linux-kernel - Re: [PATCH 2/3] riscv: Set dma_default

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Message-ID: <Y/Y8lEndecMfD+Ur@spud>
Date:   Wed, 22 Feb 2023 16:02:28 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>
Cc:     "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        mpe@...erman.id.au, paul.walmsley@...ive.com, palmer@...belt.com,
        Rob Herring <robh+dt@...nel.org>,
        Christoph Hellwig <hch@....de>, m.szyprowski@...sung.com,
        robin.murphy@....com, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH 2/3] riscv: Set dma_default_coherent to true

On Wed, Feb 22, 2023 at 03:55:19PM +0000, Jiaxun Yang wrote:
> 
> 
> > 2023年2月22日 14:50，Conor Dooley <conor@...nel.org> 写道：
> > 
> > On Wed, Feb 22, 2023 at 01:37:11PM +0000, Jiaxun Yang wrote:
> >> For riscv our assumption is unless a device states it is non-coherent,
> >> we take it to be DMA coherent.
> >> 
> >> For devicetree probed devices that have been true since very begining
> >> with OF_DMA_DEFAULT_COHERENT selected.
> >> 
> >> Signed-off-by: Jiaxun Yang <jiaxun.yang@...goat.com>
> >> ---
> >> arch/riscv/kernel/setup.c | 3 +++
> >> 1 file changed, 3 insertions(+)
> >> 
> >> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> >> index 376d2827e736..34b371180976 100644
> >> --- a/arch/riscv/kernel/setup.c
> >> +++ b/arch/riscv/kernel/setup.c
> >> @@ -300,6 +300,9 @@ void __init setup_arch(char **cmdline_p)
> >> riscv_init_cbom_blocksize();
> >> riscv_fill_hwcap();
> >> apply_boot_alternatives();
> >> +#ifdef CONFIG_RISCV_DMA_NONCOHERENT
> >> + dma_default_coherent = true;
> >> +#endif
> > 
> > Do we really need to add ifdeffery for this here?
> > It's always coherent by default, so why do we need to say set it in
> > setup_arch() when we know that, regardless of options, it is true?
> 
> Because this symbol is only a variable when:
> 
> defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE) || \
> defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU) || \
> defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL)
> 
> Which is only true if  CONFIG_RISCV_DMA_NONCOHERENT is selected.
> 
> Otherwise this symbol is defined to true and we can’t make a assignment to it.

Maybe I am just slow today, but I don't get why you need to add
ifdeffery to setup_arch() to do something that is always true.
Why can't you just set this in riscv/mm/dma-noncoherent.c? What am I
missing?

Cheers,
Conor.

> >> if (IS_ENABLED(CONFIG_RISCV_ISA_ZICBOM) &&
> >>     riscv_isa_extension_available(NULL, ZICBOM))
> >> riscv_noncoherent_supported();
> >> -- 
> >> 2.37.1 (Apple Git-137.1)
> 
> 

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