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

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Date:   Sat, 12 Sep 2020 17:53:02 -0500
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Mark Brown <broonie@...nel.org>,
        Akash Asthana <akashast@...eaurora.org>, swboyd@...omium.org,
        Andy Gross <agross@...nel.org>, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org
Subject: Re: [PATCH 1/3] spi: spi-geni-qcom: Use the FIFO even more

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?

Regards,
Bjorn

> 
> Before: It took 6.66 seconds and 59669 interrupts fired.
> After:  It took 6.66 seconds and 47992 interrupts fired.
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
> 
>  drivers/spi/spi-geni-qcom.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/spi/spi-geni-qcom.c b/drivers/spi/spi-geni-qcom.c
> index 0dc3f4c55b0b..7f0bf0dec466 100644
> --- a/drivers/spi/spi-geni-qcom.c
> +++ b/drivers/spi/spi-geni-qcom.c
> @@ -308,7 +308,7 @@ static int spi_geni_init(struct spi_geni_master *mas)
>  	 * Hardware programming guide suggests to configure
>  	 * RX FIFO RFR level to fifo_depth-2.
>  	 */
> -	geni_se_init(se, mas->tx_fifo_depth / 2, mas->tx_fifo_depth - 2);
> +	geni_se_init(se, mas->tx_fifo_depth - 3, mas->tx_fifo_depth - 2);
>  	/* Transmit an entire FIFO worth of data per IRQ */
>  	mas->tx_wm = 1;
>  	ver = geni_se_get_qup_hw_version(se);
> -- 
> 2.28.0.618.gf4bc123cb7-goog
>