linux-kernel - Re: [PATCH 12/14] arm64: Check for selected granule support

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Message-ID: <20150813172946.GD4602@e104818-lin.cambridge.arm.com>
Date:	Thu, 13 Aug 2015 18:29:47 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
Cc:	Steve Capper <steve.capper@...aro.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 12/14] arm64: Check for selected granule support

On Thu, Aug 13, 2015 at 03:45:07PM +0100, Suzuki K. Poulose wrote:
> On 13/08/15 13:28, Steve Capper wrote:
> >On 13 August 2015 at 12:34, Suzuki K. Poulose <suzuki.poulose@....com> wrote:
> >>  __enable_mmu:
> >>+       mrs     x1, ID_AA64MMFR0_EL1
> >>+       ubfx    x2, x1, #ID_AA64MMFR0_TGran_SHIFT, 4
> >>+       cmp     x2, #ID_AA64MMFR0_TGran_ENABLED
> >>+       b.ne    __no_granule_support
> >>         ldr     x5, =vectors
> >>         msr     vbar_el1, x5
> >>         msr     ttbr0_el1, x25                  // load TTBR0
> >>@@ -626,3 +643,8 @@ __enable_mmu:
> >>         isb
> >>         br      x27
> >>  ENDPROC(__enable_mmu)
> >>+
> >>+__no_granule_support:
> >>+       wfe
> >>+       b __no_granule_support
> >>+ENDPROC(__no_granule_support)
> >>--
> >>1.7.9.5
> >>
> >
> >Is is possible to tell the user that the kernel has failed to boot due
> >to the kernel granule being unsupported?
> 
> We don't have anything up at this time. The "looping address" is actually a clue
> to the (expert) user. Not sure we can do something, until we get something like DEBUG_LL(?)

No.

> Or we should let it continue and end in a panic(?). The current situation can boot a
> multi-cluster system with boot cluster having the Tgran support(which doesn't make a
> strong use case though). I will try out some options and get back to you.

If the boot CPU does not support 16KB pages, in general there isn't much
we can do since the console printing is done after we enabled the MMU.
Even mapping the UART address requires fixmap support and the PAGE_SIZE
is hard-coded in the kernel image. The DT is also mapped at run-time.

While in theory it's possible to fall back to a 4KB page size just
enough to load the DT and figure out the early console, I suggest we
just live with the "looping address" clue.

-- 
Catalin
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