linux-kernel - Re: [PATCH 2/2] spi: spi-geni-qcom: Really ensure the previous xfer is done before new one

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Date:   Tue, 15 Dec 2020 17:17:55 -0800
From:   Stephen Boyd <swboyd@...omium.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Mark Brown <broonie@...nel.org>, msavaliy@....qualcomm.com,
        Akash Asthana <akashast@...eaurora.org>,
        Roja Rani Yarubandi <rojay@...eaurora.org>,
        Alok Chauhan <alokc@...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 2/2] spi: spi-geni-qcom: Really ensure the previous xfer is done before new one

Quoting Doug Anderson (2020-12-15 15:34:59)
> On Tue, Dec 15, 2020 at 2:25 PM Stephen Boyd <swboyd@...omium.org> wrote:
> >
> > Quoting Doug Anderson (2020-12-15 09:25:51)
> > > In general when we're starting a new transfer we assume that we can
> > > program the hardware willy-nilly.  If there's some chance something
> > > else is happening (or our interrupt could go off) then it breaks that
> > > whole model.
> >
> > Right. I thought this patch was making sure that the hardware wasn't in
> > the process of doing something else when we setup the transfer. I'm
> > saying that only checking the irq misses the fact that maybe the
> > transfer hasn't completed yet or a pending irq hasn't come in yet, but
> > the fifo status would tell us that the fifo is transferring something or
> > receiving something. If an RX can't happen, then the code should clearly
> > show that an RX irq isn't expected, and mask out that bit so it is
> > ignored or explicitly check for it and call WARN_ON() if the bit is set.
> >
> > I'm wondering why we don't check the FIFO status and the irq bits to
> > make sure that some previous cancelled operation isn't still pending
> > either in the FIFO or as an irq. While this patch will fix the scenario
> > where the irq is delayed but pending in the hardware it won't cover the
> > case that the hardware itself is wedged, for example because the
> > sequencer just decided to stop working entirely.
> 
> It also won't catch the case where the SoC decided that all GPIOs are
> inverted and starts reporting highs for lows and lows for highs, nor
> does it handle the case where the CPU suddenly switches to Big Endian
> mode for no reason.  :-P
> 
> ...by that, I mean I'm not trying to catch the case where the hardware
> itself is behaving in a totally unexpected way.  I have seen no
> instances where the hardware wedges nor where the sequencer stops
> working and until I see them happen I'm not inclined to add code for
> them.  Without seeing them actually happen I'm not really sure what
> the right way to recover would be.  We've already tried "cancel" and
> "abort" and then waited at least 1 second.  If you know of some sort
> of magic "unwedge" then we should add it into handle_fifo_timeout().

I am not aware of an "unwedge" command. Presumably the cancel/abort
stuff makes the FIFO state "sane" so there's nothing to see in the FIFO
status registers. I wonder if we should keep around some "did we cancel
last time?" flag and only check the isr if we canceled out and timed
out to boot? That would be a cheap and easy check to make sure that we
don't check this each transaction.

> 
> However, super delayed interrupts due to software not servicing the
> interrupt in time is something that really happens, if rarely.  Adding
> code to account for that seems worth it and is easy to test...
> 

Agreed. The function name is wrong then as the device is not "busy". So
maybe spi_geni_isr_pending()? That would clearly describe what's being
checked.