linux-kernel - Re: [PATCH v5 2/7] iommu: Decouple iommu

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Date:   Thu, 12 Oct 2023 14:05:14 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Robin Murphy <robin.murphy@....com>, joro@...tes.org,
        will@...nel.org
Cc:     baolu.lu@...ux.intel.com, iommu@...ts.linux.dev, jgg@...dia.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/7] iommu: Decouple iommu_present() from bus ops

On 10/12/23 2:14 AM, 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

The iommu_present() is only used in below two drivers.

$ git grep iommu_present
drivers/gpu/drm/mediatek/mtk_drm_drv.c: if 
(!iommu_present(&platform_bus_type))
drivers/gpu/drm/tegra/drm.c:    if (host1x_drm_wants_iommu(dev) && 
iommu_present(&platform_bus_type)) {

Both are platform drivers and have the device pointer passed in. Just
out of curiosity, why not replacing them with device_iommu_mapped()
instead? Sorry if I overlooked previous discussion.

Best regards,
baolu

> 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.
> 
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> 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 5a3ce293a5de..7bb92e8b7a49 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -2000,9 +2000,28 @@ 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);
> +		}
> +	}
> +	return ret;
>   }
>   EXPORT_SYMBOL_GPL(iommu_present);
>