bd: Add sync function to the block device API

[geky]

Right now, all block devices must effectively sync data on read/program, or else the power resilience gauruntees of littlefs don't hold true.

This is fine with the current state of the block device, which just write through any buffers provided (since external flash is much cheaper than internal caching and speed is a secondary concern to code footprint). However, in order to provide a decent FTL a syncing mechanic is necessary to ensure storage is up to date with in-flight data.

class BlockDevice {
    /** Ensure data on storage is in sync with the driver
     *
     *  @return         0 on success or a negative error code on failure
     */
    virtual int sync();
}

cc @deepikabhavnani

[deepikabhavnani]

This is fine with the current state of the block device, which just write through any buffers provided

Since (at present) all the block devices write through buffers, sync essentially will be empty call, but what will be expected in future from sync call and what type of block device will need it?
Can we have some default implementation of sync ?

[cmonr]

FTL?

If I understand correctly, all of the block devices that we have don't require a sync, as they're currently implemented, but this is adding the function for when a block devices does need one, correct?

[geky]

@cmonr is correct. @deepikabhavnani I'm not exactly sure what else to add since the actual sync implementation is dependent on the block device.

One example: Has a low-priority thread for writing out data, sync could escalate that thread to the current priority and block until all data has been written out.

[deepikabhavnani]

👍

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 996
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/5926/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 687
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/5926/

[mbed-ci]

Test : SUCCESS

Build number : 817
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/5926/817