Re: [PATCH 7/8] dt-bindings: dma: qcom: bam-dma: Add missing required properties

[Stephan Gerhold <stephan.gerhold@linaro.org>]

On Wed, Feb 19, 2025 at 04:27:39PM -0600, Rob Herring wrote:
> On Thu, Feb 13, 2025 at 04:22:17PM +0100, Stephan Gerhold wrote:
> > On Thu, Feb 13, 2025 at 03:00:00PM +0100, Konrad Dybcio wrote:
> > > On 13.02.2025 10:13 AM, Stephan Gerhold wrote:
> > > > On Wed, Feb 12, 2025 at 10:01:59PM +0100, Konrad Dybcio wrote:
> > > >> On 12.02.2025 6:03 PM, Stephan Gerhold wrote:
> > > >>> num-channels and qcom,num-ees are required when there are no clocks
> > > >>> specified in the device tree, because we have no reliable way to read them
> > > >>> from the hardware registers if we cannot ensure the BAM hardware is up when
> > > >>> the device is being probed.
> > > >>>
> > > >>> This has often been forgotten when adding new SoC device trees, so make
> > > >>> this clear by describing this requirement in the schema.
> > > >>>
> > > >>> Signed-off-by: Stephan Gerhold <stephan.gerhold@linaro.org>
> > > >>> ---
> > > >>>  Documentation/devicetree/bindings/dma/qcom,bam-dma.yaml | 4 ++++
> > > >>>  1 file changed, 4 insertions(+)
> > > >>>
> > > >>> diff --git a/Documentation/devicetree/bindings/dma/qcom,bam-dma.yaml b/Documentation/devicetree/bindings/dma/qcom,bam-dma.yaml
> > > >>> index 3ad0d9b1fbc5e4f83dd316d1ad79773c288748ba..5f7e7763615578717651014cfd52745ea2132115 100644
> > > >>> --- a/Documentation/devicetree/bindings/dma/qcom,bam-dma.yaml
> > > >>> +++ b/Documentation/devicetree/bindings/dma/qcom,bam-dma.yaml
> > > >>> @@ -90,8 +90,12 @@ required:
> > > >>>  anyOf:
> > > >>>    - required:
> > > >>>        - qcom,powered-remotely
> > > >>> +      - num-channels
> > > >>> +      - qcom,num-ees
> > > >>>    - required:
> > > >>>        - qcom,controlled-remotely
> > > >>> +      - num-channels
> > > >>> +      - qcom,num-ees
> > > >>
> > > >> I think I'd rather see these deprecated and add the clock everywhere..
> > > >> Do we know which one we need to add on newer platforms? Or maybe it's
> > > >> been transformed into an icc path?
> > > > 
> > > > This isn't feasible, there are too many different setups. Also often the
> > > > BAM power management is tightly integrated into the consumer interface.
> > > > To give a short excerpt (I'm sure there are even more obscure uses):
> > > > 
> > > >  - BLSP BAM (UART, I2C, SPI on older SoCs):
> > > >     1. Enable GCC_BLSP_AHB_CLK
> > > >     -> This is what the bam_dma driver supports currently.
> > > > 
> > > >  - Crypto BAM: Either
> > > >     OR 1. Vote for single RPM clock
> > > >     OR 1. Enable 3 separate clocks (CE, CE_AHB, CE_AXI)
> > > >     OR 1. Vote dummy bandwidth for interconnect
> > > > 
> > > >  - BAM DMUX (WWAN on older SoCs):
> > > >     1. Start modem firmware
> > > >     2. Wait for BAM DMUX service to be up
> > > >     3. Vote for power up via the BAM-DMUX-specific SMEM state
> > > >     4. Hope the firmware agrees and brings up the BAM
> > > > 
> > > >  - SLIMbus BAM (audio on some SoCs):
> > > >     1. Start ADSP firmware
> > > >     2. Wait for QMI SLIMBUS service to be up via QRTR
> > > >     3. Vote for power up via SLIMbus-specific QMI messages
> > > >     4. Hope the firmware agrees and brings up the BAM
> > > > 
> > > > Especially for the last two, we can't implement support for those
> > > > consumer-specific interfaces in the BAM driver. Implementing support for
> > > > the 3 variants of the Crypto BAM would be possible, but it's honestly
> > > > the least interesting use case of all these. It's not really clear why
> > > > we are bothing with the crypto engine on newer SoCs at all, see e.g. [1].
> > > > 
> > > > [1]: https://lore.kernel.org/linux-arm-msm/20250118080604.GA721573@sol.localdomain/
> > > > 
> > > >> Reading back things from this piece of HW only to add it to DT to avoid
> > > >> reading it later is a really messy solution.
> > > > 
> > > > In retrospect, it could have been cleaner to avoid describing the BAM as
> > > > device node independent of the consumer. We wouldn't have this problem
> > > > if the BAM driver would only probe when the consumer is already ready.
> > > > 
> > > > But I think specifying num-channels in the device tree is the cleanest
> > > > way out of this mess. I have a second patch series ready that drops
> > > > qcom,num-ees and validates the num-channels once it's safe reading from
> > > > the BAM registers. That way, you just need one boot test to ensure the
> > > > device tree description is really correct.
> > > 
> > > Thanks for the detailed explanation!
> > > 
> > > Do you think it could maybe make sense to expose a clock/power-domain
> > > from the modem/adsp rproc and feed it to the DMUX / SLIM instances when
> > > an appropriate ping arrives? This way we'd also defer probing the drivers
> > > until the device is actually accessible.
> > > 
> > 
> > Maybe, but that would result in a cyclic dependency between the DMA
> > provider and consumer. E.g.
> > 
> > 	bam_dmux_dma: dma-controller@ {
> > 		#dma-cells = <1>;
> > 		power-domains = <&bam_dmux>;
> > 	};
> > 
> > 	remoteproc@ {
> > 		/* ... */
> > 
> > 		bam_dmux: bam-dmux {
> > 			dmas = <&bam_dmux_dma 4>, <&bam_dmux_dma 5>;
> > 			dma-names = "tx", "rx";
> > 		};
> > 	};
> > 
> > fw_devlink will likely get confused by that.
> 
> Why? We have a property to break cycles: post-init-providers
> 
> That doesn't work here?
> 

Thanks, I was not aware of that property. This looks quite useful for
fixing up some of the other cyclic dependencies we have!

Nevertheless, for this specific case, I still think we should not make
such large breaking changes at this point. As I pointed out further
below in my quoted email, this is a legacy hardware block that will
likely not get any major new users in the future. We're essentially
discussing to rework several bindings and drivers just to drop a single
straightforward "num-channels = <N>" property. A property that we will
need to keep support for anyway, to support users with older DTBs. This
effort (and risk) is really better spent elsewhere.

Thanks,
Stephan