linux-kernel - Re: [PATCH 3/3] spi: spi-geni-qcom: Slightly optimize setup of bidirectional xfters

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Date:   Sun, 13 Sep 2020 13:35:06 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Mark Brown <broonie@...nel.org>,
        Akash Asthana <akashast@...eaurora.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Andy Gross <agross@...nel.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 3/3] spi: spi-geni-qcom: Slightly optimize setup of
 bidirectional xfters

Hi,

On Sat, Sep 12, 2020 at 6:09 PM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Sat, Sep 12, 2020 at 3:54 PM Bjorn Andersson
> <bjorn.andersson@...aro.org> wrote:
> >
> > On Sat 12 Sep 16:08 CDT 2020, Douglas Anderson wrote:
> >
> > > When setting up a bidirectional transfer we need to program both the
> > > TX and RX lengths.  We don't need a memory barrier between those two
> > > writes.  Factor out the __iowmb() and use writel_relaxed().  This
> > > saves a fraction of a microsecond of setup overhead on bidirectional
> > > transfers.
> > >
> > > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> > > ---
> > >
> > >  drivers/spi/spi-geni-qcom.c | 13 ++++++++++---
> > >  1 file changed, 10 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/spi/spi-geni-qcom.c b/drivers/spi/spi-geni-qcom.c
> > > index 92d88bf85a90..6c7e12b68bf0 100644
> > > --- a/drivers/spi/spi-geni-qcom.c
> > > +++ b/drivers/spi/spi-geni-qcom.c
> > > @@ -376,15 +376,22 @@ static void setup_fifo_xfer(struct spi_transfer *xfer,
> > >       len &= TRANS_LEN_MSK;
> > >
> > >       mas->cur_xfer = xfer;
> > > +
> > > +     /*
> > > +      * Factor out the __iowmb() so that we can use writel_relaxed() for
> > > +      * both writes below and thus only incur the overhead once even if
> > > +      * we execute both of them.
> > > +      */
> >
> > How many passes through this function do we have to take before saving
> > the amount of time it took me to read this comment?
> >
> > Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>
>
> Thanks for the review!  Yeah, in Chrome OS we do a crazy amount of SPI
> transfers since our EC and security chip are connected over SPI and we
> seem to pile a whole lot of stuff into the EC.  This means we keep
> coming back to the SPI controller again and again when profiling
> things.  I'm hoping that we'll eventually be able to get DMA enabled
> here, but until then at least it's nice to make the FIFO transfers
> better...

Ugh.  Given the problem that the kernel test robot found, I'm gonna
say just drop this patch but keep the others I sent.  As per the CL
description, it's a pretty minor optimization and even though we do a
lot of SPI transfers it's probably more worth it to work towards DMA
mode than to try to find a cleaner solution for this one.

-Doug