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

[Doug Anderson]

Hi,

On Sat, Sep 12, 2020 at 3:53 PM Bjorn Andersson
<bjorn.andersson@xxxxxxxxxx> 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