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rework RP2040 PIO UART interrupt path#840

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HaoboGu merged 1 commit into
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scottlamb:fix-pio-tx-full
May 10, 2026
Merged

rework RP2040 PIO UART interrupt path#840
HaoboGu merged 1 commit into
HaoboGu:mainfrom
scottlamb:fix-pio-tx-full

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@scottlamb scottlamb commented May 10, 2026

The TX path was broken if PIO::uart_buffer().buf_tx.is_full() was true on entry; it would not drain or disable the interrupt, so the it'd repeat forever, starving other work. Best guess this was supposed to be is_empty(), but the check is unnecessary anyway.

I accidentally reproduced this quite reliably by sending several maximum-length messages in a row. :-/

While here, I adjusted both TX and RX loops to transfer up to 8 bytes per availability check rather than 1 by using the FLEVEL register instead of FSTAT.

@scottlamb
rework RP2040 PIO UART interrupt path
19326f7 
The TX path was broken if `PIO::uart_buffer().buf_tx.is_full()` was
true on entry; it would not drain or disable the interrupt, so the
it'd repeat forever, starving other work. Best guess this was supposed
to be `is_empty()`, but the check is unnecessary anyway.

I accidentally reproduced this quite reliably by sending several
maximum-length messages in a row. :-/

While here, I adjusted both TX and RX loops to transfer up to 8 bytes
per availability check rather than 1 by using the FLEVEL register
instead of FSTAT.
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github-actions Bot commented May 10, 2026

Size Report

Example main PR Diff .text .data .bss
use_config/nrf52832_ble 356.2 KiB 356.2 KiB +0.00% ⬆️ +8 0 0
use_config/nrf52840_ble 406.3 KiB 406.3 KiB +0.00% ⬇️ -8 0 0
use_config/nrf52840_ble_split (central) 482.2 KiB 482.2 KiB +0.00% ⬇️ -8 0 0
use_config/nrf52840_ble_split (peripheral) 307.2 KiB 307.2 KiB +0.00% ⬆️ +4 0 0
use_config/pi_pico_w_ble 640.2 KiB 640.2 KiB +0.00% 0 0 0
use_config/rp2040 146.6 KiB 146.7 KiB +0.00% ⬆️ +4 0 0
use_config/rp2040_split (central) 158.4 KiB 158.4 KiB +0.00% ⬆️ +8 0 0
use_config/rp2040_split (peripheral) 25.9 KiB 25.9 KiB -0.01% ⬇️ -4 0 0
use_config/stm32f1 62.3 KiB 62.3 KiB +0.00% 0 0 0
use_config/stm32h7 99.7 KiB 99.7 KiB +0.00% ⬇️ -4 0 0
use_rust/nrf52832_ble 345.3 KiB 345.3 KiB +0.00% 0 0 0
use_rust/nrf52840_ble 401.6 KiB 401.5 KiB +0.00% ⬇️ -12 0 0
use_rust/nrf52840_ble_split (central) 490.5 KiB 490.5 KiB +0.00% ⬆️ +8 0 0
use_rust/nrf52840_ble_split (peripheral) 304.5 KiB 304.5 KiB +0.00% ⬆️ +4 0 0
use_rust/pi_pico_w_ble 640.7 KiB 640.8 KiB +0.00% ⬆️ +32 0 0
use_rust/rp2040 146.4 KiB 146.4 KiB +0.00% ⬆️ +8 0 0
use_rust/rp2040_split (central) 157.7 KiB 157.7 KiB +0.00% ⬇️ -4 0 0
use_rust/rp2040_split (peripheral) 26.4 KiB 26.4 KiB -0.01% ⬇️ -4 0 0
use_rust/stm32f1 61.8 KiB 61.8 KiB +0.00% ⬇️ -4 0 0
use_rust/stm32h7 120.8 KiB 120.8 KiB +0.00% ⬆️ +4 0 0
use_config/nrf52832_ble — 356.2 KiB → 356.2 KiB (+0.00% ⬆️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 326056	   5192	  33464	 364712	  590a8	rmk-nrf52832

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 326048	   5192	  33464	 364704	  590a0	rmk-nrf52832

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%    +436  [ = ]       0    .debug_info
   +60%     +24  [ = ]       0    [Unmapped]
  +0.2%     +17  [ = ]       0    .debug_abbrev
  +0.0%      +8  +0.0%      +8    .text
  -0.0%      -8  [ = ]       0    .debug_line
  -0.0%    -601  [ = ]       0    .debug_str
  -0.0%    -124  +0.0%      +8    TOTAL
use_config/nrf52840_ble — 406.3 KiB → 406.3 KiB (+0.00% ⬇️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 360616	   5200	  50248	 416064	  65940	rmk-nrf52840

