linux-kernel - Re: Bogus struct page layout on 32-bit

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Date:   Fri, 16 Apr 2021 16:10:37 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Grygorii Strashko <grygorii.strashko@...com>
Cc:     Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        kernel test robot <lkp@...el.com>,
        Linux-MM <linux-mm@...ck.org>, kbuild-all@...ts.01.org,
        clang-built-linux@...glegroups.com,
        open list <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "David S. Miller" <davem@...emloft.net>,
        Matteo Croce <mcroce@...ux.microsoft.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Bogus struct page layout on 32-bit

On Fri, Apr 16, 2021 at 11:27 AM 'Grygorii Strashko' via Clang Built
Linux <clang-built-linux@...glegroups.com> wrote:
> On 10/04/2021 11:52, Ilias Apalodimas wrote:
> > +CC Grygorii for the cpsw part as Ivan's email is not valid anymore
> The TI platforms am3/4/5 (cpsw) and Keystone 2 (netcp) can do only 32bit DMA even in case of LPAE (dma-ranges are used).
> Originally, as I remember, CONFIG_ARCH_DMA_ADDR_T_64BIT has not been selected for the LPAE case
> on TI platforms and the fact that it became set is the result of multi-paltform/allXXXconfig/DMA
> optimizations and unification.
> (just checked - not set in 4.14)
>
> Probable commit 4965a68780c5 ("arch: define the ARCH_DMA_ADDR_T_64BIT config symbol in lib/Kconfig").

I completely missed this change in the past, and I don't really agree
with it either.

Most 32-bit Arm platforms are in fact limited to 32-bit DMA, even when they have
MMIO or RAM areas above the 4GB boundary that require LPAE.

> The TI drivers have been updated, finally to accept ARCH_DMA_ADDR_T_64BIT=y by using
> things like (__force u32) for example.
>
> Honestly, I've done sanity check of CPSW with LPAE=y (ARCH_DMA_ADDR_T_64BIT=y) very long time ago.

This is of course a good idea, drivers should work with any
combination of 32-bit
or 64-bit phys_addr_t and dma_addr_t.

        Arnd