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    SubjectRe: [PATCH 1/2] iommufd/selftest: Use a fwnode to distinguish devices
    On Tue, Nov 28, 2023 at 05:36:33PM +0000, Robin Murphy wrote:

    > You see this is exactly the kind of complexity I *don't* want, since the
    > only thing it would foreseeably benefit is the one special case of the
    > IOMMUFD selftest, which can far more trivially just adopt the other of the
    > two "standard" usage models we have. I've been trying to get *away* from
    > having to have boilerplate checks in all the drivers, and this would require
    > bringing back a load of the ones I've just removed :(

    I don't think we need to bring back the fwspec checks you removed, the
    loop just needs to keep the NULL check:

    + list_for_each_entry(iommu, &iommu_device_list, list) {
    + if (iommu->ops != ops && !iommu->fwnode) {
    + ops = iommu->ops;
    + ret = iommu_init_device(pinf, iommu->ops);
    + if (ret != -ENODEV)
    + return ret;
    + }
    + }

    Iterate over all the global driver ops only. Drivers with a fwnode
    will never be called without a fwspec.

    Also, does omap have problems now too? omap seems to set fwnode but
    does some slightly different open coded non-fwspec parsing that worked
    at bus time? Is it still OK? Does fwspec even find ops in omap's FW
    description (ie it looks like it make iommu-cells optional or
    something)?

    > As I said before, I really want to avoid the perf_event_init model of
    > calling round every driver saying "hey, do you want this?" since it's also
    > error-prone if any of those drivers doesn't get the boilerplate exactly
    > right and inadvertently fails to reject something it should have.

    The core missed an API that every driver needs: give me the struct
    iommu_driver* the FW has referenced.

    Instead every driver open codes something like
    arm_smmu_get_by_fwnode(), or much worse.

    If we force the drivers to say

    iommu_driver = iommu_fw_give_me_my_driver(dev, ops)

    Then we automatically have a place to do all the rejection checks we
    need, and driver's can't inadvertently skip this because they really
    can't work without the iommu_driver at all.

    Anyhow, I completed the series I talked about yesterday. It turned out
    really nice I think, especially the driver facing API is much
    cleaner. I'm just going through the last bits before I share it.

    Jason

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