linux-kernel - Re: [PATCH] drivers: pci: convert generic host controller to DT host bridge creation API

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Message-ID: <20140821230740.GJ13147@bart.dudau.co.uk>
Date:	Fri, 22 Aug 2014 00:07:40 +0100
From:	Liviu Dudau <liviu@...au.co.uk>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Liviu Dudau <Liviu.Dudau@....com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	Jingoo Han <jg1.han@...sung.com>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Suravee Suthikulanit <suravee.suthikulpanit@....com>,
	linux-pci <linux-pci@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	LAKML <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] drivers: pci: convert generic host controller to DT host
 bridge creation API

On Wed, Aug 20, 2014 at 09:39:27PM +0200, Arnd Bergmann wrote:
> On Wednesday 20 August 2014, Liviu Dudau wrote:
> > That has been the general approach of my patchset up to v9. But, as Bjorn has
> > mentioned in his v8 review and I have put in my cover letter, the regular
> > aproach means that architectures that use pci_scan_root_bus() will have to
> > drop their one liner and replace it with the more verbose of_create_pci_host_bridge()
> > followed by pci_scan_child_bus() and pci_bus_add_devices() (basically, the content
> > of pci_scan_root_bus()). For those architectures it will lead to a net increase of
> > lines of code.
> 
> I'll try to get hold of Bjorn here and discuss it with him in person. I'd
> rather see a few extra lines in each driver than the complexity of callback
> funtions.
> 
> > The patch for pci-host-generic.c is the first to use the callback setup function, but
> > not the only one. My PCI host bridge driver for Juno has the same need, and I'm betting
> > all other host bridge controllers will use it as it will be the only opportunity to
> > finish the controller setup before we start scanning the child busses. I'm trying to
> > balance ease of read vs ease of use here and it is the best version I've come up with
> > so far.
> 
> My main objection to the new approach is that it's different from most other
> subsystems doing the same thing. For a person reading the pci host driver
> implementation, when they are familiar with other device drivers, I think it's
> much clearer what is going on when smaller functions are called in sequence
> than to see one function passed into some other interface that you now have
> to read as well in order to understand when it gets called.

Would it be more clear if (when) the currently opaque sysdata becomes a structure
to be filled by the host bridge driver with a .setup member pointing to the
callback? That would be my preferred way of expressing the API, tbh, but it means
duplicating the existing pci_{create,scan}_root_bus() functions.

Best regards,
Liviu

> 
> 	Arnd
> 

-- 
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 One small step
   for me ...

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