Question: multiple SMMUv3 devices probe

[Zenghui Yu]

On 2021/4/14 9:45, richard clark wrote:
> Jean-Philippe Brucker <jean-philippe at linaro.org> 于2021年4月13日周二 下午8:58写道：
>>
>> On Tue, Apr 13, 2021 at 08:40:47PM +0800, richard clark wrote:

[...]

>>> Actually, my question is a if pci device driver probes its device
>>> before the SMMUv3 driver probing,
>>> how does that pci device using the DMA with SMMU translation?
>>
>> It doesn't - by looking at the dependencies described in device-tree
> (forget to reply all, so add the CC'ing...)
> I understand 'it doesn't' here means the pci driver will not probe its
> device before SMMU driver is on
>> (iommu-map property) or in ACPI IORT, we defer probe of the PCI device
>> until the SMMU is operational.
>>
> If that is correct, Jean, would you point me to the code snippet that
> a pci driver will defer
> its probe until the SMMU is functional? Some key points are enough...

I think it's correct. Take ACPI-based system as an example, we setup PCI
device's DMA configuration by pci_dma_configure()/acpi_dma_configure().
If as you said, the PCI device is probed before the SMMUv3 probing, we
will get *-EPROBE_DEFER* during the DMA configuration. Please have a
look at iort_pci_iommu_init()/iort_iommu_xlate(), which should have
provided the whole details for us.

The device we *are* probing is then put onto the "deferred list" (grep
it in the driver core code) and will be retried from the list by
deferred_probe_work.

Hope it helps.


Zenghui