linux-kernel - Re: [PATCH 15/26] asm-generic: ioremap_uc should behave the same with and without MMU

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Date:   Sat, 17 Aug 2019 14:07:29 -0700 (PDT)
From:   Paul Walmsley <paul.walmsley@...ive.com>
To:     Christoph Hellwig <hch@....de>
cc:     Arnd Bergmann <arnd@...db.de>, Guo Ren <guoren@...nel.org>,
        Michal Simek <monstr@...str.eu>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Guan Xuetao <gxt@....edu.cn>, x86@...nel.org,
        linux-arch@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-parisc@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-hexagon@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        openrisc@...ts.librecores.org, linux-mtd@...ts.infradead.org,
        linux-alpha@...r.kernel.org, sparclinux@...r.kernel.org,
        nios2-dev@...ts.rocketboards.org, linux-riscv@...ts.infradead.org,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 15/26] asm-generic: ioremap_uc should behave the same
 with and without MMU

On Sat, 17 Aug 2019, Christoph Hellwig wrote:

> Whatever reason there is for the existence of ioremap_uc, and the fact 
> that it returns NULL by default on architectures with an MMU applies 
> equally to nommu architectures, so don't provide different defaults.
> 
> In practice the difference is meaningless as the only portable driver
> that uses ioremap_uc is atyfb which probably doesn't show up on nommu
> devices.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>

[ ... ]

> diff --git a/include/asm-generic/io.h b/include/asm-generic/io.h
> index d02806513670..a98ed6325727 100644
> --- a/include/asm-generic/io.h
> +++ b/include/asm-generic/io.h

[ ... ]

> @@ -1004,6 +985,21 @@ static inline void __iomem *ioremap_wt(phys_addr_t offset, size_t size)
>  }
>  #endif
>  
> +/*
> + * ioremap_uc is special in that we do require an explicit architecture
> + * implementation.  In general you do now want to use this function in a
                                         ^^^ not

> + * driver and use plain ioremap, which is uncached by default.  Similarly
                ^ instead 

> + * architectures should not implement it unless they have a very good
> + * reason.
> + */

Looks like this mess is only needed on x86 with certain graphics drivers 
and conflicts between MTRR and page table-based MMU attributes.


Reviewed-by: Paul Walmsley <paul.walmsley@...ive.com>
Tested-by: Paul Walmsley <paul.walmsley@...ive.com> # rv32, rv64 boot


- Paul