[PATCH v6 0/2] iommu/arm-smmu-v3: Make the queue depths tunable, and shrink them in a kdump kernel

[Kiryl Shutsemau (Meta)]

The queues are sized from the IDR1 maxima and allocated at probe, costing
megabytes per queue per SMMU instance. A kdump capture kernel pays that out
of a small crashkernel reservation, for queues it barely uses and two of
which it switches off anyway.

Patch 1 adds a cmdq_max_n_shift module parameter, decided in a per-queue
helper and floored at one page. Patch 2 has a kdump kernel default all
three depths to one page through the same helper. An explicit
cmdq_max_n_shift still wins.

Measured per instance under QEMU on -M virt,iommu=smmuv3, through a real
panic and kexec into a capture kernel:

                        4K page          64K page
      cmdq         1 MB -> 4 KB      8 MB -> 64 KB
      evtq         1 MB -> 4 KB     16 MB -> 64 KB

Every clamped queue lands on exactly one page. cmdq_max_n_shift moves the
command queue alone and beats the kdump default; the capture kernel
attached four devices with no CMD_SYNC timeout, GERROR or context fault.
QEMU exposes no PRI queue, which takes the same path. Build-tested across
4K/16K/64K, TEGRA241_CMDQV=n, CRASH_DUMP=n and =m, every commit
warning-free.

v6:
 - Take the parameter as a log2 depth and call it cmdq_max_n_shift (Nicolin).
 - Nicolin's Reviewed-by and Tested-by from v5 are not carried over, as
   the parameter changed shape.

v5: https://lore.kernel.org/all/20260907095835.1233352-1-kas@xxxxxxxxxx/
v4: https://lore.kernel.org/all/20260902121724.3494954-1-kas@xxxxxxxxxx/
v3: https://lore.kernel.org/all/20260706084708.8072-1-kas@xxxxxxxxxx/

Kiryl Shutsemau (Meta) (2):
  iommu/arm-smmu-v3: Add a cmdq_max_n_shift module parameter
  iommu/arm-smmu-v3: Default queue depths to one page in a kdump kernel

 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c   | 75 +++++++++++++++++--
 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h   |  1 +
 .../iommu/arm/arm-smmu-v3/tegra241-cmdqv.c    |  2 +-
 3 files changed, 70 insertions(+), 8 deletions(-)


base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
-- 
2.54.0