linux-kernel - Re: [PATCH 3/6] i2c: spacemit: disable SDA glitch fix to avoid restart delay

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Message-ID: <DCC64908985C6746+aK7QkmNHa_QMRfgh@LT-Guozexi>
Date: Wed, 27 Aug 2025 17:32:02 +0800
From: Troy Mitchell <troy.mitchell@...ux.spacemit.com>
To: Andi Shyti <andi.shyti@...nel.org>, Yixun Lan <dlan@...too.org>,
	Alex Elder <elder@...cstar.com>,
	Troy Mitchell <troymitchell988@...il.com>
Cc: linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org, spacemit@...ts.linux.dev,
	Troy Mitchell <troy.mitchell@...ux.spacemit.com>
Subject: Re: [PATCH 3/6] i2c: spacemit: disable SDA glitch fix to avoid
 restart delay

On Wed, Aug 27, 2025 at 03:39:10PM +0800, Troy Mitchell wrote:
> The K1 I2C controller has an SDA glitch fix that introduces a small
> delay on restart signals. While this feature can suppress glitches
> on SDA when SCL = 0, it also delays the restart signal, which may
> cause unexpected behavior in some transfers.
> 
> The glitch itself does not affect normal I2C operation, because
> the I2C specification allows SDA to change while SCL is low.
> 
> To ensure correct transmission for every message, we disable the
> SDA glitch fix by setting the RCR.SDA_GLITCH_NOFIX bit during
> initialization.
> 
> This guarantees that restarts are issued promptly without
> unintended delays.
These are the restart waveforms before [1] and after [2] disabling the fix.

Link:
https://psce.pw/839rzq [1]
https://psce.pw/839s4q [2]

---
And I'll put above information into the commit message in the next version.

                 - Troy
> 
> Fixes: 5ea558473fa31 ("i2c: spacemit: add support for SpacemiT K1 SoC")
> Signed-off-by: Troy Mitchell <troy.mitchell@...ux.spacemit.com>
> ---
>  drivers/i2c/busses/i2c-k1.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/i2c/busses/i2c-k1.c b/drivers/i2c/busses/i2c-k1.c
> index d342752030d077953adf84a2886211de96e843c4..c1656b78f1681729ccc2ebca6e290259d14889d9 100644
> --- a/drivers/i2c/busses/i2c-k1.c
> +++ b/drivers/i2c/busses/i2c-k1.c
> @@ -14,6 +14,7 @@
>  #define SPACEMIT_ICR		 0x0		/* Control register */
>  #define SPACEMIT_ISR		 0x4		/* Status register */
>  #define SPACEMIT_IDBR		 0xc		/* Data buffer register */
> +#define SPACEMIT_IRCR		 0x18		/* Reset cycle counter */
>  #define SPACEMIT_IBMR		 0x1c		/* Bus monitor register */
>  
>  /* SPACEMIT_ICR register fields */
> @@ -76,6 +77,8 @@
>  					SPACEMIT_SR_GCAD | SPACEMIT_SR_IRF | SPACEMIT_SR_ITE | \
>  					SPACEMIT_SR_ALD)
>  
> +#define SPACEMIT_RCR_SDA_GLITCH_NOFIX		BIT(7)		/* bypass the SDA glitch fix */
> +
>  /* SPACEMIT_IBMR register fields */
>  #define SPACEMIT_BMR_SDA         BIT(0)		/* SDA line level */
>  #define SPACEMIT_BMR_SCL         BIT(1)		/* SCL line level */
> @@ -237,6 +240,14 @@ static void spacemit_i2c_init(struct spacemit_i2c_dev *i2c)
>  	val |= SPACEMIT_CR_MSDE | SPACEMIT_CR_MSDIE;
>  
>  	writel(val, i2c->base + SPACEMIT_ICR);
> +
> +	/*
> +	 * The glitch fix in the K1 I2C controller introduces a delay
> +	 * on restart signals, so we disable the fix here.
> +	 */
> +	val = readl(i2c->base + SPACEMIT_IRCR);
> +	val |= SPACEMIT_RCR_SDA_GLITCH_NOFIX;
> +	writel(val, i2c->base + SPACEMIT_IRCR);
>  }
>  
>  static inline void
> 
> -- 
> 2.50.1
>