linux-kernel - Re: [PATCH 3/3] spi: meson-spicc: Lower CS between bursts

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Message-ID: <098b923b-ff46-5ef5-9c21-19c974c9274f@linaro.org>
Date:   Thu, 17 Nov 2022 09:54:40 +0100
From:   Neil Armstrong <neil.armstrong@...aro.org>
To:     Carlo Caione <ccaione@...libre.com>,
        Kamlesh Gurudasani <kamlesh.gurudasani@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Jerome Brunet <jbrunet@...libre.com>,
        David Airlie <airlied@...il.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Kevin Hilman <khilman@...libre.com>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-spi@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 3/3] spi: meson-spicc: Lower CS between bursts

Hi,

On 17/11/2022 09:47, Carlo Caione wrote:
> On some hardware (reproduced on S905X) when a large payload is
> transmitted over SPI in bursts at the end of each burst, the clock line
> briefly fluctuates creating spurious clock transitions that are being
> recognised by the connected device as a genuine pulses, creating an
> offset in the data being transmitted.
> 
> Lower the GPIO CS between bursts to avoid the clock being interpreted as
> valid.

I'm afraid this will actually break SPI NORs for example where CS actually splits
transactions.

Isn't Amjad change enough ? The CLK pull-up should avoid this.

If it's not the case, then it's an HW issue and the CLK line pull-up is too weak and an
external pull should then be added.

> 
> Signed-off-by: Carlo Caione <ccaione@...libre.com>
> ---
>   drivers/spi/spi-meson-spicc.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/spi/spi-meson-spicc.c b/drivers/spi/spi-meson-spicc.c
> index d47f2623a60f..af8d74b53519 100644
> --- a/drivers/spi/spi-meson-spicc.c
> +++ b/drivers/spi/spi-meson-spicc.c
> @@ -291,6 +291,10 @@ static inline void meson_spicc_setup_burst(struct meson_spicc_device *spicc)
>   static irqreturn_t meson_spicc_irq(int irq, void *data)
>   {
>   	struct meson_spicc_device *spicc = (void *) data;
> +	struct spi_device *spi_dev;
> +
> +	spi_dev = spicc->message->spi;
> +	gpiod_set_value(spi_dev->cs_gpiod, 0);
>   
>   	writel_bits_relaxed(SPICC_TC, SPICC_TC, spicc->base + SPICC_STATREG);
>   
> @@ -309,6 +313,8 @@ static irqreturn_t meson_spicc_irq(int irq, void *data)
>   	/* Setup burst */
>   	meson_spicc_setup_burst(spicc);
>   
> +	gpiod_set_value(spi_dev->cs_gpiod, 1);
> +
>   	/* Start burst */
>   	writel_bits_relaxed(SPICC_XCH, SPICC_XCH, spicc->base + SPICC_CONREG);
>   
>