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From:   Rob Herring <robh+dt@kernel.org>
Date:   Thu, 27 Feb 2020 16:34:33 -0600
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Subject: Re: [PATCH v2 3/9] tty: serial: fsl_lpuart: handle EPROBE_DEFER for DMA
To:     Michael Walle <michael@walle.cc>
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On Fri, Feb 21, 2020 at 11:48 AM Michael Walle <michael@walle.cc> wrote:
>
> 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.

Rob