On 13/09/18 10:00, Geert Uytterhoeven wrote: [...]
The main issue is that if of_dma_find_controller() fails, a DMA slave driver cannot distinguish between dmac not yet probed successfully, and dmac driver not present.
...which is in fact the exact same problem that the IOMMU code has - might it make sense to give of_dma_request_slave_channel() similar (optional?) driver_deferred_probe_check_state() logic, i.e. "if my DMAC driver's not shown up by this point, assume it's not built-in and go on without it". Of course it is somewhat easier for IOMMU drivers as there's zero chance of those popping up as modules later on.
However, as we rely more and more on probe deferral, this will cause more issues in the future. I guess the proper solution is to take into account dependencies described in DT before probing devices, i.e. devices with phandles should always be probed after the targets of these phandles? Complication: - Circular dependencies, - Optional phandle targets (dmacs, oops).
Yeah, a proper dependency graph would be ideal, but it's certainly not trivial. There was an interesting proposal a couple of years ago of a half-way approach where various phandle-chasing routines could actively try to probe the target, but I think that got stymied on the fact that a single OF node may represent multiple different driver-level providers, so it's tricky to tell whether the *right* dependency has been satisfied without detailed knowledge of the individual bindings and driver implementations. That's probably still an issue for other approaches too, sadly.
Robin.
