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Date:   Wed, 24 Feb 2021 22:27:22 +0200
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Jacopo Mondi <jacopo@jmondi.org>
Cc:     Jacopo Mondi <jacopo+renesas@jmondi.org>,
        Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>,
        niklas.soderlund+renesas@ragnatech.se, geert@linux-m68k.org,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-media@vger.kernel.org, linux-renesas-soc@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 03/16] media: i2c: rdacm20: Replace goto with a loop
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References: <20210216174146.106639-1-jacopo+renesas@jmondi.org>
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Hi Jacopo,

On Mon, Feb 22, 2021 at 04:06:43PM +0100, Jacopo Mondi wrote:
> On Mon, Feb 22, 2021 at 03:05:03AM +0200, Laurent Pinchart wrote:
> > Hi Jacopo,
> >
> > Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> >
> > On Wed, Feb 17, 2021 at 01:01:26PM +0000, Kieran Bingham wrote:
> > > On 16/02/2021 17:41, Jacopo Mondi wrote:
> > > > During the camera module initialization the image sensor PID is read to
> > > > verify it can correctly be identified. The current implementation is
> > > > rather confused and uses a loop implemented with a label and a goto.
> > > >
> > > > Replace it with a more compact for() loop.
> > > >
> > > > No functional changes intended.
> > >
> > > I think there is a functional change in here, but I almost like it.
> > >
> > > Before, if the read was successful, it would check to see if the
> > > OV10635_PID == OV10635_VERSION, and if not it would print that the read
> > > was successful but a mismatch.
> > >
> > > Now - it will retry again instead, and if at the end of the retries it
> > > still fails then it's a failure.
> > >
> > > This means we perhaps don't get told if the device id is not correct in
> > > the same way, but it also means that if the VERSION was not correct
> > > because of a read error (which I believe i've seen occur), it will retry.
> >
> > I was going to ask about that, whether we can have a successful I2C read
> > operation that would return incorrect data. If we do, aren't we screwed
> > ? If there's a non-negligible probability that reads will return
> > incorrect data without any way to know about it (for other registers
> > than the version register of course), then I would consider that writes
> > could fail the same way, and that would mean an unusable device,
> > wouldn't it ?
> >
> > If, on the other hand, read failures can always (or nearly always,
> > ignoring space neutrinos and similar niceties) be detected, then I think
> > we should avoid the functional change.
> >
> > > Because there is a functional change you might want to update the
> > > commit, but I still think this is a good change overall.
> 
> I'm not sure I got your concerns to be honest :/
> yes before the code flow was like
> 
>         ret = ov10635_read();
>         if (ret < 0) {
> 
>         }
> 
>         if (ret != PID) {
> 
>         }
> 
> But the condition ret != PID implied ret < 0 so I don't really get
> what changes, apart from the fact that in the previous version we
> could have had two different error messages for the same issue, and yes,
> I saw ID mistmatch happening but the value of knowing the i2c read
> didn't fail but the read data was garbage (usually it's 0x01 when it
> fails iirc) is, well, questionable.

That's worrying :-S May we should add a warning message when the read
succeeds but the ID doesn't match, to at least have a way to track the
issue, and see if other changes get rid of this problem ?

> I'm sorry I didn't fully get this comment.

You're right, I had missed that the current code retried in case of a
version number mismatch. There's no functional change.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

> > > Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
> > >
> > > > Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
> > > > ---
> > > >  drivers/media/i2c/rdacm20.c | 27 ++++++++++-----------------
> > > >  1 file changed, 10 insertions(+), 17 deletions(-)
> > > >
> > > > diff --git a/drivers/media/i2c/rdacm20.c b/drivers/media/i2c/rdacm20.c
> > > > index 4d9bac87cba8..6504ed0bd3bc 100644
> > > > --- a/drivers/media/i2c/rdacm20.c
> > > > +++ b/drivers/media/i2c/rdacm20.c
> > > > @@ -59,6 +59,8 @@
> > > >   */
> > > >  #define OV10635_PIXEL_RATE		(44000000)
> > > >
> > > > +#define OV10635_PID_TIMEOUT		3
> > > > +
> > > >  static const struct ov10635_reg {
> > > >  	u16	reg;
> > > >  	u8	val;
> > > > @@ -452,7 +454,7 @@ static const struct v4l2_subdev_ops rdacm20_subdev_ops = {
> > > >
> > > >  static int rdacm20_initialize(struct rdacm20_device *dev)
> > > >  {
> > > > -	unsigned int retry = 3;
> > > > +	unsigned int i;
> > > >  	int ret;
> > > >
> > > >  	/* Verify communication with the MAX9271: ping to wakeup. */
> > > > @@ -501,23 +503,14 @@ static int rdacm20_initialize(struct rdacm20_device *dev)
> > > >  		return ret;
> > > >  	usleep_range(10000, 15000);
> > > >
> > > > -again:
> > > > -	ret = ov10635_read16(dev, OV10635_PID);
> > > > -	if (ret < 0) {
> > > > -		if (retry--)
> > > > -			goto again;
> > > > -
> > > > -		dev_err(dev->dev, "OV10635 ID read failed (%d)\n",
> > > > -			ret);
> > > > -		return -ENXIO;
> > > > +	for (i = 0; i < OV10635_PID_TIMEOUT; ++i) {
> > > > +		ret = ov10635_read16(dev, OV10635_PID);
> > > > +		if (ret == OV10635_VERSION)
> > > > +			break;
> > > > +		usleep_range(1000, 2000);
> > > >  	}
> > > > -
> > > > -	if (ret != OV10635_VERSION) {
> > > > -		if (retry--)
> > > > -			goto again;
> > > > -
> > > > -		dev_err(dev->dev, "OV10635 ID mismatch (0x%04x)\n",
> > > > -			ret);
> > > > +	if (i == OV10635_PID_TIMEOUT) {
> > > > +		dev_err(dev->dev, "OV10635 ID read failed (%d)\n", ret);
> > > >  		return -ENXIO;
> > > >  	}
> > > >

-- 
Regards,

Laurent Pinchart