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SubjectRe: [PATCH V4] dma: add channel request API that supports deferred probe
On Tue, Nov 26, 2013 at 10:04:22AM -0700, Stephen Warren wrote:
> From: Stephen Warren <>
> dma_request_slave_channel() simply returns NULL whenever DMA channel
> lookup fails. Lookup could fail for two distinct reasons:
> a) No DMA specification exists for the channel name.
> This includes situations where no DMA specifications exist at all, or
> other general lookup problems.
> b) A DMA specification does exist, yet the driver for that channel is not
> yet registered.
> Case (b) should trigger deferred probe in client drivers. However, since
> they have no way to differentiate the two situations, it cannot.
> Implement new function dma_request_slave_channel_reason(), which performs
> identically to dma_request_slave_channel(), except that it returns an
> error-pointer rather than NULL, which allows callers to detect when
> deferred probe should occur.
> Eventually, all drivers should be converted to this new API, the old API
> removed, and the new API renamed to the more desirable name. This patch
> doesn't convert the existing API and all drivers in one go, since some
> drivers call dma_request_slave_channel() then dma_request_channel() if
> that fails. That would require either modifying dma_request_channel() in
> the same way, or adding extra error-handling code to all affected
> drivers, and there are close to 100 drivers using the other API, rather
> than just the 15-20 or so that use dma_request_slave_channel(), which
> might be tenable in a single patch.
> acpi_dma_request_slave_chan_by_name() doesn't currently implement
> deferred probe. It should, but this will be addressed later.
> Acked-by: Dan Williams <>
> Signed-off-by: Stephen Warren <>
Applied, to topic/defer_probe, this wont be rebased so feel free to merge


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