[RFC v3 0/3] Re-introduce TX FIFO resize for larger EP bursting

From: Wesley Cheng
Date: Wed May 27 2020 - 21:46:22 EST

Changes in V3:
- Removed "Reviewed-by" tags
- Renamed series back to RFC
- Modified logic to ensure that fifo_size is reset if we pass the minimum
threshold. Tested with binding multiple FDs requesting 6 FIFOs.

Changes in V2:
- Modified TXFIFO resizing logic to ensure that each EP is reserved a
- Removed dev_dbg() prints and fixed typos from patches
- Added some more description on the dt-bindings commit message

Currently, there is no functionality to allow for resizing the TXFIFOs, and
relying on the HW default setting for the TXFIFO depth. In most cases, the
HW default is probably sufficient, but for USB compositions that contain
multiple functions that require EP bursting, the default settings
might not be enough. Also to note, the current SW will assign an EP to a
function driver w/o checking to see if the TXFIFO size for that particular
EP is large enough. (this is a problem if there are multiple HW defined
values for the TXFIFO size)

It is mentioned in the SNPS databook that a minimum of TX FIFO depth = 3
is required for an EP that supports bursting. Otherwise, there may be
frequent occurences of bursts ending. For high bandwidth functions,
such as data tethering (protocols that support data aggregation), mass
storage, and media transfer protocol (over FFS), the bMaxBurst value can be
large, and a bigger TXFIFO depth may prove to be beneficial in terms of USB
throughput. (which can be associated to system access latency, etc...) It
allows for a more consistent burst of traffic, w/o any interruptions, as
data is readily available in the FIFO.

With testing done using the mass storage function driver, the results show
that with a larger TXFIFO depth, the bandwidth increased significantly.

Test Parameters:
- Platform: Qualcomm SM8150
- bMaxBurst = 6
- USB req size = 256kB
- Num of USB reqs = 16
- USB Speed = Super-Speed
- Function Driver: Mass Storage (w/ ramdisk)
- Test Application: CrystalDiskMark


TXFIFO Depth = 3 max packets

Test Case | Data Size | AVG tput (in MB/s)
Sequential|1 GB x |
Read |9 loops | 193.60
| | 195.86
| | 184.77
| | 193.60

TXFIFO Depth = 6 max packets

Test Case | Data Size | AVG tput (in MB/s)
Sequential|1 GB x |
Read |9 loops | 287.35
| | 304.94
| | 289.64
| | 293.61

Wesley Cheng (3):
usb: dwc3: Resize TX FIFOs to meet EP bursting requirements
arm64: boot: dts: qcom: sm8150: Enable dynamic TX FIFO resize logic
dt-bindings: usb: dwc3: Add entry for tx-fifo-resize

Documentation/devicetree/bindings/usb/dwc3.txt | 2 +-
arch/arm64/boot/dts/qcom/sm8150.dtsi | 1 +
drivers/usb/dwc3/core.c | 2 +
drivers/usb/dwc3/core.h | 8 ++
drivers/usb/dwc3/ep0.c | 37 +++++++-
drivers/usb/dwc3/gadget.c | 115 +++++++++++++++++++++++++
6 files changed, 163 insertions(+), 2 deletions(-)

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