That is a good question. I should let Chris answer this one as he knows for sure. I suspect you will have a chain of mbufs but I would have to look over the code to be sure.
> On Jan 19, 2017, at 3:46 AM, Simon Ratner <si...@proxy.co> wrote: > > Hi Will, > > A related question: how does this map to large ATT_MTU and fragmented > packets at the L2CAP level (assuming no data length extension)? Does each > fragment get its own mbuf, which are then chained together, or does the > entire packet get reassembled into a single mbuf if there is room? > > > > On Wed, Jan 11, 2017 at 4:57 PM, will sanfilippo <wi...@runtime.io> wrote: > >> Yes; 76 or 80. Note that I have not actually tested with 80 byte mbuf >> blocks. That is the theory though :-) >> >>> On Jan 11, 2017, at 4:31 PM, Simon Ratner <si...@proxy.co> wrote: >>> >>> Got it; by minimum size you mean the 76/80 bytes? >>> >>> On Wed, Jan 11, 2017 at 4:17 PM, will sanfilippo <wi...@runtime.io> >> wrote: >>> >>>> Well, yes, there are “definitions” for these things. They are in various >>>> places but they are there. Using them might get a bit tricky as you have >>>> mentioned; not sure. You would have to make sure the right header files >> get >>>> included in the proper places... >>>> >>>> Anyway, here are the definitions: >>>> os mbuf header: sizeof(struct os_mbuf). Size = 16 >>>> os mbuf packet header: sizeof(struct os_mbuf_pkthdr) Size = 8 >>>> user header: sizeof(struct ble_mbuf_hdr) Size = 8 or 12 >>>> The HCI ACL data header: BLE_HCI_DATA_HSDR_SZ. 4 bytes >>>> The LL PDU header: BLE_LL_PDU_HDR_LEN. 2 bytes >>>> >>>> I would always make the size a multiple of 4 but the code should do that >>>> for you; I just like to do it so the size you see in the syscfg >> variable is >>>> the actual memory block size. >>>> >>>> Another thing I should mention: you should never add a buffer pool to >> msys >>>> smaller than the minimum size I mentioned if you are using the >> controller. >>>> This is something we will address in the future but for now it would be >>>> bad. :-) >>>> >>>> >>>> >>>>> On Jan 11, 2017, at 3:49 PM, Simon Ratner <si...@proxy.co> wrote: >>>>> >>>>> Thanks for the detailed write-up, Will - very useful. >>>>> >>>>> Are there defines for these things? >>>>> Ideally, if I want a payload size of N, I'd like to specify in >>>> syscfg.yml: >>>>> >>>>> MSYS_1_BLOCK_SIZE: '(N + MBUF_HEADER + PKT_HEADER + LL_OVERHEAD + >>>> ...)' >>>>> >>>>> And magically have optimally-sized buffers. >>>>> >>>>> >>>>> On Wed, Jan 11, 2017 at 11:00 AM, will sanfilippo <wi...@runtime.io> >>>> wrote: >>>>> >>>>>> Hello: >>>>>> >>>>>> Since this has come up on a number of different occasions I wanted to >>>> send >>>>>> out an email which discusses how the nimble stack uses mbufs. This >> will >>>> be >>>>>> a controller-centric discussion but the concepts apply to the host as >>>> well. >>>>>> >>>>>> A quick refresher on mbufs: Mynewt, and the nimble stack, use mbufs >> for >>>>>> networking stack packet data. A “packet” is simply a chain of mbufs >> with >>>>>> the first mbuf in the chain being a packet header mbuf and all others >>>> being >>>>>> “normal” mbufs. A packet header mbuf contains a mbuf header, a packet >>>>>> header and an optional user-defined header. >>>>>> >>>>>> The length of the packet (i.e. all the data contained in all the mbuf >>>>>> chains) is stored in the packet header. Each individual mbuf in the >>>> chain >>>>>> also contains a length which is the length of the data in that mbuf. >> The >>>>>> sum of all the mbuf data lengths = length of packet. >>>>>> >>>>>> The amount of overhead in an mbuf and its size determine the amount of >>>>>> data that can be carried in a mbuf. All mbufs have a 16-byte mbuf >>>> header. >>>>>> Packet header mbufs have an additional 8 bytes for the packet header >>>>>> structure and an optional user-data header. The nimble stack uses >>>> either an >>>>>> 8-byte or 12-byte user data header. If you turn on multi-advertising >>>>>> support, the user header is 12 bytes; otherwise it is 8 bytes. This >>>> means >>>>>> the total packet header mbuf overhead is 32 or 36 bytes. >>>>>> >>>>>> The total mbuf size is defined by the various MSYS_X_BLOCK_SIZE syscfg >>>>>> variables. Currently, there is one mbuf pool added to msys (MSYS_1) >>>> with a >>>>>> block size of 292 bytes. >>>>>> >>>>>> Controller constraints: >>>>>> The controller assumes that a certain minimum data size is available >> in >>>> a >>>>>> packet header mbuf. This size is equal to the largest advertising PDU, >>>> or >>>>>> 37 bytes, and must also contain the 2-byte LL PDU header (for a total >>>> of 39 >>>>>> bytes). Additionally, the controller requires an additional 4 bytes at >>>> the >>>>>> start of the packet header mbuf to prepend the HCI ACL data packet >>>> header. >>>>>> This means that the minimum mbuf size that can be allocated in any >> msys >>>>>> mbuf pool is: packet header overhead + 4 + 39 = 75 (79 for multi-adv). >>>>>> Since memory pools are always rounded up to the nearest 4 byte >> boundary, >>>>>> this means that the minimum size should be 76 (or 80) bytes. >>>>>> >>>>>> For most applications that dont use large packets, setting the mbuf >> size >>>>>> to 80 should be fine as this will accommodate the typical BLE PDU and >>>> also >>>>>> meets the minimum requirement. If your application generally uses >> larger >>>>>> packets it might be benefical to allocate large mbufs as you dont lose >>>> the >>>>>> 16-byte overhead per mbuf. >>>>>> >>>>>> Finally, here is an example of how many mbufs will be used by the >>>>>> controller for received packets. This assumes multi-advertising >> enabled >>>> (36 >>>>>> byte packet header overhead). >>>>>> >>>>>> Example 1: PDU length = 251, msys_1_block_size = 80 >>>>>> >>>>>> Controller needs to store 251 + 2 = 253 total bytes. >>>>>> >>>>>> Packet header mbuf can hold 80 - 36 - 4 bytes, or 40 bytes. >>>>>> Each additional mbuf can hold 80 - 16 bytes, or 64 bytes. >>>>>> Total mbufs = 5. First mbuf holds 40 bytes, the next three hold 64 >> bytes >>>>>> while the final mbuf holds 21 bytes (40 + 64*3 + 21 = 253). >>>>>> >>>>>> Example 2: PDU length = 251, msys_1_block_size = 112 >>>>>> Total mbufs: 3 (72 + 96 + 85) >>>>>> >>>>>> Hope this helps. >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>> >>>> >> >>
