linux-kernel - Re: [PATCH v2 4/5] media: i2c: ov5645: Return zero for s

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Date:   Mon, 17 Oct 2022 00:03:17 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     Prabhakar <prabhakar.csengg@...il.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil@...all.nl>,
        Shawn Tu <shawnx.tu@...el.com>,
        Jacopo Mondi <jacopo@...ndi.org>, linux-media@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org,
        Biju Das <biju.das.jz@...renesas.com>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH v2 4/5] media: i2c: ov5645: Return zero for s_stream(0)

Hi Sakari,

On Sun, Oct 16, 2022 at 08:19:40PM +0000, Sakari Ailus wrote:
> On Sun, Oct 16, 2022 at 02:23:13AM +0300, Laurent Pinchart wrote:
> > On Sat, Oct 15, 2022 at 09:35:12PM +0000, Sakari Ailus wrote:
> > > On Sat, Oct 15, 2022 at 09:25:37AM +0300, Laurent Pinchart wrote:
> > > > On Fri, Oct 14, 2022 at 07:34:58PM +0100, Prabhakar wrote:
> > > > > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> > > > > 
> > > > > Always return zero while stopping the stream as the caller will ignore the
> > > > > return value.
> > > > > 
> > > > > This patch drops checking the return value of ov5645_write_reg() and
> > > > > continues further in the code path while stopping stream. The user anyway
> > > > > gets an error message in case ov5645_write_reg() fails.
> > > > 
> > > > Continuing all the way to pm_runtime_put() is fine, but I don't think
> > > > the function should return 0. It's not up to the driver to decide if a
> > > > failure would be useful to signal to the caller or not.
> > > 
> > > If the function returns an error when disabling streaming, what is the
> > > expected power state of the device after this?
> > 
> > That's up to us to decide :-)
> > 
> > > The contract between the caller and the callee is that the state is not
> > > changed if there is an error.
> > 
> > For most APIs, but that's not universal.
> > 
> > > This is a special case as very few callers
> > > check the return value for streamoff operation and those that do generally
> > > just print something. I've never seen a caller trying to prevent streaming
> > > off in this case, for instance.
> > 
> > I think the stream off call should proceed and try to power off the
> > device even if an error occurs along the way, i.e. it shouldn't return
> > upon the first detected error.
> > 
> > > Of course we could document that streaming off always counts as succeeded
> > > (e.g. decreasing device's runtime PM usage_count) while it could return an
> > > informational error code. But I wonder if anyone would ever benefit from
> > > that somehow. :-)
> > 
> > I think it could be useful to propagate errors up to inform the user
> > that something wrong happened. That would involve fixing lots of drivers
> > along the call chain though, so there's no urgency for the ov5645 to do
> > so, but isn't it better to propagate the error code instead of hiding
> > the issue ?
> 
> I also don't think hiding the issue would be the best thing to do, but that
> wouldn't likely be a big problem either.
> 
> How about printing a warning in the wrapper while returning zero to the
> original caller? This would keep the API intact while still leaving a trace
> on something failing. Of course the driver is also free to print whatever
> messages it likes.

While I think error propagation could be more useful in the long run,
printing a message in the wrapper is a good idea. I like centralized
error handling, it has a tendency to go wrong when left to individual
drivers.

-- 
Regards,

Laurent Pinchart