On Fri, Jan 20, 2017 at 01:21:20PM -0800, Simon Ratner wrote: > On Fri, Jan 20, 2017 at 9:01 AM, Christopher Collins <ccoll...@apache.org> > wrote: > > On Thu, Jan 19, 2017 at 11:57:01PM -0800, Simon Ratner wrote: > > > When allocating mbufs for the payload, is there something I should do to > > > reserve enough leading space for the ACL header to make sure host doesn't > > > need to re-allocate it? > > > > Yes - you should use ble_hs_mbuf_att_pkt() to allocate all your mbufs. > > This function ensures the resulting mbuf has sufficient leading space > > for: > > * ACL data header > > * Basic L2CAP header > > * Four bytes for an ATT command > > > > And I assume the mbuf passed into gatt read handlers (ctxt->om) has been > allocated that way? I only need to worry about the mbufs I allocate myself, > such as for notify payload.
The mbufs handed up from the host are typically allocated by something else (e.g., for the combined host-controller, it is the controller which allocates them). If you pass these mbufs back to the host, there is a chance the host will need to allocate an additional buffer while prepending to the chain. The host stripped the headers from the front of the mbuf before handing it to the application, so there will be sufficient leading space to add them back. However, the problem is the variable length ATT command. Your outgoing command may require more "ATT bytes" than the one you received. Do you often pass these mbufs back to the host? I thought that would be an uncommon use case. Chris
