Re: [PATCH 2/4] dt-bindings: remoteproc: qcom: sc7180: Update memory-region requirements

[Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>]

On 14/12/2022 11:25, Sibi Sankar wrote:
> 
> 
> On 12/14/22 01:11, Krzysztof Kozlowski wrote:
>> On 13/12/2022 15:07, Sibi Sankar wrote:
>>> Update the bindings to reflect the addition of the new modem metadata
>>> carveout reference to the memory-region property.
>>>
>>> Signed-off-by: Sibi Sankar <quic_sibis@quicinc.com>
>>> ---
>>>   .../devicetree/bindings/remoteproc/qcom,sc7180-mss-pil.yaml    | 3 ++-
>>>   .../devicetree/bindings/remoteproc/qcom,sc7280-mss-pil.yaml    | 3 ++-
>>>   2 files changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/Documentation/devicetree/bindings/remoteproc/qcom,sc7180-mss-pil.yaml b/Documentation/devicetree/bindings/remoteproc/qcom,sc7180-mss-pil.yaml
>>> index e4a7da8020f4..b1402bef0ebe 100644
>>> --- a/Documentation/devicetree/bindings/remoteproc/qcom,sc7180-mss-pil.yaml
>>> +++ b/Documentation/devicetree/bindings/remoteproc/qcom,sc7180-mss-pil.yaml
>>> @@ -95,6 +95,7 @@ properties:
>>>       items:
>>>         - description: MBA reserved region
>>>         - description: modem reserved region
>>> +      - description: metadata reserved region
>>
>> Which makes the third item now required, also for all out of tree DTS
>> and other users of the bindings. Please write a bit more in commit msg
>> why this is necessary (e.g. was it broken before?). I assume the driver
>> does not break the ABI?
> 
> I'll pad the commit msg with some of the additional info from patch 4.
> commit c44094eee32f "arm64: dma: Drop cache invalidation from
> arch_dma_prep_coherent()" exposed a bug in the driver affecting SoCs
> from msm8996 on wards. The application processor accessing the
> dynamically allocated region after giving control to the modem results
> in a XPU violation. The recommended fix was to use a no-map carveout
> instead and memunmap before giving control to the modem. The future
> kernels that are paired with an older dtbs would crash during modem

Then it's an ABI break.

> bootup since we would continue to use dma_alloc_attr. But all the other
> combinations (old kernel/new dtb) will continue to work.

Does it mean that old kernel with old DTB was working? If yes, then it's
ABI break without clear benefits.

Best regards,
Krzysztof