linux-kernel - Re: [PATCH 03/16] media: i2c: rdacm20: Replace goto with a loop

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Message-ID: <YDMDPymgU/N5wd/i@pendragon.ideasonboard.com>
Date:   Mon, 22 Feb 2021 03:05:03 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Jacopo Mondi <jacopo+renesas@...ndi.org>
Cc:     Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        niklas.soderlund+renesas@...natech.se, geert@...ux-m68k.org,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/16] media: i2c: rdacm20: Replace goto with a loop

Hi Jacopo,

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.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.
> 
> Reviewed-by: Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
> 
> > Signed-off-by: Jacopo Mondi <jacopo+renesas@...ndi.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