iommu/arm-smmu-v3: Allow ATS to be always on

When a device's default substream attaches to an identity domain, the SMMU
driver currently sets the device's STE between two modes:

  Mode 1: Cfg=Translate, S1DSS=Bypass, EATS=1
  Mode 2: Cfg=bypass (EATS is ignored by HW)

When there is an active PASID (non-default substream), mode 1 is used. And
when there is no PASID support or no active PASID, mode 2 is used.

The driver will also downgrade an STE from mode 1 to mode 2, when the last
active substream becomes inactive.

However, there are PCIe devices that demand ATS to be always on. For these
devices, their STEs have to use the mode 1 as HW ignores EATS with mode 2.

Change the driver accordingly:
  - always use the mode 1
  - never downgrade to mode 2
  - allocate and retain a CD table (see note below)

Note that these devices might not support PASID, i.e. doing non-PASID ATS.
In such a case, the ssid_bits is set to 0. However, s1cdmax must be set to
a !0 value in order to keep the S1DSS field effective. Thus, when a master
requires ats_always_on, set its s1cdmax to at least 1, meaning that the CD
table will have a dummy entry (SSID=1) that will never be used.

Now for these devices, arm_smmu_cdtab_allocated() will always return true,
v.s. false prior to this change. When its default substream is attached to
an IDENTITY domain, its first CD is NULL in the table, which is a totally
valid case. Thus, add "!master->ats_always_on" to the condition.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Tested-by: Nirmoy Das <nirmoyd@nvidia.com>
Acked-by: Nirmoy Das <nirmoyd@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>