[PATCH v2 0/6] dmaengine: dw: Take Baikal-T1 SoC DW DMAC peculiarities into account

[Serge Semin]

Baikal-T1 SoC has an DW DMAC on-board to provide a Mem-to-Mem, low-speed
peripherals Dev-to-Mem and Mem-to-Dev functionality. Mostly it's compatible
with currently implemented in the kernel DW DMAC driver, but there are some
peculiarities which must be taken into account in order to have the device
fully supported.

First of all traditionally we replaced the legacy plain text-based dt-binding
file with yaml-based one. Secondly Baikal-T1 DW DMA Controller provides eight
channels, which alas have different max burst length configuration.
In particular first two channels may burst up to 128 bits (16 bytes) at a time
while the rest of them just up to 32 bits. We must make sure that the DMA
subsystem doesn't set values exceeding these limitations otherwise the
controller will hang up. In third currently we discovered the problem in using
the DW APB SPI driver together with DW DMAC. The problem happens if there is no
natively implemented multi-block LLP transfers support and the SPI-transfer
length exceeds the max lock size. In this case due to asynchronous handling of
Tx- and Rx- SPI transfers interrupt we might end up with Dw APB SSI Rx FIFO
overflow. So if DW APB SSI (or any other DMAC service consumer) intends to use
the DMAC to asynchronously execute the transfers we'd have to at least warn
the user of the possible errors. In forth it's worth to set the DMA device max
segment size with max block size config specific to the DW DMA controller. It
shall help the DMA clients to create size-optimized SG-list items for the
controller. This in turn will cause less dw_desc allocations, less LLP
reinitializations, better DMA device performance.

Finally there is a bug in the algorithm of the nollp flag detection.
In particular even if DW DMAC parameters state the multi-block transfers
support there is still HC_LLP (hardcode LLP) flag, which if set makes expected
by the driver true multi-block LLP functionality unusable. This happens cause'
if HC_LLP flag is set the LLP registers will be hardcoded to zero so the
contiguous multi-block transfers will be only supported. We must take the
flag into account when detecting the LLP support otherwise the driver just
won't work correctly.

This patchset is rebased and tested on the mainline Linux kernel 5.7-rc4:
0e698dfa2822 ("Linux 5.7-rc4")
tag: v5.7-rc4

Changelog v2:
- Rearrange SoBs.
- Move $ref to the root level of the properties. So do do with the
  constraints in the DT binding.
- Replace "additionalProperties: false" with "unevaluatedProperties: false"
  property in the DT binding file.
- Discard default settings defined out of property enum constraint.
- Set default max-burst-len to 256 TR-WIDTH words in the DT binding.
- Discard noLLP and block_size accessors.
- Set max segment size of the DMA device structure with the DW DMA block size
  config.
- Print warning if noLLP flag is set.
- Discard max burst length accessor.
- Add comment about why hardware accelerated LLP list support depends
  on both MBLK_EN and HC_LLP configs setting.
- Use explicit bits state comparison operator in noLLP flag setting.

Signed-off-by: Serge Semin <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx>
Cc: Alexey Malahov <Alexey.Malahov@xxxxxxxxxxxxxxxxxxxx>
Cc: Maxim Kaurkin <Maxim.Kaurkin@xxxxxxxxxxxxxxxxxxxx>
Cc: Pavel Parkhomenko <Pavel.Parkhomenko@xxxxxxxxxxxxxxxxxxxx>
Cc: Ramil Zaripov <Ramil.Zaripov@xxxxxxxxxxxxxxxxxxxx>
Cc: Ekaterina Skachko <Ekaterina.Skachko@xxxxxxxxxxxxxxxxxxxx>
Cc: Vadim Vlasov <V.Vlasov@xxxxxxxxxxxxxxxxxxxx>
Cc: Alexey Kolotnikov <Alexey.Kolotnikov@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
Cc: Paul Burton <paulburton@xxxxxxxxxx>
Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Rob Herring <robh+dt@xxxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxxx
Cc: dmaengine@xxxxxxxxxxxxxxx
Cc: devicetree@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx

Serge Semin (6):
  dt-bindings: dma: dw: Convert DW DMAC to DT binding
  dt-bindings: dma: dw: Add max burst transaction length property
  dmaengine: dw: Set DMA device max segment size parameter
  dmaengine: dw: Print warning if multi-block is unsupported
  dmaengine: dw: Introduce max burst length hw config
  dmaengine: dw: Take HC_LLP flag into account for noLLP auto-config

 .../bindings/dma/snps,dma-spear1340.yaml      | 173 ++++++++++++++++++
 .../devicetree/bindings/dma/snps-dma.txt      |  69 -------
 drivers/dma/dw/core.c                         |  57 +++++-
 drivers/dma/dw/dw.c                           |   1 +
 drivers/dma/dw/of.c                           |   9 +
 drivers/dma/dw/regs.h                         |  21 ++-
 include/linux/platform_data/dma-dw.h          |   4 +
 7 files changed, 256 insertions(+), 78 deletions(-)
 create mode 100644 Documentation/devicetree/bindings/dma/snps,dma-spear1340.yaml
 delete mode 100644 Documentation/devicetree/bindings/dma/snps-dma.txt

-- 
2.25.1