linux-kernel - Re: [PATCH v11 10/27] iommu/exynos: use managed device helper functions

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Message-id: <20140321134900.ef9b7ac83443ae2e0bc5565b@samsung.com>
Date:	Fri, 21 Mar 2014 13:49:00 +0900
From:	Cho KyongHo <pullip.cho@...sung.com>
To:	Tomasz Figa <tomasz.figa@...il.com>
Cc:	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>,
	Sachin Kamat <sachin.kamat@...aro.org>,
	Joerg Roedel <joro@...tes.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Grant Grundler <grundler@...omium.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 10/27] iommu/exynos: use managed device helper functions

On Thu, 20 Mar 2014 11:44:50 +0100, Tomasz Figa wrote:
> On 20.03.2014 11:03, Cho KyongHo wrote:
> > On Wed, 19 Mar 2014 13:08:42 +0100, Tomasz Figa wrote:
> >> On 19.03.2014 10:01, Sachin Kamat wrote:
> >>> On 19 March 2014 14:29, Cho KyongHo <pullip.cho@...sung.com> wrote:
> >>>> On Tue, 18 Mar 2014 16:14:53 +0100, Tomasz Figa wrote:
> >>>>> On 18.03.2014 12:09, Cho KyongHo wrote:
> >>>>>> On Fri, 14 Mar 2014 20:52:43 +0530, Sachin Kamat wrote:
> >>>>>>> Hi KyongHo,
> >>>>>>>
> >>>>>>> On 14 March 2014 10:35, Cho KyongHo <pullip.cho@...sung.com> wrote:
> >>>>>>>> This patch uses managed device helper functions in the probe().
> >>>>>>>>
> >>>>>>>> Signed-off-by: Cho KyongHo <pullip.cho@...sung.com>
> >>>>>>>> ---
> >>>>>>> [snip]
> >>>>>>>
> >>>>>>>> +       data->clk = devm_clk_get(dev, "sysmmu");
> >>>>>>>> +       if (IS_ERR(data->clk)) {
> >>>>>>>> +               dev_info(dev, "No gate clock found!\n");
> >>>>>>>> +               data->clk = NULL;
> >>>>>>>> +       }
> >>>>>>>
> >>>>>>> Why aren't you returning from here upon error?
> >>>>>>
> >>>>>> It is for the case of a System MMU which does not need clock gating.
> >>>>>>
> >>>>>
> >>>>> Are there really such cases?
> >>>>>
> >>>>
> >>>> Yes.
> >>>> Especially in the case of initial stage of new SoC development.
> >>>>
> >>>> I have experianced some software workaround for H/W restriction
> >>>> needs prevention of clock gating for some devices.
> >>>
> >>> So aren't these basically some exceptions/hacks rather than the usual way
> >>> of functioning of the device?
> >>>
> >>
> >> This actually raises a good question, whether we really need to support
> >> such early development SoC versions in mainline.
> >>
> >> Another thing is that if you need to assure that a clock is ungated, you
> >> must acquire it and prepare_enable explicitly, so I don't think this
> >> kind of handling is correct.
> >>
> > On early development step of a new SoC, clock related stuffs and
> > some device drivers like display controller are usually developed in parallel.
> >
> > In that case, -ENOENT from clk_get() must not treated as an error.
> > "[PATCH v11 20/17] iommu/exynos: allow having multiple System MMUs for a master H/W"
> > patch distinguishes -ENOENT from other error values returned by devm_clk_get().
> 
> I still don't think upstream is right place for such development hacks 
> and such assumption will mask potential errors caused by clocks 
> unspecified in DT.
> 
> If such thing is needed for development, an extra patch might be kept in 
> development tree, until clock driver is implemented or a dummy 
> fixed-rate clock might be specified in DT.
> 

Ok.

Now I understand.

Error from clk_get() will be failure of probe in the next patch series.

Thanks.

KyongHo

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