: Long write only writes 4 packets

Ulrich Myhre — Wed, 09 Mar 2016 13:27:32 GMT

I see that I have misunderstood you a bit then. Yes, it is possible to use e.g. authorization to receive data written to a characteristic, and then manually writing it to an offset with sd_ble_gatts_value_set, but you would need to keep track of the offset and make sure the data goes to the correct place.

Let's go back to the original issue at hand. Can you confirm that you send a prepare write request, the sd_ble_gattc_write call returns NRF_SUCCESS, and that you receive a BLE_GATTC_EVT_WRITE_RSP for each packet you send? And the evt->gattc_evt.gatt_status of all those events are BLE_GATT_STATUS_SUCCESS?

RE: Long write only writes 4 packets

nejcokle — Wed, 09 Mar 2016 12:43:36 GMT

Yes i normally used "BLE_GATT_OP_WRITE_REQ", but wanted to implement Long Writes. Since i can't use offsets with "BLE_GATT_OP_WRITE_REQ", do i have to handle char value update on my own? I mean, do i need to just send packets with my data, and when it's all through merge packets with offsets by 20 and manually update my characteristic value?

RE: Long write only writes 4 packets

Ulrich Myhre — Wed, 09 Mar 2016 11:50:46 GMT

You can use BLE_GATT_OP_WRITE_REQ or BLE_GATT_OP_WRITE_CMD. Write requests are limited the same way as prepare write requests, in that you have to wait for a response before you can send the next one. But the response will return with important error codes, like if the characteristic needs security. Write commands can be queued, and depending on your role and the SoftDevice, you can send up to 6 of them per connection interval. So they are much faster than other writes.

RE: Long write only writes 4 packets

nejcokle — Tue, 08 Mar 2016 17:29:10 GMT

Yes, i've seen that table. I made a buffer that's 200 bytes big, and i want to send around 90 - 100 bytes of data, so the buffer should be big enough. Hmm, never thought about that, but i don't really need to write multiple characteristics at once.. Which command do i use then if i just want to update one characteristic, and write in it with offsets? Which parameter should i use instead of "BLE_GATT_OP_PREP_WRITE_REQ"?

Yeah it's really strange there are no errors... I've been working on this problem for a few days now...

RE: Long write only writes 4 packets

Ulrich Myhre — Tue, 08 Mar 2016 16:19:16 GMT

The buffer needs to be big enough to contain all the data packets and their handles. See this table to see the layout of each packet. This means that you will have to account for 6 bytes of overhead per Prepare Write Request.

That is what long writes (also called reliable writes) are about. You queue up a lot of write requests, then write them all at once. If you do not need this functionality, it is better to just send write commands with an offset. The upside of the feature is that you can choose to write to multiple characteristics at the same time, or to keep data consistent at all times.

It is fairly strange that the write response does not generate any error though.

RE: Long write only writes 4 packets

nejcokle — Tue, 08 Mar 2016 15:43:10 GMT

ah sorry, i accidentally marked your comment as answer.. No the sender doesn't get any errors. I write to reciever and then wait for BLE_GATTC_EVT_WRITE_RSP. Also, the buffer provided for longwrite is a bit bigger than the size of data i want to send.

RE: Long write only writes 4 packets

Ulrich Myhre — Tue, 08 Mar 2016 15:07:48 GMT

Also, it is the sender that will see any errors. When you send a prepare write request, you get a BLE_GATTC_EVT_WRITE_RSP. All the GATTC events have a "gatt_status" flag that contains any error reported by the peer side. In this case, the peer knows that the queue is full, and responds autonomously.

RE: Long write only writes 4 packets

Ulrich Myhre — Tue, 08 Mar 2016 15:03:48 GMT

Okay, so the receiving side is a Nordic SoftDevice. If you look at the MSCs, you see that you initially get a memory request from the SoftDevice. You reply here with a pointer to a memory block. The size of this block decides how many prepare writes you can store until an execute is needed.

So you need to increase QUEUED_WRITE_BUFFER_SIZE to fit more into the buffer.

RE: Long write only writes 4 packets

nejcokle — Tue, 08 Mar 2016 15:01:48 GMT

When i monitor my terminal, i get:

MEM REQUEST
BLE_GATTS_EVT_WRITE
USER_MEM_RELEASE

i do not get any errors

RE: Long write only writes 4 packets

nejcokle — Tue, 08 Mar 2016 14:58:25 GMT

On a receiving side i handle my long write request like that:

case BLE_GATTS_EVT_WRITE:
  LOG_INFO("BLE_GATTS_EVT_WRITE");
break;
case BLE_EVT_USER_MEM_REQUEST:
LOG_INFO("MEM REQUEST");
							
mem_block.len = QUEUED_WRITE_BUFFER_SIZE;
mem_block.p_mem = &queued_write_buffer[0];
							
error_code = sd_ble_user_mem_reply(m_conn_handle, &mem_block);
if (error_code != NRF_SUCCESS) {
LOG_INFO("ERROR sd_ble_user_mem_reply: %u\r\n", (unsigned int)error_code);
} else {
  LOG_INFO("USER_MEM_REQUEST OK\r\n");
}
break;
case BLE_EVT_USER_MEM_RELEASE:
if ((p_ble_evt->evt.common_evt.params.user_mem_release.mem_block.p_mem == mem_block.p_mem)&&(p_ble_evt->evt.common_evt.params.user_mem_release.mem_block.len == mem_block.len))
{
LOG_INFO("USER_MEM_RELEASE\r\n");
}

RE: Long write only writes 4 packets

Ulrich Myhre — Tue, 08 Mar 2016 13:44:57 GMT

What is the receiving side? It could have a limited queue, and the GATTS_STATUS in the prepare write response event would contain an error code if that is the case. Likely "Prepare Queue Full".