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

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Message-ID: <Y/ZJ+N4m6UUmIpL0@spud>
Date:   Wed, 22 Feb 2023 16:59:36 +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 04:20:16PM +0000, Jiaxun Yang wrote:
> > 2023年2月22日 16:02，Conor Dooley <conor@...nel.org> 写道：
> > 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?
> 
> Hmm that sounds like a good idea but I was unable to find a place for this.
> 
> riscv/mm/dma-noncoherent.c are just a bunch of callbacks, there is no initialisation
> function that will always run on every boot. riscv_noncoherent_supported is only
> called conditionally.

Right, that's fair. riscv_noncoherent_supported() is for when we know we
can support non-coherent DMA, not to detect whether we can, hence the
conditional nature.
Apologies for sending you on a wild goose chase there.

> Perhaps I can add a initcall in dma-noncoherent.c but it seems to be a little bit
> overkilling.

Yah, probably would be.

> Actually I’d prefer to have a config option for the default value but it seems like
> it’s not in Christoph’s flavour.

We already have a config option that sets things up nicely for RISC-V
that you're getting rid of! ;)

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