linux-kernel - Re: [PATCH 21/21] dma-mapping: replace custom code with generic implementation

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Message-Id: <f25d74a0-9e61-44df-b4c7-f495462fa648@app.fastmail.com>
Date:   Fri, 31 Mar 2023 15:04:23 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Christoph Hellwig" <hch@....de>, "Arnd Bergmann" <arnd@...nel.org>
Cc:     linux-kernel@...r.kernel.org, "Vineet Gupta" <vgupta@...nel.org>,
        "Russell King" <linux@...linux.org.uk>,
        "Neil Armstrong" <neil.armstrong@...aro.org>,
        "Linus Walleij" <linus.walleij@...aro.org>,
        "Catalin Marinas" <catalin.marinas@....com>,
        "Will Deacon" <will@...nel.org>, guoren <guoren@...nel.org>,
        "Brian Cain" <bcain@...cinc.com>,
        "Geert Uytterhoeven" <geert@...ux-m68k.org>,
        "Michal Simek" <monstr@...str.eu>,
        "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>,
        "Dinh Nguyen" <dinguyen@...nel.org>,
        "Stafford Horne" <shorne@...il.com>,
        "Helge Deller" <deller@....de>,
        "Michael Ellerman" <mpe@...erman.id.au>,
        "Christophe Leroy" <christophe.leroy@...roup.eu>,
        "Paul Walmsley" <paul.walmsley@...ive.com>,
        "Palmer Dabbelt" <palmer@...belt.com>,
        "Rich Felker" <dalias@...c.org>,
        "John Paul Adrian Glaubitz" <glaubitz@...sik.fu-berlin.de>,
        "David S . Miller" <davem@...emloft.net>,
        "Max Filippov" <jcmvbkbc@...il.com>,
        "Robin Murphy" <robin.murphy@....com>,
        "Lad, Prabhakar" <prabhakar.mahadev-lad.rj@...renesas.com>,
        "Conor.Dooley" <conor.dooley@...rochip.com>,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        "linux-oxnas@...ups.io" <linux-oxnas@...ups.io>,
        "linux-csky@...r.kernel.org" <linux-csky@...r.kernel.org>,
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        linux-mips@...r.kernel.org,
        "linux-openrisc@...r.kernel.org" <linux-openrisc@...r.kernel.org>,
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Subject: Re: [PATCH 21/21] dma-mapping: replace custom code with generic implementation

On Tue, Mar 28, 2023, at 00:25, Christoph Hellwig wrote:
>> +static inline void arch_dma_cache_wback(phys_addr_t paddr, size_t size)
>>  {
>> +	dma_cache_wback(paddr, size);
>> +}
>>  
>> +static inline void arch_dma_cache_inv(phys_addr_t paddr, size_t size)
>> +{
>> +	dma_cache_inv(paddr, size);
>>  }
>
>> +static inline void arch_dma_cache_wback_inv(phys_addr_t paddr, size_t size)
>>  {
>> +	dma_cache_wback_inv(paddr, size);
>> +}
>
> There are the only calls for the three functions for each of the
> involved functions.  So I'd rather rename the low-level symbols
> (and drop the pointless exports for two of them) rather than adding
> these wrapppers.
>
> The same is probably true for many other architectures.

Ok, done that now.

>> +static inline bool arch_sync_dma_clean_before_fromdevice(void)
>> +{
>> +	return false;
>> +}
>>  
>> +static inline bool arch_sync_dma_cpu_needs_post_dma_flush(void)
>> +{
>> +	return true;
>>  }
>
> Is there a way to cut down on this boilerplate code by just having
> sane default, and Kconfig options to override them if they are not
> runtime decisions?

I've changed arch_sync_dma_clean_before_fromdevice() to a
Kconfig symbol now, as this is never a runtime decision.

For arch_sync_dma_cpu_needs_post_dma_flush(), I have this
version now in common code, which lets mips and arm have
their own logic and has the same effect elsewhere:

+#ifndef arch_sync_dma_cpu_needs_post_dma_flush
+static inline bool arch_sync_dma_cpu_needs_post_dma_flush(void)
+{
+       return IS_ENABLED(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU);
+}
+#endif

>> +#include <linux/dma-sync.h>
>
> I can't really say I like the #include version here despite your
> rationale in the commit log.  I can probably live with it if you
> think it is absolutely worth it, but I'm really not in favor of it.
>
>> +config ARCH_DMA_MARK_DCACHE_CLEAN
>> +	def_bool y
>
> What do we need this symbol for?  Unless I'm missing something it is
> always enable for arm32, and only used in arm32 code.

This was left over from an earlier draft and accidentally duplicates
the thing that I have in the Arm version for the existing
ARCH_HAS_DMA_MARK_CLEAN. I dropped this one and the
generic copy of the arch_dma_mark_dcache_clean() function
now, but still need to revisit the arm version, as it sounds
like it has slightly different semantics from the ia64 version.

     Arnd