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mimxrt: Fix USB CDC handling so it works reliably. #6881

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Merged
merged 1 commit into from
Feb 12, 2021
Merged

mimxrt: Fix USB CDC handling so it works reliably. #6881

merged 1 commit into from
Feb 12, 2021

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dpgeorge
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On i.MX the SysTick IRQ cannot wake the CPU from a WFI so the CPU was
blocked on WFI waiting for USB data in mp_hal_stdin_rx_chr() even though it
had already arrived (because it may arrive just after calling the check
tud_cdc_available()).  This commit fixes this problem by using SEV/WFE to
indicate that there has been a USB event.

The mp_hal_stdout_tx_strn() function is also fixed so that it doesn't
overflow the USB buffers.

@dpgeorge
mimxrt: Fix USB CDC handling so it works reliably.
f31c6b4 
On i.MX the SysTick IRQ cannot wake the CPU from a WFI so the CPU was
blocked on WFI waiting for USB data in mp_hal_stdin_rx_chr() even though it
had already arrived (because it may arrive just after calling the check
tud_cdc_available()).  This commit fixes this problem by using SEV/WFE to
indicate that there has been a USB event.

The mp_hal_stdout_tx_strn() function is also fixed so that it doesn't
overflow the USB buffers.

Signed-off-by: Damien George <[email protected]>
@dpgeorge
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@hathach FYI this fixes our i.MX port to make it 100% reliable with USB CDC transfers (as far as my tests show). It still calls tud_task() within the IRQ handler, which seems ok.

@dpgeorge dpgeorge merged commit f31c6b4 into micropython:master Feb 12, 2021
@dpgeorge dpgeorge deleted the mimxrt-fix-usb branch February 12, 2021 03:07
@hathach
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hathach commented Feb 12, 2021

Thanks @dpgeorge for the update, to be honest I don’t know MP enough to comment on the changes. But I am glad you figured it out.
For the race condition between entering wfi and usb event occurred, I have a similar PR for this in circuitpython adafruit#2956 . The solution is disable irqs then use tud_task_event_ready() to check if there is any pending events before executing wfi. In this case since tud_task() is called within isr and event is served right away. Scenario is bit different, though I just mentioned it here for reference since they share some similarities.

PS: feel free to tag me in the future if there is issue related to tinyusb 😃

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The solution is disable irqs then use tud_task_event_ready() to check if there is any pending events before executing wfi.

Yes, we do use that pattern in some places in the stm32 port. But IMO using SEV/WFE is a better way to handle atomic events.

@hathach
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hathach commented Feb 13, 2021

Glad you fixed it all. I just create an issue as reminder to implement an isr safe version where several osal calls such as mutex can be bypassed and better testing. Will update the development there, and let you know when its PR is ready.

hathach/tinyusb#651

@hathach hathach mentioned this pull request May 10, 2021
Merged
Wind-stormger pushed a commit to BPI-STEAM/micropython that referenced this pull request Sep 15, 2022
@dhalbert
Merge pull request micropython#6881 from CytronTechnologies/modify-cy…
45ee130 
…tron-maker-feather-aiot-s3-pin

Modified the pins assignment for Cytron Maker Feather AIoT S3.
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