linux-kernel - Re: [PATCH 2/4] soc: mediatek: Init MT8173 scpsys driver earlier

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Date:	Mon, 4 Jan 2016 10:52:51 +0800
From:	James Liao <jamesjj.liao@...iatek.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	<devicetree@...r.kernel.org>, <srv_heupstream@...iatek.com>,
	Kevin Hilman <khilman@...nel.org>,
	<linux-kernel@...r.kernel.org>,
	<linux-mediatek@...ts.infradead.org>,
	Sascha Hauer <kernel@...gutronix.de>,
	Matthias Brugger <matthias.bgg@...il.com>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/4] soc: mediatek: Init MT8173 scpsys driver earlier

Hi Arnd,

On Thu, 2015-12-31 at 15:45 +0100, Arnd Bergmann wrote:
> On Thursday 31 December 2015 17:16:34 James Liao wrote:
> > 
> > """
> > Take a example for our IOMMU(M4U) and SMI.  The IOMMU which is
> > subsys_init defaultly will depend on SMI. 
> > The SMI is a bridge between m4u and the Multimedia HW.  About the HW
> > block diagram and more other information please help check  [1].
> > SMI is responsible to enable/disable iommu and help transfer data for
> > each Multimedia HW,  Both have to wait until the power and the clocks is
> > enabled.  
> > 
> > So our iommu should probe done after smi, smi should be after
> > power-domain, and all the iommu consumer(display/vdec/venc/camera etc.)
> > should be after the iommu.
> > Then all the multimedia module will be delayed by power-domain who is
> > module_init currently.
> > 
> > After grep, we get some example whose  pm is not module_init:
> > core_initcall(exynos4_pm_init_power_domain);
> > subsys_initcall(imx_pgc_init);
> > 
> > So we expect move the power-domain initial more earlier too.  The
> > power-domain seems to be a basic module like ccf.
> > Is there some special reason about we should use module_init,  or do you
> > have any concern if we change it?
> > Thanks.
> 
> Ok, got it. Generally, we should try to avoid using the earlier initcall
> levels, but a few things like clock controllers, iommus etc are special
> enough that we need to make sure their dependencies are there by the
> time those are probed.
> 
> Please put your explanation above into the patch changelog and add a code
> comment about the IOMMU next to the subsys_initcall() so it doesn't
> accidentally get changed when someone tries to do a code cleanup.

OK, I'll add comments in next patch. Thanks for your comments.


Best regards,

James

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