Messages in this thread |  | | Date | Wed, 13 Nov 2013 05:15:52 +0400 | From | Evgeny Boger <> | Subject | Re: [PATCH 1/1] Add strong pullup emulation to w1-gpio master driver. |
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11/12/2013 12:01 PM, David Fries: > On Tue, Nov 12, 2013 at 05:07:14AM +0400, Evgeny Boger wrote: >> +David Fries <david@fries.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?
Sure. The push-pull GPIO on common SoC's are usually able to provide up to 10 mA of current.
> > 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@gmail.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 wasn't able to find any support for the latter statement. It looks like the strong pull-up should be enabled *after* the last bit has been sent so no need to set strong pull-up there.
However setting strong pullup for last bit makes sense just to ensure we fit to 10us time window.
On the other hand, I didn't experienced any problems with the proposed implementation.
> >>>> 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
-- Evgeny
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