linux-kernel - Re: [PATCH 4/4] arm64: gicv3: its: Increase FORCE_MAX

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Message-ID: <555DD0C0.3010607@arm.com>
Date:	Thu, 21 May 2015 13:34:08 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Robert Richter <robert.richter@...iumnetworks.com>
CC:	Catalin Marinas <Catalin.Marinas@....com>,
	Robert Richter <rric@...nel.org>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tirumalesh Chalamarla <tchalamarla@...ium.com>,
	Radha Mohan Chintakuntla <rchintakuntla@...ium.com>
Subject: Re: [PATCH 4/4] arm64: gicv3: its: Increase FORCE_MAX_ZONEORDER for
 Cavium ThunderX

[off the list]

On 21/05/15 13:13, Robert Richter wrote:
> On 21.05.15 09:35:47, Marc Zyngier wrote:
>> On 20/05/15 17:48, Catalin Marinas wrote:
>>> On Wed, May 20, 2015 at 02:31:59PM +0200, Robert Richter wrote:
>>>> On 20.05.15 13:22:13, Marc Zyngier wrote:
>>>>> On Tue, 12 May 2015 18:24:16 +0100
>>>>> Will Deacon <will.deacon@....com> wrote:
>>>>>> On Tue, May 12, 2015 at 05:20:49PM +0100, Robert Richter wrote:
>>>>>>> On 12.05.15 13:30:57, Will Deacon wrote:
>>>>
>>>>>>> For allocation of 16MB cont. phys mem of a defconfig kernel (4KB
>>>>>>> default pagesize) I see this different approaches:
>>>>>>
>>>>>> 16MB sounds like an awful lot. Is this because you have tonnes of MSIs or
>>>>>> a sparse DeviceID space or both?
>>>>>
>>>>> That's probably due to the sparseness of the DeviceID space. With some
>>>>> form of bridge number encoded on top of the BFD number, the device
>>>>> table is enormous, and I don't see a nice way to avoid it...
>>>>
>>>> Right. At the momement out of 21 bits (16MB) we currently have 2 spare
>>>> bits, which reduces the actually size used to 4MB. Though, for the
>>>> current cpu model we can reduce it at least to 8MB total.
>>>>
>>>> I will come up with an additional patch setting this to 8MB.
>>>>
>>>> As said before, I also write on a patch to use CMA.
>>>
>>> Can we not reserve a chunk of memory and pass the information to the
>>> kernel via DT (/memreserve/ and a new GIC-specific binding)?
>>
>> That would have to be done on a per-table basis then. And how would that
>> work with ACPI? I don't think the ACPI ITS table specifies anything in
>> that respect.
>>
>> We're just facing the horrible reality that linear tables are not very
>> well suited to sparse addressing. Nobody copied the VAX MMU model for a
>> reason... until now.
> 
> We could still fall back to mem alloc if the DT or ACPI does not
> provide a base address for the table.
> 
> For know I would prefer to just implement mem allocation with CMA.

I suppose your ITS implementation doesn't have support for the indirect
tables (where bit 62 of GITS_BASERn can be 1)? If it did, that would
solve most of the issues.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...
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