linux-kernel - Re: [PATCH 1/3] spi: spi-geni-qcom: Use the FIFO even more

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Message-ID: <CAD=FV=VT+7RX=vdy0Ba_AB3dyMKVGu9uwP5bS2eew2W49BdcWA@mail.gmail.com>
Date:   Sat, 12 Sep 2020 18:11:55 -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 1/3] spi: spi-geni-qcom: Use the FIFO even more

Hi,

On Sat, Sep 12, 2020 at 3:53 PM Bjorn Andersson
<bjorn.andersson@...aro.org> wrote:
>
> On Sat 12 Sep 16:07 CDT 2020, Douglas Anderson wrote:
>
> > In commit 902481a78ee4 ("spi: spi-geni-qcom: Actually use our FIFO") I
> > explained that the maximum size we could program the FIFO was
> > "mas->tx_fifo_depth - 3" but that I chose "mas->tx_fifo_depth()"
> > because I was worried about decreased bandwidth.
> >
> > Since that time:
> > * All the interconnect patches have landed, making things run at the
> >   proper speed.
> > * I've done more measurements.
> >
> > This lets me confirm that there's really no downside of using the FIFO
> > more.  Specifically I did "flashrom -p ec -r /tmp/foo.bin" on a
> > Chromebook and averaged over several runs.
>
> Wouldn't there be a downside in the form of setting the watermark that
> close to the full FIFO we have less room for being late handling the
> interrupt? Or is there some mechanism involved that will prevent
> the FIFO from being overrun?

Yeah, I had that worry too, but, as described in 902481a78ee4 ("spi:
spi-geni-qcom: Actually use our FIFO"), it doesn't seem to be a
problem.  From that commit: "We are the SPI master, so it makes sense
that there would be no problems with overruns, the master should just
stop clocking."

-Doug