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Date:   Mon, 7 Jun 2021 08:19:18 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Nick Kossifidis <mick@ics.forth.gr>
Cc:     guoren@kernel.org, anup.patel@wdc.com, palmerdabbelt@google.com,
        arnd@arndb.de, wens@csie.org, maxime@cerno.tech,
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        linux-arch@vger.kernel.org, linux-sunxi@lists.linux.dev,
        Guo Ren <guoren@linux.alibaba.com>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: [RFC PATCH v2 06/11] riscv: pgtable: Add DMA_COHERENT with
 custom PTE attributes
Message-ID: <20210607061918.GA24060@lst.de>
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On Sun, Jun 06, 2021 at 08:22:14PM +0300, Nick Kossifidis wrote:
> This patch violates the Privilege Spec section 4.4.1 that clearly states:
>
> "Bits63–54 are reserved for future standard use and must be zeroed by 
> software for forward compatibility"
>
> Standard use means that valid values can only be defined by the Priv. Spec, 
> not by the vendor (otherwise they'd be marked as "custom use" or "platform 
> use"), and since they "must" be zeroed by software we 'll be violating the 
> Privilege Spec if we do otherwise.

Yes, that is why I think it is a no-go.