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

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Message-ID: <56BA958A.1020503@ti.com>
Date:	Tue, 9 Feb 2016 19:42:34 -0600
From:	Suman Anna <s-anna@...com>
To:	Paul Walmsley <paul@...an.com>
CC:	Kishon Vijay Abraham I <kishon@...com>,
	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

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.

The IPUs and DSPs in general would also place the reset lines asserted
when suspended, as the power up sequence almost always involves
releasing a reset line with the boot-up code on the processor detecting
that it is a power restore boot.

regards
Suman