linux-kernel - Re: [PATCH 04/10] phy: dphy: Add configuration helpers

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Message-ID: <3954754.hq6Pmoh9iE@avalon>
Date:   Mon, 10 Sep 2018 17:28:11 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Maxime Ripard <maxime.ripard@...tlin.com>
Cc:     Kishon Vijay Abraham I <kishon@...com>,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        linux-media@...r.kernel.org,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Chen-Yu Tsai <wens@...e.org>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org,
        Krzysztof Witos <kwitos@...ence.com>,
        Rafal Ciepiela <rafalc@...ence.com>
Subject: Re: [PATCH 04/10] phy: dphy: Add configuration helpers

Hi Maxime,

On Monday, 10 September 2018 17:16:03 EEST Maxime Ripard wrote:
> On Fri, Sep 07, 2018 at 05:26:29PM +0300, Laurent Pinchart wrote:
> >>>> + */
> >>>> +int phy_mipi_dphy_get_default_config(unsigned long pixel_clock,
> >>>> +				     unsigned int bpp,
> >>>> +				     unsigned int lanes,
> >>>> +				     struct phy_configure_opts_mipi_dphy *cfg)
> >>>> +{
> >>>> +	unsigned long hs_clk_rate;
> >>>> +	unsigned long ui;
> >>>> +
> >>>> +	if (!cfg)
> >>>> +		return -EINVAL;
> >>> 
> >>> Should we really expect cfg to be NULL ?
> >> 
> >> It avoids a kernel panic and it's not in a hot patch, so I'd say yes?
> > 
> > A few line below you divide by the lanes parameter without checking
> > whether it is equal to 0 first, which would also cause issues.
> 
> You say that like it would be a bad thing to test for this.
> 
> > I believe that invalid values in input parameters should only be handled
> > explicitly when considered acceptable for the caller to pass such values.
> > In this case a NULL cfg pointer is a bug in the caller, which would get
> > noticed during development if the kernel panics.
> 
> In the common case, yes. In the case where that pointer is actually
> being lost by the caller somewhere down the line and you have to wait
> for a while before it happens, then having the driver inoperant
> instead of just having a panic seems like the right thing to do.

But why would it happen in the first place ? Why would the pointer be more 
likely here to be NULL than to contain, for instance, an uninitialized value, 
which we don't guard against ?

-- 
Regards,

Laurent Pinchart