linux-kernel - Re: [PATCH V5 2/3] ARM64 LPC: Add missing range exception for special ISA

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Date:   Wed, 09 Nov 2016 10:12:59 +1100
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Mark Rutland <mark.rutland@....com>,
        "zhichang.yuan" <yuanzhichang@...ilicon.com>
Cc:     catalin.marinas@....com, will.deacon@....com, robh+dt@...nel.org,
        bhelgaas@...gle.com, olof@...om.net, arnd@...db.de,
        linux-arm-kernel@...ts.infradead.org, lorenzo.pieralisi@....com,
        linux-kernel@...r.kernel.org, linuxarm@...wei.com,
        devicetree@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-serial@...r.kernel.org, minyard@....org, liviu.dudau@....com,
        zourongrong@...il.com, john.garry@...wei.com,
        gabriele.paoloni@...wei.com, zhichang.yuan02@...il.com,
        kantyzc@....com, xuwei5@...ilicon.com, marc.zyngier@....com
Subject: Re: [PATCH V5 2/3] ARM64 LPC: Add missing range exception for
 special ISA

On Tue, 2016-11-08 at 11:49 +0000, Mark Rutland wrote:
> 
> My understanding of ISA (which may be flawed) is that it's not part of
> the PCI host bridge, but rather on x86 it happens to share the IO space
> with PCI.

Sort-of. On some systems it actually goes through PCI and there's a
PCI->ISA bridge that uses substractive decoding to the legacy devices.

> So, how about this becomes:
> 
>   Hisilicon Hip06 SoCs implement a Low Pin Count (LPC) controller, which
>   provides access to some legacy ISA devices.
> 
> I believe that we could theoretically have multiple independent LPC/ISA
> busses, as is possible with PCI on !x86 systems. If the current ISA code
> assumes a singleton bus, I think that's something that needs to be fixed
> up more generically.
> 
> I don't see why we should need any architecture-specific code here. Why
> can we not fix up the ISA bus code in drivers/of/address.c such that it
> handles multiple ISA bus instances, and translates all sub-device
> addresses relative to the specific bus instance?

What in that code prevents that today ?

Cheers,
Ben.