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

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Date:	Thu, 11 Feb 2016 14:43:02 -0600
From:	Suman Anna <s-anna@...com>
To:	Kishon Vijay Abraham I <kishon@...com>,
	Paul Walmsley <paul@...an.com>
CC:	Tony Lindgren <tony@...mide.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>
Subject: Re: [PATCH v3 3/3] pci: dra7xx: use pdata callbacks to perform reset

On 02/09/2016 11:38 PM, Kishon Vijay Abraham I wrote:
> Hi,
> 
> On Wednesday 10 February 2016 07:12 AM, Suman Anna wrote:
>> Hi Paul,
>>
>> On 02/09/2016 01:36 PM, Paul Walmsley wrote:
>>> Hi Suman 
>>>
>>> On Tue, 9 Feb 2016, Suman Anna wrote:
>>>
>>>> On 02/09/2016 02:49 AM, Paul Walmsley wrote:
>>>>> On Mon, 8 Feb 2016, Suman Anna wrote:
>>>>>> On 02/07/2016 08:48 PM, Paul Walmsley wrote:
>>>>>>> On Tue, 2 Feb 2016, Kishon Vijay Abraham I wrote:
>>>>>>>
>>>>>>>> Paul, what do you think is the best way forward to perform reset?
>>>>>>>
>>>>>>> Many of the IP blocks with PRM hardreset lines are processor IP blocks. 
>>>>>>> Those often need special reset handling to ensure that WFI/HLT-like 
>>>>>>> instructions are executed after reset.  This special handling ensures that 
>>>>>>> the IP blocks' bus initiator interfaces indicate that they are in standby 
>>>>>>> to the PRCM - thus allowing power management for the rest of the chip to 
>>>>>>> work correctly.
>>>>>>>
>>>>>>> But that doesn't seem to be the case with PCIe - and maybe others - 
>>>>>>> possibly some of the MMUs?  
>>>>>>
>>>>>> Yeah, the sequencing between clocks and resets would still be the same
>>>>>> for MMUs, so, adding the custom flags for MMUs is fine.
>>>>>
>>>>> I'm curious as to whether HWMOD_CUSTOM_HARDRESET is needed for the MMUs.  
>>>>> We've stated that the main point of the custom hardreset code is to handle 
>>>>> processors that need to be placed into WFI/HLT, but it doesn't seem like 
>>>>> there would be an equivalent for MMUs.  Thoughts?
>>>>
>>>> The current OMAP IOMMU code already leverages the pdata ops for
>>>> performing the resets, so not adding the flags would also require
>>>> additional changes in the driver.
>>>>
>>>> Also, the reset lines controlling the MMUs actually also manage the
>>>> reset for all the other sub-modules other than the processor cores
>>>> within the sub-systems. We have currently different issues (see [1] for
>>>> eg. around the IPU sub-system entering RET in between), so from a PM
>>>> point of view, we do prefer to place the MMUs also in reset when we are
>>>> runtime suspended.
>>>
>>> Should we reassert hardreset in _idle() for IP blocks that don't have 
>>> HWMOD_CUSTOM_HARDRESET set on them?  Would that allow us to use this 
>>> mechanism for the uncore hardreset lines, or are there other quirks?
>>>
>>> Also - would that address the potential issue that you mentioned with the 
>>> PCIe block, or is that a different issue?
>>
>> Yeah, I think that would address the PCIe block issue in terms of reset
>> state balancing between pm_runtime_get_sync() and pm_runtime_put()
>> calls. Right now, they are unbalanced. The PCIe block is using these
>> only in probe and remove, so it should work for that IP.
> 
> As I mentioned before this would result in undesired behavior during
> suspend/resume cycle in PCIe. (This should be okay for the current mainline
> code but would break once we add suspend/resume support for PCIe).

Yeah, I was wondering if some peripheral would want only the clock to be
controlled during _idle() and not reset. Even then for the PCIe case
that you are talking about, going through a pm_runtime_get_sync(),
pm_runtime_put_sync()/pm_runtime_put() deasserts the resets everytime
_enable() is called. Right now, the code block has ignored the return
value from the _hardreset_deassert(), but if you check it and bail out,
then your get_sync() would start failing from the second invocation.

Can you elaborate more on what kind of issues you will see on
suspend/resume cycle with PCIe? Do note that _idle() gets called through
_od_suspend_no_irq() in omap_device.c if your runtime status is not
suspended. I had to manage the runtime status in the IPU/DSP
suspend/resume code to deal with the reset
(omap_device_assert_hardreset) and clock sequences in
_idle()/omap_device_idle()

regards
Suman