The core code allows the domain to be changed on the fly without a forced
stop in BLOCKED/IDENTITY. In this flow the driver should just continually
maintain the ATS with no change while the STE is updated.

ATS relies on a linked list smmu_domain->devices to keep track of which
masters have the domain programmed, but this list is also used by
arm_smmu_share_asid(), unrelated to ats.

Create two new functions to encapsulate this combined logic:
 arm_smmu_attach_prepare()
 <caller generates and sets the STE>
 arm_smmu_attach_commit()

The two functions can sequence both enabling ATS and disabling across
the STE store. Have every update of the STE use this sequence.

Installing a S1/S2 domain always enables the ATS if the PCIe device
supports it.

The enable flow is now ordered differently to allow it to be hitless:

 1) Add the master to the new smmu_domain->devices list
 2) Program the STE
 3) Enable ATS at PCIe
 4) Remove the master from the old smmu_domain

This flow ensures that invalidations to either domain will generate an ATC
invalidation to the device while the STE is being switched. Thus we don't
need to turn off the ATS anymore for correctness.

The disable flow is the reverse:
 1) Disable ATS at PCIe
 2) Program the STE
 3) Invalidate the ATC
 4) Remove the master from the old smmu_domain

Move the nr_ats_masters adjustments to be close to the list
manipulations. It is a count of the number of ATS enabled masters
currently in the list. This is stricly before and after the STE/CD are
revised, and done under the list's spin_lock.

This is part of the bigger picture to allow changing the RID domain while
a PASID is in use. If a SVA PASID is relying on ATS to function then
changing the RID domain cannot just temporarily toggle ATS off without
also wrecking the SVA PASID. The new infrastructure here is organized so
that the PASID attach/detach flows will make use of it as well in
following patches.
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Michael Shavit <mshavit@google.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/4-v9-5cd718286059+79186-smmuv3_newapi_p2b_jgg@nvidia.comSigned-off-by: Will Deacon <will@kernel.org>