On Thu, Feb 27, 2020 at 4:49 PM Li Yang [off-list ref] wrote:

On Thu, Feb 27, 2020 at 4:35 PM Rob Herring [off-list ref] wrote:

quoted

On Fri, Feb 21, 2020 at 11:48 AM Michael Walle [off-list ref] wrote:

quoted

The DMA channel might not be available at the first probe time. This is
esp. the case if the DMA controller has an IOMMU mapping.

Use the new dma_request_chan() API and handle EPROBE_DEFER errors. Also
reorder the code a bit, so that we don't prepare the whole UART just to
determine that the DMA channel is not ready yet and we have to undo all
the stuff. Try to map the DMA channels earlier.

Changing this means you never probe successfully if you boot a kernel
with the DMA driver disabled (or it's IOMMU disabled). Some other
drivers request DMA in open() and can work either way.

We got this exact issue previously with another driver.  When the
required DMA driver is disabled, the DMA framework cannot figure out
this situation and keeps returning EPROBE_DEFER.  I'm wondering if we
should update the DMA framework to use your deferred probe timeout
mechanism.  Is it still only used for debug purpose?

It's undergoing some rework ATM to not just be for debug. However,
it's not really going to help you if you care about the console
because waiting for the timeout will be too late to register the
console.

Rob

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