linux-kernel - Re: [PATCH v4 2/7] iommu: Decouple iommu

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Date:   Mon, 2 Oct 2023 11:17:04 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     joro@...tes.org, will@...nel.org, iommu@...ts.linux.dev,
        baolu.lu@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/7] iommu: Decouple iommu_present() from bus ops

On Mon, Oct 02, 2023 at 02:49:10PM +0100, Robin Murphy wrote:
> Much as I'd like to remove iommu_present(), the final remaining users
> are proving stubbornly difficult to clean up, so kick that can down the
> road and just rework it to preserve the current behaviour without
> depending on bus ops. Since commit 57365a04c921 ("iommu: Move bus setup
> to IOMMU device registration"), any registered IOMMU instance is already
> considered "present" for every entry in iommu_buses, so it's simply a
> case of validating the bus and checking we have at least once IOMMU.
> 
> Signed-off-by: Robin Murphy <robin.murphy@....com>
> 
> ---
> 
> v3: Tweak to use the ops-based check rather than group-based, to
>     properly match the existing behaviour
> v4: Just look for IOMMU instances instead of managed devices
> ---
>  drivers/iommu/iommu.c | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index f7793d1b5c3e..ef7feb0acc34 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -1988,9 +1988,28 @@ int bus_iommu_probe(const struct bus_type *bus)
>  	return 0;
>  }
>  
> +/**
> + * iommu_present() - make platform-specific assumptions about an IOMMU
> + * @bus: bus to check
> + *
> + * Do not use this function. You want device_iommu_mapped() instead.
> + *
> + * Return: true if some IOMMU is present and aware of devices on the given bus;
> + * in general it may not be the only IOMMU, and it may not have anything to do
> + * with whatever device you are ultimately interested in.
> + */
>  bool iommu_present(const struct bus_type *bus)
>  {
> -	return bus->iommu_ops != NULL;
> +	bool ret = false;
> +
> +	for (int i = 0; i < ARRAY_SIZE(iommu_buses); i++) {
> +		if (iommu_buses[i] == bus) {
> +			spin_lock(&iommu_device_lock);
> +			ret = !list_empty(&iommu_device_list);
> +			spin_unlock(&iommu_device_lock);
> +		}

Add here:

return ret;

> +	}
> +	return ret;

And this becomes

return false

?

Regardless

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason