linux-kernel - Re: [PATCH] bus: sunxi-rsb: Return correct data when mixing 16-bit and 8-bit reads

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Message-ID: <20200220173213.s2ytf3zdi6q3bxli@gilmour.lan>
Date:   Thu, 20 Feb 2020 18:32:13 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Ondrej Jirman <megous@...ous.com>
Cc:     linux-sunxi@...glegroups.com, Chen-Yu Tsai <wens@...e.org>,
        Samuel Holland <samuel@...lland.org>,
        Stephen Boyd <swboyd@...omium.org>,
        "moderated list:ARM/Allwinner sunXi SoC support" 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bus: sunxi-rsb: Return correct data when mixing 16-bit
 and 8-bit reads

On Wed, Feb 19, 2020 at 02:09:50AM +0100, Ondrej Jirman wrote:
> When doing a 16-bit read that returns data in the MSB byte, the
> RSB_DATA register will keep the MSB byte unchanged when doing
> the following 8-bit read. sunxi_rsb_read() will then return
> a result that contains high byte from 16-bit read mixed with
> the 8-bit result.
>
> The consequence is that after this happens the PMIC's regmap will
> look like this: (0x33 is the high byte from the 16-bit read)
>
> % cat /sys/kernel/debug/regmap/sunxi-rsb-3a3/registers
> 00: 33
> 01: 33
> 02: 33
> 03: 33
> 04: 33
> 05: 33
> 06: 33
> 07: 33
> 08: 33
> 09: 33
> 0a: 33
> 0b: 33
> 0c: 33
> 0d: 33
> 0e: 33
> [snip]
>
> Fix this by masking the result of the read with the correct mask
> based on the size of the read. There are no 16-bit users in the
> mainline kernel, so this doesn't need to get into the stable tree.
>
> Signed-off-by: Ondrej Jirman <megous@...ous.com>
> ---
>  drivers/bus/sunxi-rsb.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/bus/sunxi-rsb.c b/drivers/bus/sunxi-rsb.c
> index b8043b58568ac..8ab6a3865f569 100644
> --- a/drivers/bus/sunxi-rsb.c
> +++ b/drivers/bus/sunxi-rsb.c
> @@ -316,6 +316,7 @@ static int sunxi_rsb_read(struct sunxi_rsb *rsb, u8 rtaddr, u8 addr,
>  {
>  	u32 cmd;
>  	int ret;
> +	u32 mask;
>
>  	if (!buf)
>  		return -EINVAL;
> @@ -323,12 +324,15 @@ static int sunxi_rsb_read(struct sunxi_rsb *rsb, u8 rtaddr, u8 addr,
>  	switch (len) {
>  	case 1:
>  		cmd = RSB_CMD_RD8;
> +		mask = 0xffu;
>  		break;
>  	case 2:
>  		cmd = RSB_CMD_RD16;
> +		mask = 0xffffu;
>  		break;
>  	case 4:
>  		cmd = RSB_CMD_RD32;
> +		mask = 0xffffffffu;
>  		break;
>  	default:
>  		dev_err(rsb->dev, "Invalid access width: %zd\n", len);
> @@ -345,7 +349,7 @@ static int sunxi_rsb_read(struct sunxi_rsb *rsb, u8 rtaddr, u8 addr,
>  	if (ret)
>  		goto unlock;
>
> -	*buf = readl(rsb->regs + RSB_DATA);
> +	*buf = readl(rsb->regs + RSB_DATA) & mask;

Thanks for debugging this and the extensive commit log.

I guess it would be cleaner to just use GENMASK(len * 8, 0) here?

Maxime

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