Re: [PATCH v4 5/5] spi: spi-geni-qcom: Don't keep a local state variable

From: Doug Anderson <dianders@chromium.org>
To: Stephen Boyd <swboyd@chromium.org>
Cc: Mark Brown <broonie@kernel.org>,
	Alok Chauhan <alokc@codeaurora.org>,
	skakit@codeaurora.org, Andy Gross <agross@kernel.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-spi <linux-spi@vger.kernel.org>
Subject: Re: [PATCH v4 5/5] spi: spi-geni-qcom: Don't keep a local state variable
Date: Thu, 18 Jun 2020 17:34:55 -0700	[thread overview]
Message-ID: <CAD=FV=WC_EaktmUgev_13roVYyTsmHukYO0oJODhjVVryVPpzQ@mail.gmail.com> (raw)
In-Reply-To: <159252347502.62212.15886549130634139267@swboyd.mtv.corp.google.com>

Hi,

On Thu, Jun 18, 2020 at 4:37 PM Stephen Boyd <swboyd@chromium.org> wrote:
>
> Quoting Doug Anderson (2020-06-18 15:00:10)
> > On Thu, Jun 18, 2020 at 2:52 PM Stephen Boyd <swboyd@chromium.org> wrote:
> > >
> > > -----8<----
> > > diff --git a/drivers/spi/spi-geni-qcom.c b/drivers/spi/spi-geni-qcom.c
> > > index d8f03ffb8594..670f83793aa4 100644
> > > --- a/drivers/spi/spi-geni-qcom.c
> > > +++ b/drivers/spi/spi-geni-qcom.c
> > > @@ -121,6 +121,10 @@ static void handle_fifo_timeout(struct spi_master *spi,
> > >         spin_lock_irq(&mas->lock);
> > >         reinit_completion(&mas->cancel_done);
> > >         writel(0, se->base + SE_GENI_TX_WATERMARK_REG);
> > > +       /*
> > > +        * Make sure we don't finalize a spi transfer that timed out but
> > > +        * came in while cancelling.
> > > +        */
> > >         mas->cur_xfer = NULL;
> > >         mas->tx_rem_bytes = mas->rx_rem_bytes = 0;
> > >         geni_se_cancel_m_cmd(se);
> >
> > Sure.  It gets the point across, though
> > spi_finalize_current_transfer() is actually pretty harmless if you
> > call it while cancelling.  It just calls a completion.  I'd rather say
> > something like "If we're here because the SPI controller was calling
> > handle_err() then the transfer is done and we shouldn't hold onto it
> > anymore".
> >
>
> Agreed it's mostly harmless. I thought the concern was that 'cur_xfer'
> may reference a freed piece of memory so it's best to remove ownership
> of the pointer from here so that the irq handler doesn't try to finalize
> a transfer that may no longer exist. "Shouldn't hold onto it anymore"
> doesn't tell us why it shouldn't be held onto, leaving it to the reader
> to figure out why, which isn't good.

Right.  The point is that 'cur_xfer' isn't valid anymore after
handle_err() finishes so we shouldn't hold the pointer.  I'm OK with
your wording and am happy if Mark squashes it when he applies or I can
send out a new version soon.

-Doug