linux-kernel - Re: [RFC PATCH 08/17] powerpc/e500: Remove conditional "lwsync" substitution

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Message-Id: <FCE81732-AD2B-4EF2-BC31-E24D0F9D41A9@kernel.crashing.org>
Date:	Thu, 10 Nov 2011 14:34:32 -0600
From:	Kumar Gala <galak@...nel.crashing.org>
To:	"Moffett, Kyle D" <Kyle.D.Moffett@...ing.com>
Cc:	Scott Wood <scottwood@...escale.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Baruch Siach <baruch@...s.co.il>,
	Timur Tabi <B04825@...escale.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [RFC PATCH 08/17] powerpc/e500: Remove conditional "lwsync" substitution


On Nov 10, 2011, at 2:27 PM, Moffett, Kyle D wrote:

> On Nov 10, 2011, at 12:03, Scott Wood wrote:
>> On Thu, Nov 10, 2011 at 10:42:25AM -0600, Kumar Gala wrote:
>>> 
>>> On Nov 10, 2011, at 10:31 AM, Scott Wood wrote:
>>> 
>>>> On Thu, Nov 10, 2011 at 07:40:04AM -0600, Kumar Gala wrote:
>>>>> Nak, we can run an e500mc in a mode that is compatible with e500v1/v2.
>>>>> I see no reason to change the support we have there.
>>>> 
>>>> What "mode" do you mean?  DCBZ32?  We don't support using that currently,
>>>> and I'd imagine the performance implication would be such that you'd
>>>> never want to do it unless it's the only way to make some piece of legacy
>>>> software work.
>>> 
>>> Correct, DCBZ32, we've had customers that go down this path.
>> 
>> For running legacy software, or for multiplatform Linux kernels?
>> 
>> And if you're willing to toss performance away for this goal, why do you
>> need lwsync? :-)
>> 
>> DCBZ32 is not a "mode that is compatible with v1/v2", BTW.  It only
>> affects cache block size (for dcbz/dcba only), not SPE versus FP, not
>> changes in power management, not changes in machine check handling, etc.
>> 
>> Using DCBZ32 for the kernel would also complicate switching the kernel to
>> dcbzl, to support enabling DCBZ32 for certain userspace apps (a more
>> likely use case) without making it systemwide.
> 
> So, as far as I can tell the kernel doesn't even try to touch DCBZ32.

Correct, it was my thinking I'd get there an add this one day, that day never came.

> Even if it did, if you are building a new kernel that includes this patch,
> surely you can actually build a proper e500mc kernel instead of trying to
> build a new kernel to run on hardware it wasn't designed to run on, right?
> 
> I think the bigger issue is the fact that building a PPC_BOOK3E_64 kernel
> with both e5500 and PowerPC A2 support turned on will not actually run on
> both.  Before my v1-patch-series, machine-check handling is messed up for
> PowerPC A2, and afterwards cacheline sizes are messed up for e5500.

That might be, but who is asking or wanting to run a BOOK3E_64 kernel on both.  I'm guessing there are a number of issues with this.

> Does this mean that PPC_BOOK3E_64 needs to be split into two separate
> Book 3-III families the same way that 32-bit has been split?  Is there
> another way around it?

No idea, we have to ask Ben how much he cares.  I don't see any FSL customers pushing us to run the same kernel on A2 and P5020 (or future FSL devices).

- k--
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