linux-kernel - Re: [PATCH v3 3/3] pci: dra7xx: use pdata callbacks to perform reset

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Message-ID: <56AA84FE.7020001@ti.com>
Date:	Thu, 28 Jan 2016 15:15:42 -0600
From:	Suman Anna <s-anna@...com>
To:	Tony Lindgren <tony@...mide.com>
CC:	Kishon Vijay Abraham I <kishon@...com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	<richardcochran@...il.com>, Russell King <linux@....linux.org.uk>,
	<linux-omap@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <nsekhar@...com>,
	Paul Walmsley <paul@...an.com>
Subject: Re: [PATCH v3 3/3] pci: dra7xx: use pdata callbacks to perform reset

On 01/28/2016 12:31 PM, Tony Lindgren wrote:
> * Suman Anna <s-anna@...com> [160127 15:17]:
>> On 01/27/2016 12:56 PM, Tony Lindgren wrote:
>>> * Suman Anna <s-anna@...com> [160127 10:17]:
>>>> On 01/27/2016 11:31 AM, Tony Lindgren wrote:
>>>>> Why do you need another reset here? Can't you just implement PM runtime
>>>>> in the driver and do the usual pm_runtime_put_sync followed by
>>>>> pm_runtime_disable?
>>>>
>>>> The omap_hwmod_enable/disable code does not deal with hardresets (PRCM
>>>> reset lines) and so the pm_runtime_get_sync/put_sync only end up dealing
>>>> with clocks, and we need to invoke the reset functions separately.
>>>> Modules with softresets in SYSCONFIG are ok, as they are dealt with
>>>> properly.
>>>
>>> Hmm _reset() in omap_hwmod.c has this to call _assert_hardreset:
>>>
>>> 	if (oh->class->reset) {
>>> 		r = oh->class->reset(oh);
>>> 	} else {
>>> 		if (oh->rst_lines_cnt > 0) {
>>> 			for (i = 0; i < oh->rst_lines_cnt; i++)
>>> 				_assert_hardreset(oh, oh->rst_lines[i].name);
>>> 			return 0;
>>> 		} else {
>>> 			r = _ocp_softreset(oh);
>>> 			if (r == -ENOENT)
>>> 				r = 0;
>>> 		}
>>> 	}
>>
>> Right, hwmod code does the initial reset.
>>
>>> Care to explain what exactly the problem with the hwmod code not doing
>>> the reset on init?
>>
>> And we only need to deassert the reset in probe. Technically, we don't
>> need to assert first and deassert in probe, and that was a design choice
>> made by Kishon.
> 
> OK so if hwmod code has already done the reset, then why would you need
> to deassert reset in the device driver probe?

So the _reset() above asserts the reset for IPs with PRCM reset lines,
but module is not enabled (no register accesses even if clocks enabled).
The _enable() code bails out if there are PRCM reset lines (there are
varied IPs with resets including processors, and we really cannot enable
and idle it without loading some code that would have executed WFI).

> 
>>> And why do you need to do another reset in dra7xx_pcie_remove()?
>>
>> Primarily to restore the reset state back to what it was after the
>> driver remove gets called. We cannot call deassert twice without calling
>> a assert in between. Kishon had originally added the assert and deassert
>> only in probe, but nothing in remove, they ought to be deassert in probe
>> and assert in remove to match initial hardware state, and to also make
>> it work across multiple probe/remove.
> 
> I don't understand this part either.. Usually you just power up and init
> the registers to a sane state in a device driver probe and on exit just
> power down the device.

Yes, in the case of IPs with hard-reset lines, that init is left to the
drivers.

> 
>>>>> Basically I'm wondering how come we need these platform data callbacks
>>>>> at all.
>>>>
>>>> The hardresets are controlled through the
>>>> omap_device_assert(deassert)_hardreset functions, and since these are
>>>> limited to mach-omap2, we are invoking them through platform data callbacks.
>>>
>>> Right.. But I'm wondering about the why you need to do this in the
>>> driver at all part :)
>>
>> The initial reset at init time is okay, but hwmod _enable() bails out if
>> the resets lines are asserted. This was a change made long time back, I
>> believe to deal with the problems around the DSP enabling sequences. As
>> such, pm_runtime_get_sync() and put_sync() do not deassert and assert
>> the resets.
> 
> OK if the hwmod code does not deassert reset lines properly on enable,
> then that sounds like a bug that should be fixed instead of adding
> device specific work arounds.

As I said above, not all IPs with hard-reset lines have the same power
on/power off sequences.. IPs with only SYSCONFIG based soft-reset all
pretty much follow the PRCM HW_Auto idling, so dealing with them is
rather straightforward in the common hwmod code. I have had to do rather
funky stuff in our product kernels when doing suspend/resume on IOMMUs,
remoteprocs.

> Sorry to keep dragging this on a bit longer, but I think we need to
> hear Paul's comments on this one.

Yeah, it would be good to restart this discussion, as I will be adding
the DT support for the remoteproc devices a bit later.

regards
Suman