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Subject[PATCH v4 00/11] dmaengine: dw: Take Baikal-T1 SoC DW DMAC peculiarities into account
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
- 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.

Changelog v3:
- Use the block_size found for the very first channel instead of looking for
the maximum of maximum block sizes.
- Don't define device-specific device_dma_parameters object, since it has
already been defined by the platform device core.
- Add more details into the property description about what limitations
snps,max-burst-len defines.
- Move commit fb7e3bbfc830 ("dmaengine: dw: Take HC_LLP flag into account for
noLLP auto-config") to the head of the series.
- Add a new patch "dmaengine: Introduce min burst length capability" as a
result of the discussion with Vinod and Andy regarding the burst length
- Add a new patch "dmaengine: Introduce max SG list entries capability"
suggested by Andy.
- Add a new patch "dmaengine: Introduce DMA-device device_caps callback" as
a result of the discussion with Vinud and Andy in the framework of DW DMA
burst and LLP capabilities.
- Add a new patch "dmaengine: dw: Add dummy device_caps callback" as a
preparation commit before setting the max_burst and max_sg_nents
DW DMA capabilities.
- Override the slave channel max_burst capability instead of calculating
the minimum value of max burst lengths and setting the DMA-device
generic capability.
- Add a new patch "dmaengine: dw: Initialize max_sg_nents with nollp flag".
This is required to fix the DW APB SSI issue of the Tx and Rx DMA
channels de-synchronization.

Changelog v4:
- Use explicit if-else statement when assigning the max_sg_nents field.
- Clamp the dst and src burst lengths in the generic dwc_config() method
instead of doing that in the encode_maxburst() callback.
- Define max_burst with u32 type in struct dw_dma_platform_data.
- Perform of_property_read_u32_array() with the platform data
max_burst member passed directly.
- Add a new patch "dmaengine: dw: Initialize min_burst capability",
which initializes the min_burst capability with 1.
- Fix of->if typo. It should be definitely "of" in the max_sg_list
capability description.

Signed-off-by: Serge Semin <>
Cc: Alexey Malahov <>
Cc: Maxim Kaurkin <>
Cc: Pavel Parkhomenko <>
Cc: Ramil Zaripov <>
Cc: Ekaterina Skachko <>
Cc: Vadim Vlasov <>
Cc: Alexey Kolotnikov <>
Cc: Thomas Bogendoerfer <>
Cc: Arnd Bergmann <>
Cc: Andy Shevchenko <>
Cc: Rob Herring <>

Serge Semin (11):
dt-bindings: dma: dw: Convert DW DMAC to DT binding
dt-bindings: dma: dw: Add max burst transaction length property
dmaengine: Introduce min burst length capability
dmaengine: Introduce max SG list entries capability
dmaengine: Introduce DMA-device device_caps callback
dmaengine: dw: Take HC_LLP flag into account for noLLP auto-config
dmaengine: dw: Set DMA device max segment size parameter
dmaengine: dw: Add dummy device_caps callback
dmaengine: dw: Initialize min_burst capability
dmaengine: dw: Introduce max burst length hw config
dmaengine: dw: Initialize max_sg_nents capability

.../bindings/dma/snps,dma-spear1340.yaml | 176 ++++++++++++++++++
.../devicetree/bindings/dma/snps-dma.txt | 69 -------
drivers/dma/dmaengine.c | 5 +
drivers/dma/dw/core.c | 47 ++++-
drivers/dma/dw/of.c | 5 +
drivers/dma/dw/regs.h | 3 +
include/linux/dmaengine.h | 14 ++
include/linux/platform_data/dma-dw.h | 4 +
8 files changed, 253 insertions(+), 70 deletions(-)
create mode 100644 Documentation/devicetree/bindings/dma/snps,dma-spear1340.yaml
delete mode 100644 Documentation/devicetree/bindings/dma/snps-dma.txt


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