linux-kernel - Re: [PATCH] i2c: mpc: Restore reread of I2C status register

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Date:   Thu, 8 Jul 2021 02:03:41 +0000
From:   Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To:     "wsa@...nel.org" <wsa@...nel.org>,
        Hamish Martin <Hamish.Martin@...iedtelesis.co.nz>
CC:     "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] i2c: mpc: Restore reread of I2C status register


On 2/07/21 3:27 pm, Chris Packham wrote:
> Prior to commit 1538d82f4647 ("i2c: mpc: Interrupt driven transfer") the
> old interrupt handler would reread MPC_I2C_SR after checking the CSR_MIF
> bit. When the driver was re-written this was removed as it seemed
> unnecessary. However as it turns out this is necessary for i2c devices
> which do clock stretching otherwise we end up thinking the bus is still
> busy when processing the interrupt.
>
> Fixes: 1538d82f4647 ("i2c: mpc: Interrupt driven transfer")
> Signed-off-by: Chris Packham <chris.packham@...iedtelesis.co.nz>

Just a heads up that this hasn't totally fixed the issue. Just made it 
less likely to occur. I'm now wondering if we should be treating MCF as 
a busy bit and waiting for it to clear (with approrpriate timeouts) 
instead of just flagging an error immediately.

> ---
>   drivers/i2c/busses/i2c-mpc.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/i2c/busses/i2c-mpc.c b/drivers/i2c/busses/i2c-mpc.c
> index dcca9c2396db..6d5014ebaab5 100644
> --- a/drivers/i2c/busses/i2c-mpc.c
> +++ b/drivers/i2c/busses/i2c-mpc.c
> @@ -635,6 +635,8 @@ static irqreturn_t mpc_i2c_isr(int irq, void *dev_id)
>   
>   	status = readb(i2c->base + MPC_I2C_SR);
>   	if (status & CSR_MIF) {
> +		/* Read again to allow register to stabilise */
> +		status = readb(i2c->base + MPC_I2C_SR);
>   		writeb(0, i2c->base + MPC_I2C_SR);
>   		mpc_i2c_do_intr(i2c, status);
>   		return IRQ_HANDLED;