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

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Message-ID: <159251712230.62212.10744179843753723398@swboyd.mtv.corp.google.com>
Date:   Thu, 18 Jun 2020 14:52:02 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Mark Brown <broonie@...nel.org>,
        Alok Chauhan <alokc@...eaurora.org>, skakit@...eaurora.org,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-spi <linux-spi@...r.kernel.org>
Subject: Re: [PATCH v4 5/5] spi: spi-geni-qcom: Don't keep a local state variable

Quoting Doug Anderson (2020-06-18 13:09:47)
> On Thu, Jun 18, 2020 at 11:05 AM Stephen Boyd <swboyd@...omium.org> wrote:
> >
> > Quoting Douglas Anderson (2020-06-18 08:06:26)
> > > @@ -126,20 +120,23 @@ static void handle_fifo_timeout(struct spi_master *spi,
> > >         struct geni_se *se = &mas->se;
> > >
> > >         spin_lock_irq(&mas->lock);
> > > -       reinit_completion(&mas->xfer_done);
> > > -       mas->cur_mcmd = CMD_CANCEL;
> > > -       geni_se_cancel_m_cmd(se);
> > > +       reinit_completion(&mas->cancel_done);
> > >         writel(0, se->base + SE_GENI_TX_WATERMARK_REG);
> > > +       mas->cur_xfer = NULL;
> >
> > BTW, is this necessary? It's subtlely placed here without a comment why.
> 
> I believe so.  Now that we don't have the "cur_mcmd" we rely on
> cur_xfer being NULL to tell the difference between a "done" for chip
> select vs. a "done" for transfer.
> 
> * When we start a transfer we set "cur_xfer" to a non-NULL pointer.
> When the transfer finishes we set it to NULL again.
> 
> * When we start a chip select transfer we _don't_ explicitly set it to
> NULL because it should already be NULL.
> 
> * When we are aborting a transfer we need to NULL so we can handle the
> chip select that will come next.
> 
> I suppose it's possible that we could get by without without NULLing
> it because I believe when the "abort" IRQ finally fires then it will
> include a "DONE" and that would presumably NULL it out.  ...but I
> guess if both the cancel and abort timed out and no IRQ ever fired
> then nothing would have NULLed it and the next chip select would be
> confused.

I was going to say that we should set it NULL when starting CS but that
is not as important as clearing it out when a cancel/abort is processing
so that a stale transfer isn't kept around.

> 
> Prior to getting rid of "cur_mcmd" this all wasn't needed because
> "cur_xfer" was only ever looked at if "cur_mcmd" was set to
> "CMD_XFER".
> 
> 
> One part of my change that is technically not related to the removal
> of "cur_mcmd" is the part where I do "mas->tx_rem_bytes =
> mas->rx_rem_bytes = 0;".  I can split that as a separate change if you
> want but it seemed fine to just clean up this extra bit of state here.
> 

How about a comment like this?

-----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);