[jira] [Created] (MYNEWT-555) BLE Host - Avoid mbuf copy on chr read

["Christopher Collins (JIRA)" <ji...@apache.org>]

Christopher Collins created MYNEWT-555:
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             Summary: BLE Host - Avoid mbuf copy on chr read
                 Key: MYNEWT-555
                 URL: https://issues.apache.org/jira/browse/MYNEWT-555
             Project: Mynewt
          Issue Type: Bug
          Components: Nimble
            Reporter: Christopher Collins
            Assignee: Christopher Collins
             Fix For: v1_0_0_rel


When the host receives any form of characteristic read request, an extraneous mbuf gets allocated.  The sequence is:

* Allocate new mbuf.
* Pass new mbuf to application code so app can fill in characteristic value.
* Copy characteristic value out of new mbuf into response mbuf
* Free new mbuf

The reason for the extra allocation is to simplify reads that specify non-zero offsets. In the case of a GATT read, the peer may read from different offsets of the characteristic. In an effort to simplify the API, I decided the requested offset should be hidden from the application.  Instead, the application always provides the full characteristic value, and the host copies the requested portion out of the user-filled mbuf and into the ACL data packet.

The fix is to try to reuse the response to hold the characteristic data.  This can be done if the read request specifies an offset of 0.  For non-zero offsets, the old behavior remains: allocate an extra mbuf and copy the data.

Additional context: https://lists.apache.org/thread.html/78358745716cac9512b3c659dfe9b9b7bbb1354ee4c38d6da137bcc0@%3Cdev.mynewt.apache.org%3E



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