RE: [EXT] Re: The problem about arm64: io: Relax implicit barriers in default I/O accessors

-----Original Message-----
From: Will Deacon <will@kernel.org>
Sent: Monday, August 9, 2021 8:51 AM
To: Frank Li <frank.li@nxp.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>; Zhi Li <redacted>;
Shenwei Wang [off-list ref]; Han Xu [off-list ref]; Nitin Garg
[off-list ref]; Jason Liu [off-list ref]; linux-arm-
kernel@lists.infradead.org
Subject: Re: [EXT] Re: The problem about arm64: io: Relax implicit barriers
in default I/O accessors

Caution: EXT Email

On Thu, Jul 22, 2021 at 07:14:19PM +0000, Frank Li wrote:

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On Wed, Jun 23, 2021 at 03:48:10PM +0000, Frank Li wrote:

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I think you had a support case open with Arm [1] which I'm not

able

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to

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access -- please can you ask them about the two examples above?

Still not get feedback from ARM.

Just wondering if you were able to solve this without the need to

change

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Linux?

Sorry for late reply

For CCI-500 and 550, ARM removed support for barrier transactions but

CCI-

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400 supports barrier transactions. With CCI-400 it is a valid

configuration

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to have SYSBARDISABLE LOW in Cortex-A processors. This change in Linux
kernel is assuming that the SYSBARDISABLE is set to HIGH hence its not
correct change for all products having various versions of ARM CCI IP.

Frank Li

Deacon:

      Did you plan fix this problem by changing dma_wmb()?

No. As far as I understand this problem, you're driving SYSBARDISABLE
'low' yet you have your own bus fabric downstream of the CCI which doesn't
respect barrier transactions. Even if we bodge dma_wmb(), store-release to
non-cacheable memory cannot be made to work on your system as you're
effectively putting some of your non-coherent DMA devices into a separate
outer-shareable domain from the CPUs.

Does it means the Linux expect all DMA devices in outer-shareable domain instead
of system shared domain?  

Frank

So you have two options:

  1. Drive SYSBARDISABLE 'high' so that the CPU handles ordering for you

- or -

  2. Quirk Linux so that we patch dma_wmb() when we detect your system at
     runtime (so we can extend this in future if we need to emit a
different
     sequence for store release)

(1) is definitely the easiest option if it's possible.

Will

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