Re: [PATCH v3 04/18] driver core: platform: Add driver dma ownership management

[Jason Gunthorpe]

On Tue, Dec 07, 2021 at 05:25:04AM -0800, Christoph Hellwig wrote:
> On Tue, Dec 07, 2021 at 09:16:27AM -0400, Jason Gunthorpe wrote:
> > Yes, the suggestion was to put everything that 'if' inside a function
> > and then of course a matching undo function.
> 
> Can't we simplify things even more?  Do away with the DMA API owner
> entirely, and instead in iommu_group_set_dma_owner iterate over all
> devices in a group and check that they all have the no_dma_api flag
> set (plus a similar check on group join).  With that most of the
> boilerplate code goes away entirely in favor of a little more work at
> iommu_group_set_dma_owner time.

Robin suggested something like this already.

The locking doesn't work out, we can't nest device_lock()'s safely
without ABBA deadlocks, and can't touch the dev->driver without the
device_lock.

Jason