linux-kernel - Re: [PATCH stable v5.10 0/7] arm64: Default to 32-bit wide ZONE

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Date:   Thu, 04 Mar 2021 15:05:32 +0100
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Jing Xiangfeng <jingxiangfeng@...wei.com>
Cc:     catalin.marinas@....com, will@...nel.org,
        akpm@...ux-foundation.org, paul.walmsley@...ive.com,
        palmer@...belt.com, aou@...s.berkeley.edu, rppt@...nel.org,
        lorenzo.pieralisi@....com, guohanjun@...wei.com,
        sudeep.holla@....com, rjw@...ysocki.net, lenb@...nel.org,
        song.bao.hua@...ilicon.com, ardb@...nel.org,
        anshuman.khandual@....com, bhelgaas@...gle.com, guro@...com,
        robh+dt@...nel.org, stable@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, frowand.list@...il.com,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-riscv@...ts.infradead.org,
        wangkefeng.wang@...wei.com
Subject: Re: [PATCH stable v5.10 0/7] arm64: Default to 32-bit wide ZONE_DMA

Hi Greg.

On Thu, 2021-03-04 at 14:46 +0100, Greg KH wrote:
> On Wed, Mar 03, 2021 at 03:33:12PM +0800, Jing Xiangfeng wrote:
> > Using two distinct DMA zones turned out to be problematic. Here's an
> > attempt go back to a saner default.
> 
> What problem does this solve?  How does this fit into the stable kernel
> rules?

We changed the way we setup memory zones in arm64 in order to cater for
Raspberry Pi 4's weird DMA constraints: ZONE_DMA spans the lower 1GB of memory
and ZONE_DMA32 the rest of the 32bit address space. Since you can't allocate
memory that crosses zone boundaries, this broke crashkernel allocations on big
machines. This series fixes all this by parsing the HW description and checking
for DMA constrained buses. When not found, the unnecessary zone creation is
skipped.

That said, I have no clue whether this falls or not into the stable kernel
rules.

Regards,
Nicolas


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