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Date:	Sun, 2 Feb 2014 17:56:09 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jean-Francois Moine <moinejf@...e.fr>
Cc:	dri-devel@...ts.freedesktop.org, Dave Airlie <airlied@...il.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Rob Clark <robdclark@...il.com>
Subject: Re: [PATCH v5 02/23] drm/i2c: tda998x: check more I/O errors

On Sun, Feb 02, 2014 at 06:30:00PM +0100, Jean-Francois Moine wrote:
> On Sun, 2 Feb 2014 16:20:58 +0000
> Russell King - ARM Linux <linux@....linux.org.uk> wrote:
> 
> > On Sat, Jan 25, 2014 at 06:14:45PM +0100, Jean-Francois Moine wrote:
> > > This patch adds more error checking inn I2C I/O functions.
> > > In case of I/O error, this permits to avoid writing in bad controller
> > > pages, a bad chipset detection or looping when getting the EDID.
> > 
> > I've just looked at this again, and spotted something:
> > 
> > > -static uint8_t
> > > +static int
> > >  reg_read(struct tda998x_priv *priv, uint16_t reg)
> > >  {
> > >  	uint8_t val = 0;
> > > -	reg_read_range(priv, reg, &val, sizeof(val));
> > > +	int ret;
> > > +
> > > +	ret = reg_read_range(priv, reg, &val, sizeof(val));
> > > +	if (ret < 0)
> > > +		return ret;
> > 
> > So yes, this can return negative numbers.
> > 
> > > @@ -1158,8 +1184,11 @@ tda998x_encoder_init(struct i2c_client *client,
> > >  	tda998x_reset(priv);
> > >  
> > >  	/* read version: */
> > > -	priv->rev = reg_read(priv, REG_VERSION_LSB) |
> > > -			reg_read(priv, REG_VERSION_MSB) << 8;
> > > +	ret = reg_read(priv, REG_VERSION_LSB) |
> > > +		(reg_read(priv, REG_VERSION_MSB) << 8);
> > > +	if (ret < 0)
> > > +		goto fail;
> > > +	priv->rev = ret;
> > 
> > Two issues here:
> > 
> > 1. The additional parens are /really/ not required.
> > 2. What if reg_read(priv, REG_VERSION_MSB) returns a negative number?
> > 
> > If we're going to the extent of attempting to make the read/write
> > functions return errors, we should at least handle errors generated
> > by them properly, otherwise it's pointless making them return errors.
> > 
> > 	ret = reg_read(priv, REG_VERSION_LSB);
> > 	if (ret < 0)
> > 		goto fail;
> > 
> > 	priv->rev = ret;
> > 
> > 	ret = reg_read(priv, REG_VERSION_MSB);
> > 	if (ret < 0)
> > 		goto fail;
> > 
> > 	priv->rev |= ret << 8;
> > 
> > If you want it to look slightly nicer:
> > 
> > 	int rev_lo, rev_hi;
> > 
> > 	rev_lo = reg_read(priv, REG_VERSION_LSB);
> > 	rev_hi = reg_read(priv, REG_VERSION_MSB);
> > 	if (rev_lo < 0 || rev_hi < 0) {
> > 		ret = rev_lo < 0 ? rev_lo : rev_hi;
> > 		goto fail;
> > 	}
> > 
> > 	priv->rev = rev_lo | rev_hi << 8;
> > 
> > I'm happy to commit such a change after this patch to clean it up, or if
> > you want to regenerate your patch 2 and post /just/ that incorporating
> > this change.
> 
> I think that my code works correctly: when there is an error, the
> result of reg_read() is minus the error code, and this error code is
> always lower than 8388607 (0x7fffff). Then, reg_read() << 8 will always
> be negative.

The issue I'm pointing out is not whether it will be interpreted as an
error or not, it's whether the value is a correct error code.  If we
don't care about the reason why an error occurs, we might as well just
return -1.

I've added my own patch which adjusts it as per above to my tree anyway,
so I'm not that worried about this.

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