Message-ID: <7f0bb7ce-92ee-4b22-8692-a851adf756a4@oss.qualcomm.com>
Date: Thu, 13 Feb 2025 17:06:24 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Stephan Gerhold <stephan.gerhold@...aro.org>,
        Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Cc: Vinod Koul <vkoul@...nel.org>, Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley
 <conor+dt@...nel.org>, Andy Gross <agross@...nel.org>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Yuvaraj Ranganathan <quic_yrangana@...cinc.com>,
        Anusha Rao <quic_anusha@...cinc.com>,
        Md Sadre Alam
 <quic_mdalam@...cinc.com>,
        linux-arm-msm@...r.kernel.org, dmaengine@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Luca Weiss <luca.weiss@...rphone.com>
Subject: Re: [PATCH 7/8] dt-bindings: dma: qcom: bam-dma: Add missing required
 properties

On 13.02.2025 4:22 PM, 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@...aro.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.
> 
> At the end my thought process here is the following:
> 
>  1. BAM-DMA is a legacy block at this point, it doesn't look like there
>     are any new use cases being added on new SoCs
>  2. We need to preserve compatibility with the old bindings anyway
>  3. I trimmed it down to having to specify just "num-channels"
>  4. Everything else is read from the hardware registers, and
>     num-channels gets validated when the first DMA channel is requested
> 
> I think it's the best we can do here at this point.

Alright, let's go this route then.

Konrad

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