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 360624	   5200	  50248	 416072	  65948	rmk-nrf52840

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1% +2.57Ki  [ = ]       0    .debug_info
  +0.2% +1.03Ki  [ = ]       0    .debug_loc
  +0.0%    +706  [ = ]       0    .debug_str
  +0.1%    +411  [ = ]       0    .debug_line
   +12%      +6  [ = ]       0    [Unmapped]
  -0.0%      -8  [ = ]       0    .debug_aranges
  -0.0%      -8  -0.0%      -8    .text
  -0.0%     -20  [ = ]       0    .debug_frame
  -0.0%     -25  [ = ]       0    .strtab
  -0.0%     -32  [ = ]       0    .debug_ranges
  -0.0%     -32  [ = ]       0    .symtab
  +0.1% +4.57Ki  -0.0%      -8    TOTAL
use_config/nrf52840_ble_split (central) — 482.2 KiB → 482.2 KiB (+0.00% ⬇️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 441164	   6524	  46056	 493744	  788b0	central

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 441172	   6524	  46056	 493752	  788b8	central

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1% +1.90Ki  [ = ]       0    .debug_info
  +0.1% +1.00Ki  [ = ]       0    .debug_loc
  +0.0%    +105  [ = ]       0    .debug_line
  +7.5%      +5  [ = ]       0    [Unmapped]
  -0.0%      -8  [ = ]       0    .debug_aranges
  -0.0%      -8  -0.0%      -8    .text
  -0.0%     -20  [ = ]       0    .debug_frame
  -0.0%     -25  [ = ]       0    .strtab
  -0.0%     -32  [ = ]       0    .debug_ranges
  -0.0%     -32  [ = ]       0    .symtab
  -0.0% -1.18Ki  [ = ]       0    .debug_str
  +0.0% +1.71Ki  -0.0%      -8    TOTAL
use_config/nrf52840_ble_split (peripheral) — 307.2 KiB → 307.2 KiB (+0.00% ⬆️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 282128	   5888	  26520	 314536	  4cca8	peripheral

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 282124	   5888	  26520	 314532	  4cca4	peripheral

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%      +4  +0.0%      +4    .text
  -0.0%      -2  [ = ]       0    .debug_line
  -3.1%      -2  [ = ]       0    [Unmapped]
  -0.0%     -32  [ = ]       0    .debug_loc
  -0.0%    -664  [ = ]       0    .debug_info
  -0.0% -1.00Ki  [ = ]       0    .debug_str
  -0.0% -1.68Ki  +0.0%      +4    TOTAL
use_config/pi_pico_w_ble — 640.2 KiB → 640.2 KiB (+0.00%)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 600944	      0	  54632	 655576	  a00d8	rmk-pi-pico-w

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 600944	      0	  54632	 655576	  a00d8	rmk-pi-pico-w

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0% +1.37Ki  [ = ]       0    .debug_str
  -0.0%     -33  [ = ]       0    .debug_line
  -0.0%    -154  [ = ]       0    .debug_info
  +0.0% +1.19Ki  [ = ]       0    TOTAL
use_config/rp2040 — 146.6 KiB → 146.7 KiB (+0.00% ⬆️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 134928	      0	  15244	 150172	  24a9c	rmk-rp2040

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 134924	      0	  15244	 150168	  24a98	rmk-rp2040

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%    +156  [ = ]       0    .debug_line
  +0.0%     +88  [ = ]       0    .debug_info
   +64%     +25  [ = ]       0    [Unmapped]
  +0.0%      +4  +0.0%      +4    .text
  -0.0%     -12  [ = ]       0    .debug_loc
  -0.0%    -393  [ = ]       0    .debug_str
  -0.0%    -132  +0.0%      +4    TOTAL
use_config/rp2040_split (central) — 158.4 KiB → 158.4 KiB (+0.00% ⬆️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 146156	      0	  16008	 162164	  27974	central

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 146148	      0	  16008	 162156	  2796c	central

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%    +910  [ = ]       0    .debug_str
  +0.1%    +140  [ = ]       0    .debug_line
  +0.0%     +69  [ = ]       0    .debug_info
  +0.0%     +22  [ = ]       0    .debug_loc
  +0.0%      +8  +0.0%      +8    .text
  -0.0%      -1  [ = ]       0    .strtab
 -18.2%      -8  [ = ]       0    [Unmapped]
  +0.0% +1.11Ki  +0.0%      +8    TOTAL
use_config/rp2040_split (peripheral) — 25.9 KiB → 25.9 KiB (-0.01% ⬇️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
  24016	     56	   2412	  26484	   6774	peripheral

