[rtl872x] hal: make USB CDC a bit quicker by scheduling TX from timer callback delayed by 1ms #2672
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scott-brust
approved these changes
Jul 20, 2023
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I tested serial logging in a tight loop and can see this PR speeds things up considerably.
Without PR, about 21 ms between lines:
[2023-07-20 15:09:16.717] 0000014181 [app] INFO: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890
[2023-07-20 15:09:16.738] 0000014202 [app] INFO: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890
[2023-07-20 15:09:16.759] 0000014223 [app] INFO: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890
[2023-07-20 15:09:16.781] 0000014244 [app] INFO: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890
[2023-07-20 15:09:16.802] 0000014266 [app] INFO: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890
[2023-07-20 15:09:16.823] 0000014287 [app] INFO: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890
With PR, about 8ms between lines
[2023-07-20 15:04:24.525] 0000004632 [app] INFO: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890
[2023-07-20 15:04:24.533] 0000004640 [app] INFO: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890
[2023-07-20 15:04:24.542] 0000004649 [app] INFO: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890
[2023-07-20 15:04:24.550] 0000004657 [app] INFO: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890
[2023-07-20 15:04:24.559] 0000004665 [app] INFO: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890
[2023-07-20 15:04:24.567] 0000004673 [app] INFO: abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890
| @@ -415,29 +422,34 @@ int CdcClassDriver::startTx() { | |||
| return 0; | |||
| } | |||
|
|
|||
| if (holdoff) { | |||
| stopTxTimer(); | |||
| return startTxTimer(); | |||
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What is the use case for holdoff = true? This just delays the start of transmission by 1ms?
… callback delayed by 1ms
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Problem
USB stack on RTL872x platforms is pretty slow due to processing of interrupts in a task thread and weird scheduling. As a consequence this makes USB CDC (Serial) pretty slow especially on Device->Host path.
Solution
Trigger Device->Host transfers from a high-priority timer thread and delay them by 1ms (RTOS tick), which helps with task scheduling and allows to put more data into TX buffer before the transfer is scheduled. Previously writing 1 character (which is what the wiring APIs do) into an empty TX buffer was immediately triggering TX transaction and due to weird scheduling most of the transfers were just 1 byte.
Steps to Test
wiring/usbserialExample App
N/A
References
N/A
Completeness