Date:	Tue, 2 Jun 2015 21:54:10 -0400
From:	Matt Campbell <mattrcampbell@...il.com>
To:	David Fries <David@...es.net>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [patch 2/2] w1: unlock correct lock on error in w1_seq_show()

Thanks for the response, I will try to answer your questions.

The other function besides temp conversion and EEPROM write that
requires a delay is the initial CHAIN on command. The datasheet says
to "Wait for chain to charge" but it does not offer ANY indication on
how to do so.  I did discover that my sleep was a sleep(0) long after
I submitted the patch, but as you observed it was not hurting
anything.

The 64 was, in all honesty, a starting point that I picked for lack of
finding a better option.  I only currently have 10 sensors available
for me to test with, so I was not able to do any significant tests to
see what would happen if I raised that limit and the loop did not exit
properly at the end of the chain.  I was also not able to find an edge
case that would cause the loop to not find a match.  The w1_seq file
only shows for the one type sensor that supports it so in the absence
of a bus error, a match is guaranteed.

The output of the function in the event the pins are hooked up as GPIO
and not a chain is undefined.  Since the chips are very tiny,
prototyping this function would require me to design and order some
boards that I never was able to use.  To my knowledge it is not
possible to determine that things are wired this way in the software.

Apologies, this is my first foray in submitting any sort of kernel
code so I am not sure of exactly how I would submit a fixup patch for
gregkh's tree.  If someone wants to walk me through that off list I
would be most appreciative!
--m

Matt Campbell
MattRCampbell@...il.com


On Tue, Jun 2, 2015 at 9:07 PM, David Fries <David@...es.net> wrote:
> The source code is as good as it gets, that's why I posted those two
> lines from the w1.h file.  Maybe it could be expanded upon, mutex is
> more for adding or removing slaves and the like modifications to
> w1_master data structure, where bus_mutex is what make sure something
> else doesn't try to talk on the bus while the current device is using
> it.  What really matters is how other code is using the locks.  For
> instance if you look at w1_therm.c bus_mutex is the only one used.
> Then you look at what your routine is doing, and what the existing
> routine is doing, they are both reading/writing from the one wire bus,
> and then return their output to the user, so then use the same lock.
> If it's the wrong lock then it's at least consistent and easier to
> cleanup when someone figures out it's the wrong lock.  If you think
> it's the wrong lock raise a flag and ask.
>
> My patch review comments.
>
> The temperature sensor uses a strong pullup or delay of 750 ms to do
> the temperature conversion.  A quick read through of this sensor has
> temperature conversion and EEPROM write as the only operations
> requiring a significant time delay, everything else is in the us, so I
> would expect this to not require any delay.
>
> Read the comments to w1_next_pullup in w1_io.c as pullup_duration will
> always be 0 in msleep(sl->master->pullup_duration); which isn't likely
> what you expect.  Apparently it is working for you so that further
> reinforces my previous point.
>
> Where did the 64 come from?  I have 14 temperature sensors in my
> single family residence right now, and it would take another 7 to
> cover each area/room.  I could imagine an actual industrial setting to
> far exceed the 64 count, but I don't work in that area so I don't know
> the practical limits.  How long does it take to read if it loops
> through 64 times without finding a match?  What is reg_num value if a
> sensor doesn't reply?  If it is all 00's or all ff's would that be a
> better test for the end of the line?  How long would it take to loop
> 16384 times?  Maybe you stop if it reaches 16384 or if the end of line
> value is read 64 times.  I also assume this is pretty much a static
> answer, so you aren't going to be querying it except once per device,
> once per program setup and rarely or infrequently after that so the
> time shouldn't matter too much.  Maybe the program queries again if a
> sensor is detected or removed.
>
> I would like to see some documentation (comment) at the top of
> w1_seq_show as to what the sysfs number returns means.  I can see the
> variable is named seq, and it returns it as a signed number.  What
> does a negative value mean?  Are they expected?  Will the first device
> in the chain be 0 or 1, and what defines what end that is?  What kind
> of value is returned if the pins are connected as GPIO and not in the
> direct sequence configuration?  Maybe it needs a comment that all
> sensors of this type must be in the direct sequence configuration for
> this routine to give meaningful values.  "Place all devices in CHAIN
> state" that will address all devices including other types of devices,
> would you expect this to cause a problem with other devices?
>
> Line up the constants after #define to the same tab location before
> w1_seq_show.
>
> Do you plan to send in a mutex fixup patch?  In my experience gregkh's
> tree only accepts patches to what is already there, he won't swap out a
> patch he already has (he could as it isn't in Linus's tree, but he
> doesn't).
>
> On Tue, Jun 02, 2015 at 08:35:41AM -0400, Matt Campbell wrote:
>> David, not arguing just curious how you decide which lock to use?  Is there
>> a standard documented?
>>
>> --m
>>
>> Matt Campbell
>> MattRCampbell@...il.com
>>
>> On Tue, Jun 2, 2015 at 8:12 AM, David Fries <David@...es.net> wrote:
>>
>> > Thanks for including me, I'm going to have to nack this though, it is
>> > an error, but bus_mutex is correct, mutex is incorrect, so the other
>> > two lock/unlock in that routine are the ones that need to be changed.
>> >
>> > w1.h
>> >  * @mutex:              protect most of w1_master
>> >  * @bus_mutex:          pretect concurrent bus access
>> >
>> > On Mon, Jun 01, 2015 at 12:56:32PM +0300, Dan Carpenter wrote:
>> > > Smatch complains that we don't unlock "master->mutex" on this error
>> > > path.  It looks like it is a typo and we unlock ->bus_mutext where
>> > > ->mutex was intended.
>> > >
>> > > Fixes: d9411e57dc7f ('w1: Add support for DS28EA00 sequence to w1-therm')
>> > > Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
>> > >
>> > > diff --git a/drivers/w1/slaves/w1_therm.c b/drivers/w1/slaves/w1_therm.c
>> > > index 3351be6..79ecaf8 100644
>> > > --- a/drivers/w1/slaves/w1_therm.c
>> > > +++ b/drivers/w1/slaves/w1_therm.c
>> > > @@ -412,7 +412,7 @@ static ssize_t w1_seq_show(struct device *device,
>> > >       c -= snprintf(buf + PAGE_SIZE - c, c, "%d\n", seq);
>> > >       return PAGE_SIZE - c;
>> > >  error:
>> > > -     mutex_unlock(&sl->master->bus_mutex);
>> > > +     mutex_unlock(&sl->master->mutex);
>> > >       return -EIO;
>> > >  }
>> > >
>> >
>> > --
>> > David Fries <david@...es.net>
>> >
>
> --
> David Fries <david@...es.net>
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