Function start 

pub unsafe fn start(ch: Channel, transfer: Transfer)

Program channel ch with transfer and start it.

The same three stores every SDK driver makes: address, size, a compiler-only barrier that publishes ordinary RAM stores made before the call (it emits no instruction), then control. The caller still enables the channel (enable_channel) and waits for completion (wait_done).

Safety

From the control store until the channel goes idle (wait_done returns true, or abort stops it), the DMA controller reads or writes RAM with no regard for Rust’s borrow rules. The caller must guarantee that for that whole window:

• every word the transfer can write (a device-to-RAM channel, such as MDEC-out, CD-ROM, GPU readback or the ordering-table clear) lies in memory the caller owns exclusively, with no live Rust reference to it;
• every word the transfer can read (a RAM-to-device channel, or each node of a linked list and the nodes its tags link to) is live, initialised and not written by anyone else;
• the extent is the one size and the sync mode describe, remembering that a zero block count means 65,536 blocks;
• the channel is idle when this is called (silicon ignores a control write to a busy channel, so the old transfer would keep running).