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

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Date:   Thu, 20 Feb 2020 18:43:19 +0100
From:   Ondřej Jirman <megous@...ous.com>
To:     Maxime Ripard <maxime@...no.tech>
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

Hi,

On Thu, Feb 20, 2020 at 06:32:13PM +0100, Maxime Ripard wrote:
> 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?

GENMASK most probably fails with value (32,0) I think.

#define GENMASK(h, l) \
	(((~UL(0)) - (UL(1) << (l)) + 1) & \
	 (~UL(0) >> (BITS_PER_LONG - 1 - (h))))

would give ~0 >> -1

regards,
	o.

> Maxime