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
  24020	     56	   2412	  26488	   6778	peripheral

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%      +3  [ = ]       0    .debug_str
  +2.7%      +1  [ = ]       0    [Unmapped]
  -0.1%      -4  -0.1%      -4    .rodata
  [ = ]       0  -0.0%      -4    TOTAL
use_config/stm32f1 — 62.3 KiB → 62.3 KiB (+0.00%)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
  56312	     28	   7432	  63772	   f91c	rmk-stm32f1

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
  56312	     28	   7432	  63772	   f91c	rmk-stm32f1

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  -1.6%      -1  [ = ]       0    [Unmapped]
  -0.0%     -14  [ = ]       0    .debug_line
  -0.0%    -134  [ = ]       0    .debug_info
  -0.0%    -143  [ = ]       0    .debug_str
  -0.0%    -292  [ = ]       0    TOTAL
use_config/stm32h7 — 99.7 KiB → 99.7 KiB (+0.00% ⬇️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
  92116	    268	   9664	 102048	  18ea0	rmk-stm32h7

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
  92120	    268	   9664	 102052	  18ea4	rmk-stm32h7

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  -0.0%      -4  -0.0%      -4    .text
  -0.0%    -123  [ = ]       0    .debug_info
  -0.1%    -139  [ = ]       0    .debug_line
  -0.0%    -298  [ = ]       0    .debug_str
  -0.0%    -564  -0.0%      -4    TOTAL
use_rust/nrf52832_ble — 345.3 KiB → 345.3 KiB (+0.00%)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 315944	   5200	  32456	 353600	  56540	rmk-nrf52832

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 315944	   5200	  32456	 353600	  56540	rmk-nrf52832

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%    +277  [ = ]       0    .debug_info
  +1.4%    +110  [ = ]       0    .debug_abbrev
  +0.0%     +10  [ = ]       0    .debug_loc
  +5.7%      +3  [ = ]       0    [Unmapped]
  -0.0%      -9  [ = ]       0    .debug_line
  -0.0%    -507  [ = ]       0    .debug_str
  -0.0%    -116  [ = ]       0    TOTAL
use_rust/nrf52840_ble — 401.6 KiB → 401.5 KiB (+0.00% ⬇️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 360256	   5200	  45728	 411184	  64630	rmk-nrf52840

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 360268	   5200	  45728	 411196	  6463c	rmk-nrf52840

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1% +2.37Ki  [ = ]       0    .debug_info
  +0.2% +1.03Ki  [ = ]       0    .debug_loc
  +0.1%    +207  [ = ]       0    .debug_line
   +29%     +15  [ = ]       0    [Unmapped]
  -0.0%      -8  [ = ]       0    .debug_aranges
  -0.0%     -12  -0.0%     -12    .text
  -0.0%     -20  [ = ]       0    .debug_frame
  -0.0%     -25  [ = ]       0    .strtab
  -0.0%     -32  [ = ]       0    .debug_ranges
  -0.0%     -32  [ = ]       0    .symtab
  -0.0%    -915  [ = ]       0    .debug_str
  +0.0% +2.60Ki  -0.0%     -12    TOTAL
use_rust/nrf52840_ble_split (central) — 490.5 KiB → 490.5 KiB (+0.00% ⬆️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 443984	   6524	  51792	 502300	  7aa1c	central

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 443976	   6524	  51792	 502292	  7aa14	central

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
   +40%     +19  [ = ]       0    [Unmapped]
  +0.0%      +8  +0.0%      +8    .text
  +0.0%      +7  [ = ]       0    .debug_line
  -0.0%     -31  [ = ]       0    .debug_loc
  -0.0%    -851  [ = ]       0    .debug_str
  -0.0%    -964  [ = ]       0    .debug_info
  -0.0% -1.77Ki  +0.0%      +8    TOTAL
use_rust/nrf52840_ble_split (peripheral) — 304.5 KiB → 304.5 KiB (+0.00% ⬆️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 281148	   5328	  25288	 311764	  4c1d4	peripheral

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 281144	   5328	  25288	 311760	  4c1d0	peripheral

