linux-kernel - Re: [PATCH 02/14] dmaengine: dma-jz4780: Separate chan/ctrl registers

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Date:   Sat, 7 Jul 2018 12:57:56 +0530
From:   PrasannaKumar Muralidharan <prasannatsmkumar@...il.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Vinod Koul <vkoul@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        Zubair Lutfullah Kakakhel <Zubair.Kakakhel@...tec.com>,
        Mathieu Malaterre <malat@...ian.org>,
        Daniel Silsby <dansilsby@...il.com>, dmaengine@...r.kernel.org,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Linux-MIPS <linux-mips@...ux-mips.org>
Subject: Re: [PATCH 02/14] dmaengine: dma-jz4780: Separate chan/ctrl registers

On 6 July 2018 at 03:15, Paul Cercueil <paul@...pouillou.net> wrote:
>
>
>> Paul,
>>
>> On 3 July 2018 at 18:02, Paul Cercueil <paul@...pouillou.net> wrote:
>>>
>>>  The register area of the JZ4780 DMA core can be split into different
>>>  sections for different purposes:
>>>
>>>  * one set of registers is used to perform actions at the DMA core level,
>>>  that will generally affect all channels;
>>>
>>>  * one set of registers per DMA channel, to perform actions at the DMA
>>>  channel level, that will only affect the channel in question.
>>>
>>>  The problem rises when trying to support new versions of the JZ47xx
>>>  Ingenic SoC. For instance, the JZ4770 has two DMA cores, each one
>>>  with six DMA channels, and the register sets are interleaved:
>>>  <DMA0 chan regs> <DMA1 chan regs> <DMA0 ctrl regs> <DMA1 ctrl regs>
>>>
>>>  By using one memory resource for the channel-specific registers and
>>>  one memory resource for the core-specific registers, we can support
>>>  the JZ4770, by initializing the driver once per DMA core with different
>>>  addresses.
>>
>>
>> As per my understanding device tree should be modified only when
>> hardware changes. This looks the other way around. It must be possible
>> to achieve what you are trying to do in this patch without changing
>> the device tree.
>
>
> I would agree that devicetree has an ABI that we shouldn't break if
> possible.
>
> However DTS support for all the Ingenic SoCs/boards is far from being
> complete, and more importantly, all Ingenic-based boards compile the DTS
> file within the kernel; so breaking the ABI is not (yet) a problem, and
> we should push the big changes right now while it's still possible.

Completely agree with you in this. Let's wait and see what DT maintainer's view.