linux-kernel - Re: [PATCH v2 2/4] dt-bindings: dma: sf-pdma: add new compatible name

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Date:   Thu, 5 Oct 2023 11:54:29 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Rob Herring <robh@...nel.org>
Cc:     shravan chippa <shravan.chippa@...rochip.com>,
        green.wan@...ive.com, vkoul@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, palmer@...belt.com,
        paul.walmsley@...ive.com, conor+dt@...nel.org,
        dmaengine@...r.kernel.org, devicetree@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        nagasuresh.relli@...rochip.com, praveen.kumar@...rochip.com,
        Conor Dooley <conor.dooley@...rochip.com>
Subject: Re: [PATCH v2 2/4] dt-bindings: dma: sf-pdma: add new compatible name

On Wed, Oct 04, 2023 at 08:30:21AM -0500, Rob Herring wrote:
> On Tue, Oct 03, 2023 at 09:52:13AM +0530, shravan chippa wrote:
> > From: Shravan Chippa <shravan.chippa@...rochip.com>
> > 
> > Add new compatible name microchip,mpfs-pdma to support
> > out of order dma transfers
> > 
> > Reviewed-by: Conor Dooley <conor.dooley@...rochip.com>
> > Signed-off-by: Shravan Chippa <shravan.chippa@...rochip.com>
> > ---
> >  .../bindings/dma/sifive,fu540-c000-pdma.yaml         | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/Documentation/devicetree/bindings/dma/sifive,fu540-c000-pdma.yaml b/Documentation/devicetree/bindings/dma/sifive,fu540-c000-pdma.yaml
> > index a1af0b906365..974467c4bacb 100644
> > --- a/Documentation/devicetree/bindings/dma/sifive,fu540-c000-pdma.yaml
> > +++ b/Documentation/devicetree/bindings/dma/sifive,fu540-c000-pdma.yaml
> > @@ -27,10 +27,14 @@ allOf:
> >  
> >  properties:
> >    compatible:
> > -    items:
> > -      - enum:
> > -          - sifive,fu540-c000-pdma
> > -      - const: sifive,pdma0
> > +    oneOf:
> > +      - items:
> > +          - const: microchip,mpfs-pdma # Microchip out of order DMA transfer
> > +          - const: sifive,fu540-c000-pdma # Sifive in-order DMA transfer

IIRC I asked for the comments here to be removed on the previous
version, and my r-b was conditional on that.
The device specific compatible has merit outside of the ordering, which
may just be a software policy decision.

> This doesn't really make sense. microchip,mpfs-pdma is compatible with 
> sifive,fu540-c000-pdma and sifive,fu540-c000-pdma is compatible with 
> sifive,pdma0, but microchip,mpfs-pdma is not compatible with 
> sifive,pdma0? (Or replace "compatible with" with "a superset of")

TBH, I am not sure why it was done this way. Probably because the driver
contains both sifive,pdma0 and sifive,fu540-c000-pdma. Doing
compatible = "microchip,mpfs-pdma", "sifive,fu540-c000-pdma", "sifive,pdma0";
thing would be fine.

> Any fallback is only useful if an OS only understanding the fallback 
> will work with the h/w. Does this h/w work without the driver changes?

Yes. 
I've been hoping that someone from SiFive would come along, and in
response to this patchset, tell us _why_ the driver does not make use of
out-of-order transfers to begin with.

Thanks,
Conor.

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