linux-kernel - Re: [PATCH 7/7] powerpc: Added PCI MSI support using the HSTA module

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Message-ID: <1541869.UgdfUgWYNB@mexican>
Date:	Tue, 25 Feb 2014 16:54:58 +1100
From:	Alistair Popple <alistair@...ple.id.au>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Arnd Bergmann <arnd@...db.de>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 7/7] powerpc: Added PCI MSI support using the HSTA module

On Sat, 22 Feb 2014 07:41:26 Benjamin Herrenschmidt wrote:
> On Fri, 2014-02-21 at 15:33 +0100, Arnd Bergmann wrote:

[...]

> 
> Should we (provided it's possible in HW) create two ranges instead ? One
> covering RAM and one covering MSIs ? To avoid stray DMAs whacking random
> HW registers in the chip ...
> 

The thought occurred to me but I figured if we had stray DMAs then they could 
already whack random bits of system memory which would likely break your 
system anyway so I wasn't sure how much we'd gain. I guess whacking random HW 
registers is arguably a bit worse though.

I did a bit of digging into the HW documentation and it looks like it _may_ be 
possible to create a second range that would limit access to a subset of HW 
registers, although there doesn't seem to be much flexibility. Personally I'm 
not sure it justifies the work, but I'm happy to look into it a bit more if 
you feel it's important?

- Alistair

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