linux-kernel - Re: [PATCH 2/2] spi: spi-fsl-dspi: Initialize completion before possible interrupt

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Message-ID: <20200614151247.GA2494@kozik-lap>
Date:   Sun, 14 Jun 2020 17:12:47 +0200
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Vladimir Oltean <vladimir.oltean@....com>,
        linux-spi <linux-spi@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        Wolfram Sang <wsa@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH 2/2] spi: spi-fsl-dspi: Initialize completion before
 possible interrupt

On Sun, Jun 14, 2020 at 04:43:28PM +0300, Vladimir Oltean wrote:
> On Sun, 14 Jun 2020 at 16:39, Vladimir Oltean <olteanv@...il.com> wrote:
> >
> > On Sun, 14 Jun 2020 at 14:18, Krzysztof Kozlowski <krzk@...nel.org> wrote:
> > >
> > > On Sun, Jun 14, 2020 at 02:14:15PM +0300, Vladimir Oltean wrote:
> > > > On Sun, 14 Jun 2020 at 13:56, Krzysztof Kozlowski <krzk@...nel.org> wrote:
> > > > >
> > > > > If interrupt fires early, the dspi_interrupt() could complete
> > > > > (dspi->xfer_done) before its initialization happens.
> > > > >
> > > > > Fixes: 4f5ee75ea171 ("spi: spi-fsl-dspi: Replace interruptible wait queue with a simple completion")
> 
> Also please note that this patch merely replaced an
> init_waitqueue_head with init_completion. But the "bug" (if we can
> call it that) originates from even before.

Yeah, I know, the Fixes is not accurate. Backport to earlier kernels
would be manual so I am not sure if accurate Fixes matter.

> 
> > > > > Cc: <stable@...r.kernel.org>
> > > > > Signed-off-by: Krzysztof Kozlowski <krzk@...nel.org>
> > > > > ---
> > > >
> > > > Why would an interrupt fire before spi_register_controller, therefore
> > > > before dspi_transfer_one_message could get called?
> > > > Is this master or slave mode?
> > >
> > > I guess practically it won't fire.  It's more of a matter of logical
> > > order and:
> > > 1. Someone might fix the CONFIG_DEBUG_SHIRQ_FIXME one day,
> >
> > And what if CONFIG_DEBUG_SHIRQ_FIXME gets fixed? I uncommented it, and
> > still no issues. dspi_interrupt checks the status bit of the hw, sees
> > there's nothing to do, and returns IRQ_NONE.

Indeed, still the logical way of initializing is to do it before any
possible use.

> >
> > > 2. The hardware is actually initialized before and someone could attach
> > >    to SPI bus some weird device.
> > >
> >
> > Some weird device that does what?

You never know what people will connect to a SoM :).

Wolfram made actually much better point - bootloaders are known to
initialize some things and leaving them in whatever state, assuming that
Linux kernel will redo any initialization properly.

Best regards,
Krzysztof