linux-kernel - Re: [PATCH 1/1] Add strong pullup emulation to w1-gpio master driver.

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Message-ID: <20131112080146.GF5509@spacedout.fries.net>
Date:	Tue, 12 Nov 2013 02:01:46 -0600
From:	David Fries <david@...es.net>
To:	Evgeny Boger <eugenyboger@...il.com>
Cc:	Evgeniy Polyakov <zbr@...emap.net>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] Add strong pullup emulation to w1-gpio master driver.

On Tue, Nov 12, 2013 at 05:07:14AM +0400, Evgeny Boger wrote:
> +David Fries <david@...es.net>
> 
> Hi David,
> 
> Would you please comment on this?

On Mon, Nov 11, 2013 at 06:36:54PM +0400, Evgeny Boger wrote:
>  Strong pullup is emulated by driving pin logic high after write
>  command when
>  using tri-state push-pull GPIO.

Not knowing the hardware involved, is driving the logic high a
stronger pullup than the normal weak pullup input high?  Meaning it
was already being left high, just with a lessor pullup and this will
provide a stronger one?

On Tue, Nov 12, 2013 at 03:09:36AM +0400, Evgeniy Polyakov wrote:
> > + msleep(pdata->pullup_duration);
> 
> This doesn't look like a good idea - kernel will sleep for that long
> not doing usual w1 job

Not speaking for Evgeny Boger, but I'm thinking that's intended here.
The original strong pullup code change 6a158c0de791a81 I wrote will
msleep in w1_post_write when a hardware pullup isn't available, while
the hardware ds2490 ds9490r_set_pullup sleeps for the strong pullup
using spu_sleep variable.  The user requests a strong pullup for a
given time and any other operations on the bus will interrupt the
strong pullup, so locking any other operations sounds desired.

> 11/12/2013 05:03 AM, Evgeniy Polyakov:
> >Hi
> >
> >12.11.2013, 03:32, "Evgeny Boger" <eugenyboger@...il.com>:
> >>>Why did you drop this check? It has nothing with w1-gpio driver
> >>This check prevents master from implementing "set_pullup"  provided it does support only "write_bit" method.
> >>The comment above states that
> >>>  w1_io.c would need to support calling set_pullup before - * the last write_bit operation of a w1_write_8 which it currently - * doesn't.
> >>which is kind of strange, since it describes what w1_io.c actually does support.
> >>
> >>w1_write_8 (w1_io.c:154, https://github.com/torvalds/linux/blob/master/drivers/w1/w1_io.c#L154):
> >>>                 for (i = 0; i < 8; ++i) {
> >>>                         if (i == 7)
> >>>                                 w1_pre_write(dev);
> >>>                         w1_touch_bit(dev, (byte >> i) & 0x1);
> >>>                 }
> >>It seems like w1_write_8() calls w1_pre_write(), which in turn calls set_pullup() just before the last write_bit().

I'm not seeing any harm in removing this check and clear
master->set_pullup.  It doesn't seem correct for this code to override
a master that claims to provide something of a stronger pullup.  It's
been about five years since I wrote that code, I think it was just to
protect against a stupid master.

With this patch the last w1_write_bit will go logic 1, for 64 or 10 us
before returning, then w1_gpio_set_pullup is called to enable the
strong pullup.  What I wouldn't know is if in that last bit if the
logic 1 would be a go up to the strong pullup, or if it would finish
that time slot with a weak pullup and then go to a strong pullup.  I
would have to dig into the timing specifications much more than I have
time to right now to say what is supposed to happen.  The 18b20
datasheet lists, "The DQ line must be switched over to the strong
pullup within 10 us maximum after issuing any protocol that involves
copying the E2 memory or initiates temperature conversions."  It isn't
clear where that 10 us starts from.  You might try to dig around and
see if that last bit written should go to weak pullup 1 or strong
pullup 1.  It would take more changes if it should go right to a
strong pullup.

> >>I'm not sure why this check was there in the first place.
> >Please add author of those lines to clarify things.
> >This doesn't look obvious to me

-- 
David Fries <david@...es.net>    PGP pub CB1EE8F0
http://fries.net/~david/
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