Re: [PATCH 4/7] dt-bindings: dma: allwinner,sun50i-a64-dma: Add compatible for H616

[Andre Przywara <andre.przywara@arm.com>]

On Thu, 25 Jan 2024 22:37:34 +0800
Chen-Yu Tsai <wens@kernel.org> wrote:

Hi,

> On Tue, Jan 23, 2024 at 8:41 AM Andre Przywara <andre.przywara@arm.com> wrote:
> >
> > On Tue, 23 Jan 2024 01:05:15 +0800
> > Chen-Yu Tsai <wens@kernel.org> wrote:
> >
> > Hi,
> >  
> > > From: Chen-Yu Tsai <wens@csie.org>
> > >
> > > The DMA controllers found on the H616 and H618 are the same as the one
> > > found on the H6. The only difference is the DMA endpoint (DRQ) layout.  
> >
> > That does not seem to be entirely true: The H616 encodes the two lowest
> > bits in DMA_DESC_ADDR_REG differently: on the H6 they must be 0 (word
> > aligned), on the H616 these contain bits [33:32] of the address of the
> > DMA descriptor. The manual doesn't describe the descriptor format in
> > much detail, but ec31c5c59492 suggests that those two bits are put in
> > the "para" word of the descriptor.  
> 
> Good catch. So, same as the A100 I believe?

Yes, that's what I got as well.

> > The good thing it that this encoding is backwards compatible, so I
> > think the fallback string still holds: Any driver just implementing the
> > H6 encoding would be able to drive the H616.
> >
> > I think the A100 was mis-described, as mentioned here:
> > https://lore.kernel.org/linux-arm-kernel/29e575b6-14cb-73f1-512d-9f0f934490ea@arm.com/
> > I think we should:
> > - make the A100 use: "allwinner,sun50i-a100-dma", "sun50i-h6-dma"
> > - make the H616 use: "allwinner,sun50i-h616-dma", "allwinner,sun50i-a100-dma", "sun50i-h6-dma"
> >
> > Does that make sense?  
> 
> I wouldn't call that exactly backward compatible. Say the driver forgot to
> clear the two bits. It would work fine on the H6, but the accessed address
> could be way off on the A100 and H616.

I don't know the exact boundaries of "compatible" here, but the H6 manual
pretty clearly states "The descriptor address must be word-aligned."
But since the A100 compatible is known and supported for a while, that
doesn't really matter, practically speaking, I guess.

One could check how the H6 DMA controller reacts to those bits not being
0, not sure if I find the time, though.

Cheers,
Andre.

> > Cheers,
> > Andre
> >  
> > > Since the number of channels and endpoints are described with additional
> > > generic properties, just add a new H616-specific compatible string and
> > > fallback to the H6 one.
> > >
> > > Signed-off-by: Chen-Yu Tsai <wens@csie.org>
> > > ---
> > >  .../bindings/dma/allwinner,sun50i-a64-dma.yaml    | 15 +++++++++++----
> > >  1 file changed, 11 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/Documentation/devicetree/bindings/dma/allwinner,sun50i-a64-dma.yaml b/Documentation/devicetree/bindings/dma/allwinner,sun50i-a64-dma.yaml
> > > index ec2d7a789ffe..e5693be378bd 100644
> > > --- a/Documentation/devicetree/bindings/dma/allwinner,sun50i-a64-dma.yaml
> > > +++ b/Documentation/devicetree/bindings/dma/allwinner,sun50i-a64-dma.yaml
> > > @@ -28,6 +28,9 @@ properties:
> > >        - items:
> > >            - const: allwinner,sun8i-r40-dma
> > >            - const: allwinner,sun50i-a64-dma
> > > +      - items:
> > > +          - const: allwinner,sun50i-h616-dma
> > > +          - const: allwinner,sun50i-h6-dma
> > >
> > >    reg:
> > >      maxItems: 1
> > > @@ -59,10 +62,14 @@ required:
> > >  if:
> > >    properties:
> > >      compatible:
> > > -      enum:
> > > -        - allwinner,sun20i-d1-dma
> > > -        - allwinner,sun50i-a100-dma
> > > -        - allwinner,sun50i-h6-dma
> > > +      oneOf:
> > > +        - enum:
> > > +            - allwinner,sun20i-d1-dma
> > > +            - allwinner,sun50i-a100-dma
> > > +            - allwinner,sun50i-h6-dma
> > > +        - items:
> > > +            - const: allwinner,sun50i-h616-dma
> > > +            - const: allwinner,sun50i-h6-dma
> > >
> > >  then:
> > >    properties:  
> >  
>