linux-kernel - Re: [PATCH v6 06/30] PCI: Combine PCI domain and bus number in u32 arg

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Message-ID: <20150312194927.GC7346@google.com>
Date:	Thu, 12 Mar 2015 14:49:27 -0500
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Yijing Wang <wangyijing@...wei.com>
Cc:	Jiang Liu <jiang.liu@...ux.intel.com>, linux-pci@...r.kernel.org,
	Yinghai Lu <yinghai@...nel.org>, linux-kernel@...r.kernel.org,
	Marc Zyngier <marc.zyngier@....com>,
	linux-arm-kernel@...ts.infradead.org,
	Russell King <linux@....linux.org.uk>, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Tony Luck <tony.luck@...el.com>, linux-ia64@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Guan Xuetao <gxt@...c.pku.edu.cn>, linux-alpha@...r.kernel.org,
	linux-m68k@...ts.linux-m68k.org, Liviu Dudau <liviu@...au.co.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v6 06/30] PCI: Combine PCI domain and bus number in u32
 arg

On Thu, Mar 12, 2015 at 08:14:40PM +0800, Yijing Wang wrote:
> On 2015/3/12 9:29, Bjorn Helgaas wrote:
> > On Mon, Mar 09, 2015 at 10:34:03AM +0800, Yijing Wang wrote:
> >> Currently, we use int type for bus number in
> >> pci_create_root_bus(), pci_scan_root_bus() and
> >> pci_scan_bus_legacy. Because PCI bus number
> >> always <= 255, so we could change the bus number
> >> argument type to u32, and combine PCI domain and
> >> bus number in one. 
> > 
> > This makes no sense.  Or rather, it only states the obvious: an 8-bit value
> > and a 16-bit value will both fit in a 32-bit value.  But it doesn't say
> > *why* you think it's a good idea to pass a single value that contains both
> > domain and bus numbers.  The obvious thing is to pass two separate values,
> > and you don't say why passing a single combined value is better.
> 
> Sorry for my poor description for this patch, I combined the domain and bus, because
> I think now we have too many args at pci_scan_root_bus() or other scan functions,
> 
> struct pci_bus *pci_scan_root_bus(struct device *parent, int bus,
> 		struct pci_ops *ops, void *sysdata, struct list_head *resources)
> 
> Now we have five args yet, plus the new introduced domain and pci_host_bridge_ops,
> it will become 7.
> 
> I thought introduced a new structure which contain the necessary info to scan root bus/ host bridge,
> 
> E.g
> 
> struct pci_scan_info {
> 	int bus;
> 	struct device *parent;
> 	struct pci_ops *ops;
> 	void *sysdata;
> 	struct list_head *resource;
> 	int domain;
> 	struct pci_host_bridge_ops;
> }
> 
> Do you like this one or keep it like now ?
> 
> pci_scan_root_bus(struct device *parent, int domain, int bus,
> 		struct pci_ops *ops, void *sysdata, struct list_head *resources, struct pci_host_bridge_ops *ops)

I don't think reducing the number of arguments is a good argument for
squashing some of them together.

I don't really want to add a structure like that because it adds management
complexity for all the callers because it contains per-bridge things (bus,
parent, domain, resource, sysdata).  Things like struct pci_ops and struct
pci_host_bridge_ops are much simpler because drivers can statically
allocate a single copy and use it for multiple devices.

I think it might make sense to put the struct pci_ops pointer inside struct
pci_host_bridge_ops.  That would get rid of one of the arguments.

You might also be able to get rid of the "bus" argument, since the caller
should be passing an IORESOURCE_BUS resource in the resource list, and
"bus" should be the same as res->start.

Bjorn
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