[GitHub] [mynewt-nimble] haukepetersen opened a new pull request #477: npl/riot: reduce default MSYS_1_BLOCK_SIZE to minimum

[GitBox <gi...@apache.org>]

haukepetersen opened a new pull request #477: npl/riot: reduce default MSYS_1_BLOCK_SIZE to minimum
URL: https://github.com/apache/mynewt-nimble/pull/477
 
 
   The default configuration of `MYNEWT_VAL_MSYS_1_BLOCK_SIZE` for the RIOT port uses a pretty large value, preventing NimBLE to run with this configuration on `nrf51`-based boards. Also, I can't quite explain, why the value of `292` is actually chosen, it seems quite random?!
   
   So with this PR I want to reduce the default `MSYS_1_BLOCK_SIZE` to the minimum possible for the typical GATT server/client and (non-extended) advertising scenarios. The idea is, to have the minimum value as default, and set it to something larger from inside the RIOT configuration if needed (e.g. for IP-over-BLE).
   
    As I see it, this should be `70` bytes:
   - 24 bytes `mbuf` overhead (`sizeof(struct os_mbuf) + sizeof(struct os_mbuf_pkthdr)`)
   - 46 bytes BLE packet size (4 access address, 2 ll-header, 37 PDU, 3 CRC) -> BT4.0, without data packet length extension
   
   I gave this setting a quick try and the RIOT-based GATT server example works as expected. BUT I can't really tell this is a coincidence or if my calculations/assumptions are actually correct. So maybe someone could shed some light on the following questions:
   - would `70` bytes work?
   - how is `MYNEWT_VAL_BLE_ATT_PREFERRED_MTU` related and does it have any impact? It seems not be used anywhere in the code...
   - could we also restrict `MYNEWT_VAL_MSYS_1_BLOCK_COUNT` do some lower default value?

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