linux-kernel - Re: [PATCH 2/2] arm-nommu: Add use_reserved_mem() to check if device support reserved memory

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Message-ID: <031034fb-b109-7410-3ff8-e78cd12a5552@arm.com>
Date:   Tue, 9 Jun 2020 16:43:54 +0100
From:   Vladimir Murzin <vladimir.murzin@....com>
To:     Christoph Hellwig <hch@....de>,
        dillon min <dillon.minfei@...il.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        linux@...linux.org.uk, Kate Stewart <kstewart@...uxfoundation.org>,
        allison@...utok.net, info@...ux.net, tglx@...utronix.de,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        linux-stm32@...md-mailman.stormreply.com,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] arm-nommu: Add use_reserved_mem() to check if device
 support reserved memory

On 6/9/20 4:36 PM, Christoph Hellwig wrote:
> On Tue, Jun 09, 2020 at 11:22:24PM +0800, dillon min wrote:
>> Hi Vladimir,
>>
>> Thanks for reviewing.
>>
>> Hi Christoph Hellwig,
>>
>> I just want to know if kernel dma mapping/direct is focused on
>> platforms with MMU.
>> leave arch code to handle dma coherent memory management themself for
>> no-MMU platform.
> 
> No, I'd really like to consolidate everything that isn't overly
> arch specific eventually.
> 
>>
>> so, you just return error code in kernel/dma/mapping.c,direct.c
>> without CONFIG_MMU defined ?
>> which means dma-direct mapping doesn't support !CONFIG_MMU is not a
>> bug, but design as it's.
>> or, just return error code currently, will add dma direct mapping
>> support for !CONFIG_MMU in the
>> future?
>>
>> As Vladimir Murzin's suggestion has changes in kernel code, I need
>> your input to get
>> the design goal about dma-direct mapping, thanks.
> 
> Can someone repost the whole patch?
> 

Happy to repost as separate patch once dillon confirms it actually works.

Meanwhile, I'm trying to understand at which point we lost this 
functionality for NOMMU... maybe it will become different patch :)

Cheers
Vladimir