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

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Date:	Wed, 20 May 2015 13:22:13 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Will Deacon <will.deacon@....com>
Cc:	Robert Richter <robert.richter@...iumnetworks.com>,
	Robert Richter <rric@...nel.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Tirumalesh Chalamarla <tchalamarla@...ium.com>,
	Radha Mohan Chintakuntla <rchintakuntla@...ium.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 4/4] arm64: gicv3: its: Increase FORCE_MAX_ZONEORDER for
 Cavium ThunderX

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:
> > > On Mon, May 11, 2015 at 10:14:38AM +0100, Robert Richter wrote:
> > > > On 05.05.15 11:53:29, Will Deacon wrote:
> > > > > On Sun, May 03, 2015 at 09:49:32PM +0100, Robert Richter wrote:
> > > > > > From: Radha Mohan Chintakuntla <rchintakuntla@...ium.com>
> > > > > > 
> > > > > > In case of ARCH_THUNDER, there is a need to allocate the GICv3 ITS table
> > > > > > which is bigger than the allowed max order. So we are forcing it only in
> > > > > > case of 4KB page size.
> > > > > 
> > > > > Does this problem disappear if the ITS driver uses dma_alloc_coherent
> > > > > instead? That would also allow us to remove the __flush_dcache_area abuse
> > > > > from the driver.
> > > > 
> > > > __get_free_pages() is also used internally in dma_alloc_coherent().
> > > > 
> > > > There is another case if the device brings dma mem with it. I am not
> > > > sure if it would be possible to assign some phys memory via devicetree
> > > > to the interrupt controller and then assign that range for its table
> > > > allocation.
> > > > 
> > > > Another option would be to allocate a hugepage. This would require
> > > > setting up hugepages during boottime. I need to figure out whether
> > > > that could work.
> > 
> > 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...

	M.
-- 
Without deviation from the norm, progress is not possible.
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