linux-kernel - Re: [PATCH 1/1] riscv: set ARCH_DMA_DEFAULT_COHERENT if RISCV_DMA

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Message-ID: <15b89089-3dcd-4668-8d8b-d12ff0684c29@inbox.ru>
Date: Mon, 25 Dec 2023 09:47:21 +0300
From: Maxim Kochetkov <fido_max@...ox.ru>
To: Christoph Hellwig <hch@....de>, Jiaxun Yang <jiaxun.yang@...goat.com>
Cc: Conor Dooley <conor@...nel.org>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, robh@...nel.org, mpe@...erman.id.au,
 aou@...s.berkeley.edu, palmer@...belt.com, paul.walmsley@...ive.com
Subject: Re: [PATCH 1/1] riscv: set ARCH_DMA_DEFAULT_COHERENT if
 RISCV_DMA_NONCOHERENT is not set



On 23.12.2023 07:59, Christoph Hellwig wrote:
> On Fri, Dec 22, 2023 at 04:01:43PM +0000, Jiaxun Yang wrote:
>>>
>>> But arch_setup_dma_ops() is called only from of_dma_configure_id() and
>>> acpi_dma_configure_id(). So it works only for DT and ACPI devices. What
>>> about platform_device?
>>
>> Ah I see, that's the problem, in MIPS's use case all DMA capable devices
>> are following platform's default coherency. For RISC-V we assume all device
>> are enabled by ACPI or DT.
>>
>> Perhaps you can override it in driver, but that will make drivers platform
>> dependent.
>>
>> I'll leave this question to Christoph.
> 
> I've already said it.  You must not have DMA capable devices that aren't
> declared in ACPI or OF, just like on any modern Linux platform.

Ok. I've got the point. Thank you for clarification.

> 
> What devices are you concerned about anyway Maxim?

I have 3rd party external out of tree camera driver. csi, isp, dewarp 
components are OF, but common media device is created as 
platform_device. I will convert is to OF.