linux-kernel - Re: [PATCH v11 17/27] iommu/exynos: remove calls to Runtime PM API functions

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Message-id: <20140320210705.19aa5192f55f263ccf271b32@samsung.com>
Date:	Thu, 20 Mar 2014 21:07:05 +0900
From:	Cho KyongHo <pullip.cho@...sung.com>
To:	Tomasz Figa <tomasz.figa@...il.com>
Cc:	Grant Grundler <grundler@...omium.org>,
	Tomasz Figa <t.figa@...sung.com>,
	Linux DeviceTree <devicetree@...r.kernel.org>,
	Linux Samsung SOC <linux-samsung-soc@...r.kernel.org>,
	Prathyush <prathyush.k@...sung.com>,
	Joerg Roedel <joro@...tes.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Sachin Kamat <sachin.kamat@...aro.org>,
	Linux IOMMU <iommu@...ts.linux-foundation.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Varun Sethi <Varun.Sethi@...escale.com>,
	Antonios Motakis <a.motakis@...tualopensystems.com>,
	Linux ARM Kernel <linux-arm-kernel@...ts.infradead.org>,
	Rahul Sharma <rahul.sharma@...sung.com>
Subject: Re: [PATCH v11 17/27] iommu/exynos: remove calls to Runtime PM API
 functions

On Wed, 19 Mar 2014 19:51:21 +0100, Tomasz Figa wrote:
> On 19.03.2014 19:37, Grant Grundler wrote:
> > On Wed, Mar 19, 2014 at 10:30 AM, Tomasz Figa <t.figa@...sung.com> wrote:
> > ...
> >> As I said, AFAIK the trend is to get rid of ordering by initcalls and make
> >> sure that drivers can handle missing dependencies properly, even for
> >> "services" such as DMA, GPIO, clocks and so on, which after all are provided
> >> by normal drivers like other.
> >
> > Ok - I'm not following the general kernel dev trends. initcall()
> > levels are easy to understand and implement. So I would not be in a
> > hurry to replace them.
> >
> 
> Well, initcall level is still a way to satisfy most of dependencies, 
> i.e. all client devices with higher initcall levels will probe 
> successfully. However the other case needs to be handled as well - in 
> this case the IOMMU binding code needs to defer probe of client driver 
> if respective IOMMU is not yet available.

I now understand what is deferred probing you mentioned.
However, I worry that many existing drivers are not ready
for deferred probing.

But still I wonder if System MMU driver need to be probed in the same
initcall level.

> >>> ps. I've written IOMMU support for four different IOMMUs on three
> >>> operating systems (See drivers/parisc for two linux examples). But I
> >>> still feel like I at best have 80% understanding of how this one is
> >>> organized/works. Abstract descriptions and convoluted code have been
> >>> handicapping me (and lack of time to dig further).
> >>
> >>
> >> Well, this is one of my concerns with this driver. It isn't easy to read
> >> (and so review, maintain, extend and debug found issues).
> >
> > My postscript comment was more to explain why I'm not confident in my
> > opinion - not a reason to reject the patch series.  I still consider
> > the whole series as a step forward. But I'm not the expert here.
> 
> I fully agree with you. Other than the issues mentioned in review, the 
> patches are definitely a step forward. I'd even say that all the patches 
> that have nothing to do with device tree could be merged in their 
> current form and the code refined later. It doesn't mean that patches 
> shouldn't be reviewed now and issues spotted reported, even if they 
> could be fixed later - this is for the IOMMU subsystem maintainer to decide.
> 
> As for patches related to DT support, more care needs to be taken, as 
> bindings should be designed with stability in mind, so the refining 
> process should happen at review stage.
> 
> > Right now, with ~30 patches posted by the exynos iommu (official?)
> > maintainer, no one else who has a clue will attempt to fix or clean up
> > those kinds of problems.  i.e. it's useful to enable others to fix
> > what are essentially unspecified "design pattern" issues.
> 
> Agreed.

Let me wait for the way of binding System MMU and its master developed by Marek.

Regards,

KyongHo
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