From: Christoph Hellwig <hch@lst.de>
To: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Maxim Kochetkov <fido_max@inbox.ru>,
linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
robh@kernel.org, mpe@ellerman.id.au, aou@eecs.berkeley.edu,
palmer@dabbelt.com, paul.walmsley@sifive.com,
Conor Dooley <conor@kernel.org>,
hch@lst.de
Subject: Re: [PATCH 1/1] riscv: set ARCH_DMA_DEFAULT_COHERENT if RISCV_DMA_NONCOHERENT is not set
Date: Fri, 22 Dec 2023 05:14:28 +0100 [thread overview]
Message-ID: <20231222041428.GA2803@lst.de> (raw)
In-Reply-To: <f31d929c-fa0a-4046-be05-38e92afa5d92@flygoat.com>
On Thu, Dec 21, 2023 at 10:27:33PM +0000, Jiaxun Yang wrote:
>
>
> 在 2023/12/21 20:29, Conor Dooley 写道:
>> + Christoph
>>
>> I don't think this patch is correct. Regardless of whether we support
>> cache management operations, DMA is assumed to be coherent unless
>> peripherals etc are specified to otherwise in DT (or however ACPI deals
>> with that kind of thing).
>>
>> What problem are you trying to solve here?
>>
>> On Thu, Dec 21, 2023 at 09:51:52PM +0300, Maxim Kochetkov wrote:
>>> Not all the RISCV are DMA coherent by default.
>
> Sorry for chime in here.
> IMO if your platform is not coherent by default, just insert
> "dma-noncoherent"
> at devicetree root node.
Exactly. ARCH_DMA_DEFAULT_COHERENTis a setting that just says for
a given architecture assumes coherent unless otherwise specified,
which has historically been the case for mips. Not setting it means
non-coherent unless specified, which has historially been the case
for arm.
RISC-V starte out without support for non-coherent DMA, and high ups
in RISCV still told me in 2019 that RISC-V doesn't need cache
management instructions because no new hardware would ever not be
dma coherent. Yeah, right..
Anyay, Linux for RISC-V has historically been coherent only and then
coherent default, so this option is wrong, and you need to mark
you platform as non-coherent by inserting dma-noncoherent somewhere.
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next prev parent reply other threads:[~2023-12-22 4:14 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-12-21 18:51 [PATCH 1/1] riscv: set ARCH_DMA_DEFAULT_COHERENT if RISCV_DMA_NONCOHERENT is not set Maxim Kochetkov
2023-12-21 20:29 ` Conor Dooley
2023-12-21 22:27 ` Jiaxun Yang
2023-12-22 4:14 ` Christoph Hellwig [this message]
2023-12-22 14:39 ` Maxim Kochetkov
2023-12-22 14:54 ` Conor Dooley
2023-12-22 15:04 ` Christoph Hellwig
2023-12-22 15:38 ` Maxim Kochetkov
2023-12-22 15:45 ` Jiaxun Yang
2023-12-22 15:53 ` Maxim Kochetkov
2023-12-22 16:01 ` Jiaxun Yang
2023-12-23 4:59 ` Christoph Hellwig
2023-12-25 6:47 ` Maxim Kochetkov
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