Re: [PATCH v1 3/3] ARM64 LPC: update binding doc

From: Arnd Bergmann <arnd@arndb.de>
To: Rongrong Zou <zourongrong@huawei.com>
Cc: linux-arm-kernel@lists.infradead.org,
	Rongrong Zou <zourongrong@gmail.com>,
	devicetree@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Corey Minyard <minyard@acm.org>,
	gregkh@linuxfoundation.org, Will Deacon <will.deacon@arm.com>,
	linux-kernel@vger.kernel.org, linuxarm@huawei.com,
	benh@kernel.crashing.org, liviu.dudau@arm.com
Subject: Re: [PATCH v1 3/3] ARM64 LPC: update binding doc
Date: Tue, 05 Jan 2016 13:19:48 +0100	[thread overview]
Message-ID: <2550495.K9prJVsVEi@wuerfel> (raw)
In-Reply-To: <568BB035.1050801@huawei.com>

On Tuesday 05 January 2016 19:59:49 Rongrong Zou wrote:
> 在 2016/1/5 0:34, Arnd Bergmann 写道:
> > On Tuesday 05 January 2016 00:04:19 Rongrong Zou wrote:
> >> 在 2016/1/4 19:13, Arnd Bergmann 写道:
> >>> On Sunday 03 January 2016 20:24:14 Rongrong Zou wrote:
> >>>> 在 2015/12/31 23:00, Rongrong Zou 写道:
> >> Ranges property can set empty, but this means 1:1 translation. the I/O
> >> port range is translated to MMIO address 0x00000001 00000000 to
> >> 0x00000001 00000004, it looks wrong else. I wonder if anyone get legacy
> >> I/O port resource from dts.
> >
> > As I said, nothing should really require the ranges property here, unless
> > you have a valid IORESOURCE_MEM translation. The code that requires
> > the ranges to be present is wrong.
> >
> 
> I think the openfirmware(DT) do not support for those unmapped I/O ports, because I
> must get resource by calling of_address_to_resource(), which have to call
> pci_address_to_pio() when resource type is IORESOURCE_IO. I'm sorry I have no
> better idea for this now. Maybe liviu can give me some opinions.

I think on x86 it works (or used to work, few people use open firmware on
x86 these days, and it may be broken), and the pci_address_to_pio() call
behaves differently when PCI_IOBASE is set. x86 never maps I/O ports into
memory mapped I/O addresses, they have their own way of accessing them
just like your platform.

> /**
>   * of_address_to_resource - Translate device tree address and return as resource
>   *
>   * Note that if your address is a PIO address, the conversion will fail if
>   * the physical address can't be internally converted to an IO token with
>   * pci_address_to_pio(), that is because it's either called to early or it
>   * can't be matched to any host bridge IO space
>   */
> int of_address_to_resource(struct device_node *dev, int index,
> 			   struct resource *r)

The problem here seems to be that the code assumes that either the I/O ports
are always mapped or they are never mapped (no PCI_IOBASE). We need to extend
it because now we can have the combination of the two.

> >> For ipmi driver, I can get I/O port resource by DMI rather than dts.
> >
> > No, the ipmi driver uses the resource that belongs to the platform
> > device already, you can't rely on DMI data to be present there.
> 
> Ipmi has a lot of way to be discovered(ACPI, DMI, hardcoded, hot-add,
> openfirmware and a few other), I think we just use one of them, not all of them.
> It depend on vendor's hardware solution actually.

I don't think we should mix multiple methods here: if the bus is described
in DT, all its children should be there as well. Otherwise you get into problems
e.g. if you have multiple instances of the LPC bus and the Linux I/O addresses
for one or more of them have an offset to the bus specific addresses.

The bus probe code decides what the Linux I/O port numbers are, but DMI
and other methods have no idea of the mapping. As long as there is only
one instance, using the first 0x1000 addresses with a 1:1 mapping saves
us a bit of trouble, but I'd be worried about relying on that assumption
too much.

	Arnd