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

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Date:   Sat, 12 Sep 2020 18:09:05 -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 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...

-Doug