[PATCH v4 0/3] bus: mhi: Add loopback driver

[Sumit Kumar <sumit.kumar@oss.qualcomm.com>]

The MHI specification defines a LOOPBACK channel that is already
implemented by MHI-based devices (modems, WLAN) deployed in the field.
The endpoint firmware echoes back whatever the host sends on this channel.
Without a host-side driver, there is no way to exercise this channel to
validate MHI data path integrity between host and endpoint.

This series adds drivers to exercise the LOOPBACK channel from both the
host and endpoint sides. The host driver (patch 1) binds to the LOOPBACK
channel and provides a sysfs interface for configuring transfer parameters,
triggering a test, and reading the result. The sysfs interface is stable
ABI because the wire protocol is fixed by the endpoint firmware already
deployed in the field and cannot be changed.

The endpoint driver (patch 3) echoes received data back to the host using
a workqueue for asynchronous processing. Patch 2 introduces the
mhi_ep_queue_buf() API needed by the endpoint driver for raw buffer
queuing without an skb dependency.

Signed-off-by: Sumit Kumar <sumit.kumar@oss.qualcomm.com>
---
Changes in v4:
- Fix MHI_LOOPBACK_MAX_TRE_SIZE: change SZ_64K to (SZ_64K - 1) since the TRE
  length field is 16 bits and cannot encode 65536 (sashiko)
- Move mhi_prepare_for_transfer() to probe() so ring->el_size is initialized
  before num_tre_store() calls mhi_get_free_desc_count() (sashiko)
- Add mhi_unprepare_from_transfer() in mhi_loopback_remove() (sashiko)
- Add NULL guard in all sysfs show/store callbacks against post-remove drvdata
  race with devres teardown (sashiko)
- Add KMALLOC_MAX_SIZE check before kzalloc() to prevent page allocator WARN
  on large tre_count * tre_size values (sashiko)
- Fix start_store() to use __free(kfree) locals instead of goto-based cleanup
  to comply with cleanup.h guard+goto mixing rule (sashiko)
- Change buf_left and read_offset from u32 to size_t in mhi_ep_queue() to
  avoid truncation of size_t len parameter (sashiko)
- Add zero-length guard in mhi_ep_loopback_ul_callback() before kmemdup() to
  handle 0-byte transfers returning ZERO_SIZE_PTR (sashiko)
- Add NULL guard in mhi_ep_loopback_ul_callback() against post-remove drvdata
  race (sashiko)
- Link to v3: https://lore.kernel.org/r/20260610-loopback_mhi-v3-0-a733c0cef61a@oss.qualcomm.com

Changes in v3:
- Move ep driver to drivers/bus/mhi/ep/clients/loopback.c (Mani)
- Move host driver to drivers/bus/mhi/host/clients/loopback.c; keep
  module name mhi_loopback (Bjorn, Mani)
- Add ABI documentation in Documentation/ABI/testing/sysfs-bus-mhi-devices-loopback
  (Bjorn, Mani)
- Rename sysfs attribute 'size' to 'tre_size'; add 'max_tre_size' attribute
- Update Kconfig title to 'MHI LOOPBACK client driver' and describe that
  the driver binds to the MHI LOOPBACK channel defined in the MHI spec
  (Mani).
- Fix memory leak in ep loopback DL transfer error path.
- Rename mhi_ep_skb_completion() to mhi_ep_buf_completion().
- Document buffer ownership semantics in mhi_ep_queue_buf() kernel-doc
- Fix use-after-free in host loopback
- Fix completion race: arm completion before queuing recv TREs
- Fix teardown race: synchronize mhi_loopback_remove() with start_store()
  via lb_mutex
- Fix u32 multiplication overflow in total_size: use size_mul()
- Replace kmalloc+memcpy with kmemdup in ep loopback UL callback
- Update mhi_ep_queue_buf() kernel-doc: note per-TRE callback behavior
  when buffer length spans multiple host DL TREs
- Move mhi_prepare_for_transfer()/mhi_unprepare_from_transfer() into
  start_store() to avoid holding the channel open when idle
- Link to v2: https://lore.kernel.org/r/20251104-loopback_mhi-v2-0-727a3fd9aa74@oss.qualcomm.com

Changes in v2:
- Use __free(kfree) macro for buffers
- Removed NET layer socket buffer dependency, now using buffer and len
- Created a New Api for queuing buffers for clients which do not use skb
- Link to v1: https://lore.kernel.org/r/20250923-loopback_mhi-v1-0-8618f31f44aa@oss.qualcomm.com

---
Sumit Kumar (3):
      bus: mhi: host: clients: Add loopback driver with sysfs interface
      bus: mhi: ep: Add mhi_ep_queue_buf() API for raw buffer queuing
      bus: mhi: ep: clients: Add loopback driver for data path testing

 .../ABI/testing/sysfs-bus-mhi-devices-loopback     |  51 ++++
 MAINTAINERS                                        |   1 +
 drivers/bus/mhi/ep/Kconfig                         |   2 +
 drivers/bus/mhi/ep/Makefile                        |   1 +
 drivers/bus/mhi/ep/clients/Kconfig                 |  16 +
 drivers/bus/mhi/ep/clients/Makefile                |   2 +
 drivers/bus/mhi/ep/clients/loopback.c              | 128 ++++++++
 drivers/bus/mhi/ep/main.c                          |  29 +-
 drivers/bus/mhi/host/Kconfig                       |   1 +
 drivers/bus/mhi/host/Makefile                      |   1 +
 drivers/bus/mhi/host/clients/Kconfig               |  17 ++
 drivers/bus/mhi/host/clients/Makefile              |   2 +
 drivers/bus/mhi/host/clients/loopback.c            | 329 +++++++++++++++++++++
 include/linux/mhi_ep.h                             |  15 +
 14 files changed, 586 insertions(+), 9 deletions(-)
---
base-commit: e6b9dce0aeeb91dfc0974ab87f02454e24566182
change-id: 20250903-loopback_mhi-dee55ff0d462

Best regards,
-- 
Sumit Kumar <sumit.kumar@oss.qualcomm.com>