linux-kernel - Re: [PATCH v2 3/9] tty: serial: fsl_lpuart: handle EPROBE

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Message-ID: <CAL_JsqL8QGKARtRAfjCMyk4Pp7EWhFMV8JQpveHoJ2OyH5kBPA@mail.gmail.com>
Date:   Thu, 27 Feb 2020 16:34:33 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Michael Walle <michael@...le.cc>
Cc:     "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Shawn Guo <shawnguo@...nel.org>, Li Yang <leoyang.li@....com>,
        Jiri Slaby <jslaby@...e.com>, Peng Fan <peng.fan@....com>,
        Vabhav Sharma <vabhav.sharma@....com>
Subject: Re: [PATCH v2 3/9] tty: serial: fsl_lpuart: handle EPROBE_DEFER for DMA

On Fri, Feb 21, 2020 at 11:48 AM Michael Walle <michael@...le.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