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Date:   Mon, 20 Feb 2017 20:52:41 +0000
From:   Sudip Mukherjee <sudipm.mukherjee@...il.com>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-kernel@...r.kernel.org, linux-fbdev@...r.kernel.org
Subject: Re: [PATCH] backlight: report error on failure

On Sun, Feb 19, 2017 at 08:26:18AM +0000, Daniel Thompson wrote:
> On Sat, Feb 18, 2017 at 04:31:43PM +0000, Sudip Mukherjee wrote:
> > On Monday 30 January 2017 11:04 PM, Sudip Mukherjee wrote:
> > > On Wednesday 18 January 2017 09:23 PM, Sudip Mukherjee wrote:
> > > > It is possible to update the backlight power and the brightness using
> > > > the sysfs and on writing it either returns the count or if the callback
> > > > function does not exist then returns the error code 'ENXIO'.
> > > > 
> > > > We have a situation where the userspace client is writing to the sysfs
> > > > to update the power and since the callback function exists the client
> > > > receives the return value as count and considers the operation to be
> > > > successful. That is correct as the write to the sysfs was successful.
> > > > But there is no way to know if the actual operation was done or not.
> > > > 
> > > > backlight_update_status() returns the error code if it fails. Pass that
> > > > to the userspace client who is trying to update the power so that the
> > > > client knows that the operation failed.
> > > > 
> > > > Signed-off-by: Sudip Mukherjee <sudip.mukherjee@...ethink.co.uk>
> > > > ---
> > > 
> > > A gentle ping.
> > 
> > Hi Andrew,
> > 
> > Its more than one month now that this patch was submitted. Can you please
> > take it through your tree... else it will miss the merge window.
> 
> Sorry you've been waiting so long for a review. Your "gentle ping" did
> result in a few minor tweaks to the maintainers but what it didn't
> (until today) provoke was  review!

what tweaks? I can see that now you are the new maintainer.Is that the
result of my ping? :)

> 
> To be honest, the ABI changes in the patch meant I wanted to check a 
> few things first. However prompted by this mail I've just replied with 
> the review I *should* have sent out a week ago [asking you rather than 
> me to fill in some details about the likely effect of the ABI change ;-) ].
> 
> BTW, I did do some archeology eariler in the week and I think the kernel
> has been swallowing error codes here for >10 years, so I'm curious if this
> is a theoretic bug fix or are you aware of some tangible problem in 
> userspace that results from it?

I will reply to your other mail later this week. I am on a short holiday
now with very limited access.

But the problem that prompted me to send this patch is the current project
that I am working on now. And we faced this there. The userspace code is
writing to the sysfs node to poweron the backlight and reported success.
But sometimes we noticed that backlight was not actally powered on. And
that lead me to check the code and noticed that it is swallowing all the
errors.

Regards
Sudip

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