CONFIG_BT_MESH_DFD_SRV_TARGETS_MAX questions

[AndyM]

Hello

I've been experimenting in increasing the number of targets that a mesh FOTA distribution server can support on nRF52840

The CONFIG_BT_MESH_DFD_SRV_TARGETS_MAX has a maximum  65535

But for practical purposes I'd be happy if I could have at least 50-60.

Ran into build asserts in zephyr\subsys\bluetooth\mesh\dfd_srv.c

"The Firmware Distribution Receivers List message does not fit into the maximum " "outgoing SDU size"

The comparison is done to the  BT_MESH_TX_SDU_MAX which is built as follows

#define BT_MESH_TX_SDU_MAX        MAX((BT_MESH_TX_SEG_MAX *     \

                       BT_MESH_APP_SEG_SDU_MAX),    \

                      BT_MESH_APP_UNSEG_SDU_MAX)

CONFIG_BT_MESH_TX_SEG_MAX is limited to 32

BT_MESH_APP_SEG_SDU_MAX is defined as 12

BT_MESH_APP_UNSEG_SDU_MAX is defined as 15

By trial and error I was able to set the number of targets to 39 with these configurations

ONFIG_BT_MESH_MSG_CACHE_SIZE=256

CONFIG_BT_MESH_ADV_BUF_COUNT=128

CONFIG_BT_MESH_TX_SEG_MAX=32

CONFIG_BT_MESH_RX_SEG_MAX=32

CONFIG_BT_MESH_TX_SEG_MSG_COUNT=64

CONFIG_BT_MESH_RX_SEG_MSG_COUNT=64

CONFIG_BT_MESH_DFD_SRV_TARGETS_MAX=39

Is this it? Or is there another way to increase the number of supported targets?

Thank you

 

[AndyM]

Update

Experimented with CONFIG_BT_MESH_TX_SEG_MAX and CONFIG_BT_MESH_DFD_SRV_TARGETS_MAX
Was able to increase CONFIG_BT_MESH_DFD_SRV_TARGETS_MAX to 40 with CONFIG_BT_MESH_TX_SEG_MAX set to 20

However, new problem
Image transfer fails with status 3 (internal error)

Distributor log shows multiple messages " <wrn> bt_mesh_transport: SDU canceled"

which come from zephyr\subsys\bluetooth\mesh\transport.c 

ack = net_buf_simple_pull_be32(buf);

If the ack is not in the buffer this message is printed

More experimentation shows that any  CONFIG_BT_MESH_TX_SEG_MAX value greater than 10 causes "SDU canceled" message and a failure to transfer the image

Which means that the CONFIG_BT_MESH_DFD_SRV_TARGETS_MAX can only be 22 - any greater value requires CONFIG_BT_MESH_TX_SEG_MAX  to be increased otherwise there is a build assert  described above. 

 

Any ideas?

Thanks