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

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Date:   Thu, 27 Feb 2020 17:03:42 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Li Yang <leoyang.li@....com>
Cc:     Michael Walle <michael@...le.cc>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <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>, 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 Thu, Feb 27, 2020 at 4:49 PM Li Yang <leoyang.li@....com> wrote:
>
> On Thu, Feb 27, 2020 at 4:35 PM Rob Herring <robh+dt@...nel.org> wrote:
> >
> > 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.
>
> 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