linux-kernel - Re: [PATCH 7/7] dt-bindings: dmaengine: pl330: Add new quirks

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Message-ID: <CAOSNQF0jJLc78_1aGYY3=csJc7WqqvydwxmQ22rvXpLruQ-XRg@mail.gmail.com>
Date:   Mon, 8 May 2023 17:28:50 +0530
From:   Joy Chakraborty <joychakr@...gle.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Vinod Koul <vkoul@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, manugautam@...gle.com,
        danielmentz@...gle.com, sjadavani@...gle.com
Subject: Re: [PATCH 7/7] dt-bindings: dmaengine: pl330: Add new quirks

On Fri, May 5, 2023 at 5:53 PM Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> On 05/05/2023 11:44, Joy Chakraborty wrote:
> > On Thu, May 4, 2023 at 8:38 PM Krzysztof Kozlowski
> > <krzysztof.kozlowski@...aro.org> wrote:
> >>
> >> On 04/05/2023 16:57, Joy Chakraborty wrote:
> >>> Add 2 new quirks added to the driver "arm,pl330-optimize-dev2mem-axsize"
> >>> and "arm,pl330-periph-single-dregs"
> >>
> >> This we can see from the diff. You need to answer why?
> >>
> >
> > Sure will change it to:
> > "
> > Addition of following quirks :
> > - "arm,pl330-periph-use-diff-axsize"
> >    AxSize of transactions to peripherals are limited by the peripheral
> > address width which inturn limits the AxSize used for transactions
> > towards memory.
> >    This quirk will make transactions to memory use the maximum
> > possible bus width(AxSize), store data in MFIFO and use narrow
> > multi-beat transactions to move data to peripherals.
> >    This only applies to transfers between memory and peripherals where
> > bus widths available are different for memory and the peripheral.
> > - "arm,pl330-periph-complete-with-singles" :
> >    When transfer sizes are not a multiple of a block of burst
> > transfers (AxLen * AxSize configured at the peripheral), certain
> > peripherals might choose not to set the burst request at the
> > peripheral request interface of the DMA.
> >    This quirk moves the remaining bytes to the peripheral using single
> > transactions.
> > "
>
> This does not answer why. You just copied again the patch contents.
>
Hi Krzysztof,
Both the changes could be useful for SOC's which have PL330 integrated
with a peripheral but I am not sure if all SOC's need/want this change
hence wanted to keep it as a DT knob to avoid any regressions.
But like you say it might not be the right thing to do.

> >
> >>>
> >>> Signed-off-by: Joy Chakraborty <joychakr@...gle.com>
> >>> ---
> >>>  Documentation/devicetree/bindings/dma/arm,pl330.yaml | 8 ++++++++
> >>>  1 file changed, 8 insertions(+)
> >>>
> >>> diff --git a/Documentation/devicetree/bindings/dma/arm,pl330.yaml b/Documentation/devicetree/bindings/dma/arm,pl330.yaml
> >>> index 4a3dd6f5309b..0499a7fba88d 100644
> >>> --- a/Documentation/devicetree/bindings/dma/arm,pl330.yaml
> >>> +++ b/Documentation/devicetree/bindings/dma/arm,pl330.yaml
> >>> @@ -53,6 +53,14 @@ properties:
> >>>      type: boolean
> >>>      description: quirk for performing burst transfer only
> >>>
> >>> +  arm,pl330-optimize-dev2mem-axsize:
> >>> +    type: boolean
> >>> +    description: quirk for optimizing AxSize used between dev<->mem
> >>
> >> This tells me nothing... Neither what it is about nor why this is
> >> property of a board or PL330 hardware implementation. Please describe
> >> hardware, not drivers.
> >>
> >
> > Will change the name to "arm,pl330-periph-use-diff-axsize" and add description:
> > "
> > Quirk to use different AxSize for bursts while accessing source and
> > destination when moving data between memory and peripheral.
> > Maximum possible bus width is used as AxSize for transactions towards
> > memory and transactions towards peripherals use AxSize as per
> > peripheral address width.
> > "
>
> Still no answer. Why this is property of a board or PL330 hardware
> implementation?
> I also asked to describe hardware but I still see "quirk to ...". We use
> "quirk" as concept in Linux driver. Describe the hardware, not Linux driver.
>

This comes to use when the bus width requirement between peripheral
and memory is different, but buswidth is something we read from HW
registers so this can be enabled by default.

>
> >
> >>> +
> >>> +  arm,pl330-periph-single-dregs:
> >>> +    type: boolean
> >>> +    description: quirk for using dma-singles for peripherals in _dregs()
> >>
> >> Same concerns.
> >>

An example of such a case is given in the ARM TRM for PL330, so maybe
we can have this by default as well.
Link : https://developer.arm.com/documentation/ddi0424/d/functional-overview/peripheral-request-interface/dmac-length-management#:~:text=DMAC%20length%20management-,Example%202.3,-shows%20a%20DMAC

> >
> > Will change the name to  "arm,pl330-periph-complete-with-singles" and
> > add description:
> > "
> > Quirk to use dma singles n times instead of an n beat burst to
> > complete a transfer when the transfer size is not a multiple of the
>
> No, how you wrote it sounds like driver. Don't add driver quirks to DT.
>
> Best regards,
> Krzysztof
>

Hi Vinod Kaul,

Do you think it is feasible to enable these changes by default instead
of a DT property ?

Thanks
Joy