linux-kernel - Re: [BUG 5.14] arm64/mm: dma memory mapping fails (in some cases)

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Message-ID: <20210921082007.GA29121@lst.de>
Date:   Tue, 21 Sep 2021 10:20:07 +0200
From:   Christoph Hellwig <hch@....de>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Mike Rapoport <rppt@...nel.org>, Christoph Hellwig <hch@....de>,
        David Hildenbrand <david@...hat.com>,
        Robin Murphy <robin.murphy@....com>,
        Alex Bee <knaerzche@...il.com>, Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [BUG 5.14] arm64/mm: dma memory mapping fails (in some cases)

On Mon, Sep 20, 2021 at 11:57:58AM +0100, Catalin Marinas wrote:
> > As this WARN_ON(pfn_valid()) is only present in dma_map_resource() it's
> > probably safe to drop it entirely. 
> 
> I agree, we should drop it. IIUC dma_map_resource() does not create any
> kernel mapping to cause problems with attribute aliasing. You'd need a
> prior devm_ioremap_resource() if you want access to that range from the
> CPU side. For arm64 at least, the latter ends up with a
> pfn_is_map_memory() check.

It doesn't create any new mappings.  The only real issue is that it
does the wrong thing for RAM in a way that might not be noticed on
simple (x86/PC) platforms.