RE: [External Mail] [PATCH v2 1/7] net: wwan: t9xx: Add PCIe core

[Wu. JackBB (GSM)]

Hi Jakub,

Addressing sashiko AI code review comments for this patch, as
requested by you in the patch 3/7 review:
https://patchwork.kernel.org/project/netdevbpf/patch/20260610-t9xx_driver_v1-v2-3-c65addf23b3f@xxxxxxxxxx/#27006088

Q1: Does this code perform an incorrect double byte-swap on big-endian
architectures? The hardware bits are manually swapped using cpu_to_le32()
and cast back to u32. This value is later passed to mtk_pci_write32(),
which utilizes iowrite32(). Since iowrite32() internally handles
host-to-little-endian conversion, swapping the value beforehand will cause
a double swap on big-endian platforms.

  This driver targets MediaTek T9xx PCIe WWAN modems, which exist
  exclusively on x86 platforms (little-endian). On little-endian,
  cpu_to_le32() is a no-op and LE32_TO_U32() is a simple cast, so
  no byte-swap occurs. The hardware register layout assumes LE host
  ordering. While the pattern is technically incorrect for big-endian,
  this hardware is not available on BE platforms and the Kconfig
  dependency (depends on PCI) combined with the device's PCIe-only
  nature effectively restricts this to x86/ARM64-LE.

Q2: Does this call to ffs() yield the wrong channel index on big-endian
systems? hw_bits has already been endian-swapped in
mtk_pci_ext_h2d_evt_hw_bits(). Using ffs() on an endian-swapped value
produces a completely incorrect bit index.

  Same as Q1 — on little-endian platforms, no swap occurs, so ffs()
  operates on the correct value. Additionally, all callers of
  mtk_pci_send_ext_evt() pass known-valid channel values through the
  is_power_of_2(ch) check, and the SET_HW_BITS mapping covers all
  valid channels.

Q3: Does clearing the top-level interrupt status at the end of the handler
without a subsequent read loop lead to permanently lost hardware events?
If a new hardware event triggers during the worker's execution, clearing
the write-1-to-clear (W1C) interrupt status after the hardware event
statuses were read at the start will discard the newly asserted events.

  No events are lost. mtk_pci_clear_irq() clears the MSI-X interrupt
  status register (BIT(irq_id) in REG_MSIX_ISTATUS_HOST_GRP0_0), NOT
  the MHCCIF event registers. The MHCCIF events are level-triggered
  from the modem's EP2RC registers — the modem sets event bits and
  they remain set until the modem clears them or the host acknowledges.

  The flow is:
    1. Hardware: modem sets EP2RC event bits -> MSI-X interrupt fires
    2. ISR: masks MSI-X bit, schedules work
    3. Worker: reads EP2RC event status, dispatches callbacks
    4. Worker: clears MSI-X status (BIT(irq_id)), unmasks MSI-X

  If a new MHCCIF event arrives during step 3, the EP2RC register
  gets the new bit set. The MSI-X clear in step 4 only clears the
  MSI-X pending bit, not the MHCCIF source. After unmask, the MHCCIF
  event source re-asserts the MSI-X interrupt because the EP2RC bits
  are still set, triggering a new ISR -> new work -> new read of the
  updated EP2RC register.

Q4: Does this code incorrectly attempt to unmap a bitmask instead of a
single BAR index? MTK_REQUESTED_BARS is defined as a bitmask, but
pcim_iounmap_region() expects a specific BAR index. Furthermore, is
this manual unmapping redundant since the memory was mapped with
pcim_iomap_regions() and will automatically be cleaned up by devres?

  Valid on both points. Fixed in v3 by removing mtk_pci_bar_exit()
  entirely. pcim_iomap_regions() is devres-managed so manual unmap
  is redundant.

Q5: Does this logic mistakenly drop hardware interrupts when fewer than 32
MSI-X vectors are allocated? If fewer than 32 vectors are allocated
(for example, irq_cnt = 1), GENMASK(0, 0) evaluates to BIT(0). A
hardware interrupt source like MHCCIF (bit 28) will result in
BIT(28) & BIT(0) == 0. Could this lead to a complete loss of critical
device events?

  No issue. This is the MSI-X merged mode design. When fewer vectors
  are allocated than interrupt sources, mtk_pci_set_msix_merged()
  configures the hardware to merge multiple interrupt sources into
  the available vectors. With irq_cnt=1, the hardware maps ALL
  interrupt sources to vector 0. The GENMASK filters the irq_state
  to only the allocated vector bits, then checks if the current
  handler's vector bit is set. This correctly handles both full
  allocation (32 vectors, 1:1 mapping) and merged mode (fewer
  vectors, N:1 mapping).

Q6: Will this manual pci_disable_device() call trigger an atomic underflow?
Since the device was initialized using pcim_enable_device(), devres will
automatically disable the device during cleanup. Manually calling
pci_disable_device() drops the enable_cnt to zero. When devres executes
its cleanup, it will call pci_disable_device() again, leading to an
underflow and a kernel warning.

  Valid. Fixed in v3 by removing pci_disable_device() from both the
  probe error path and mtk_pci_remove(). pcim_enable_device() handles
  cleanup via devres.

Q7: Does performing a PCIe Link Down Reset (PLDR) before freeing IRQs cause
PCIe Unsupported Request (UR) faults? mtk_pci_pldr() power-cycles the
device and clears its BARs. Directly after this, mtk_pci_free_irq() is
called, which attempts to access the device's MSI-X MMIO table.

  No UR faults. mtk_pci_pldr() calls ACPI PXP._OFF then PXP._ON.
  The _ON call restores power to the device, bringing the PCIe link
  back up with BARs accessible. pci_free_irq_vectors() writes to PCI
  config space (MSI-X capability enable bit) which is always
  accessible while the device is on the bus. The MSI-X table MMIO
  access for masking individual entries goes through the
  re-established BAR mapping.

Q8: Will this manual pci_disable_device() call in mtk_pci_remove() trigger
an atomic underflow for the same reason as in mtk_pci_probe()?
Is a call to mtk_pci_dev_exit() missing from the remove path?

  Two issues, both valid and fixed in v3:
  1. pci_disable_device() underflow: same as Q6, removed.
  2. Missing mtk_pci_dev_exit(): added in patch 3/7 v3 (where
     mtk_pci_dev_init is introduced).

Thanks.


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