Re: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma interfaces

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Sep 27, 2021 at 09:42:58AM +0000, Tian, Kevin wrote:

> +static int iommu_dev_viable(struct device *dev, void *data)
> +{
> +	enum dma_hint hint = *data;
> +	struct device_driver *drv = READ_ONCE(dev->driver);
> +
> +	/* no conflict if the new device doesn't do DMA */
> +	if (hint == DMA_FOR_NONE)
> +		return 0;
> +
> +	/* no conflict if this device is driver-less, or doesn't do DMA */
> +	if (!drv || (drv->dma_hint == DMA_FOR_NONE))
> +		return 0;

While it is kind of clever to fetch this in the drv like this, the
locking just doesn't work right.

The group itself needs to have an atomic that encodes what state it is
in. You can read the initial state from the drv, under the
device_lock, and update the atomic state

Also, don't call it "hint", there is nothing hinty about this, it has
definitive functional impacts.

Greg will want to see a definiate benefit from this extra global code,
so be sure to explain about why the BUG_ON is bad, and how driver core
involvement is needed to fix it properly.

Jason