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%     +50  [ = ]       0    .debug_info
   +65%     +26  [ = ]       0    [Unmapped]
  +0.0%      +4  +0.0%      +4    .text
  -0.0%      -1  [ = ]       0    .debug_loc
  -0.0%      -2  [ = ]       0    .debug_line
  -0.0%    -349  [ = ]       0    .debug_str
  -0.0%    -272  +0.0%      +4    TOTAL
use_rust/pi_pico_w_ble — 640.7 KiB → 640.8 KiB (+0.00% ⬆️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 601380	      0	  54776	 656156	  a031c	rmk-pi-pico-w

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 601348	      0	  54776	 656124	  a02fc	rmk-pi-pico-w

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1% +2.32Ki  [ = ]       0    .debug_str
  +0.0%    +203  [ = ]       0    .debug_loc
  +0.1%    +150  [ = ]       0    .strtab
  +0.0%     +85  [ = ]       0    .debug_info
  +0.0%     +45  [ = ]       0    .debug_line
  +0.0%     +32  [ = ]       0    .symtab
  +0.0%     +32  +0.0%     +32    .text
  +0.1%     +28  [ = ]       0    .debug_frame
  +0.0%      +8  [ = ]       0    .debug_aranges
  +0.0%      +8  [ = ]       0    .debug_ranges
   +12%      +5  [ = ]       0    [Unmapped]
  +0.0% +2.90Ki  +0.0%     +32    TOTAL
use_rust/rp2040 — 146.4 KiB → 146.4 KiB (+0.00% ⬆️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 134800	      0	  15124	 149924	  249a4	rmk-rp2040

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 134792	      0	  15124	 149916	  2499c	rmk-rp2040

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%    +129  [ = ]       0    .debug_line
  +0.0%     +73  [ = ]       0    .debug_str
  +0.0%     +39  [ = ]       0    .debug_info
   +50%     +21  [ = ]       0    [Unmapped]
  +0.0%     +10  [ = ]       0    .debug_loc
  +0.0%      +8  +0.0%      +8    .text
  +0.0%    +280  +0.0%      +8    TOTAL
use_rust/rp2040_split (central) — 157.7 KiB → 157.7 KiB (+0.00% ⬇️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 145656	      0	  15788	 161444	  276a4	central

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 145660	      0	  15788	 161448	  276a8	central

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%     +36  [ = ]       0    .debug_line
  +0.0%      +7  [ = ]       0    .debug_str
  +5.6%      +3  [ = ]       0    [Unmapped]
  -0.0%      -1  [ = ]       0    .strtab
  -0.0%      -4  -0.0%      -4    .text
  -0.0%    -297  [ = ]       0    .debug_info
  -0.0%    -256  -0.0%      -4    TOTAL
use_rust/rp2040_split (peripheral) — 26.4 KiB → 26.4 KiB (-0.01% ⬇️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
  24316	     56	   2676	  27048	   69a8	peripheral

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
  24320	     56	   2676	  27052	   69ac	peripheral

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%      +2  [ = ]       0    .debug_str
  +3.5%      +2  [ = ]       0    [Unmapped]
  -0.1%      -4  -0.1%      -4    .rodata
  [ = ]       0  -0.0%      -4    TOTAL
use_rust/stm32f1 — 61.8 KiB → 61.8 KiB (+0.00% ⬇️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
  55848	     28	   7408	  63284	   f734	rmk-stm32f1

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
  55852	     28	   7408	  63288	   f738	rmk-stm32f1

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%     +16  [ = ]       0    .debug_str
 +10.0%      +4  [ = ]       0    [Unmapped]
  -0.0%      -4  -0.0%      -4    .text
  +0.0%     +16  -0.0%      -4    TOTAL
use_rust/stm32h7 — 120.8 KiB → 120.8 KiB (+0.00% ⬆️)

cargo size (PR):

   text	   data	    bss	    dec	    hex	filename
 107972	    324	  15452	 123748	  1e364	rmk-stm32h7

cargo size (main):

   text	   data	    bss	    dec	    hex	filename
 107968	    324	  15452	 123744	  1e360	rmk-stm32h7

Bloaty diff (PR vs main):

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%    +132  [ = ]       0    .debug_line
  +0.0%      +4  +0.0%      +4    .text
  +0.0%      +1  [ = ]       0    .strtab
  -9.3%      -4  [ = ]       0    [Unmapped]
  -0.0%    -199  [ = ]       0    .debug_info
  -0.0%    -870  [ = ]       0    .debug_str
  -0.0%    -936  +0.0%      +4    TOTAL

HaoboGu
HaoboGu approved these changes May 10, 2026
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@HaoboGu HaoboGu left a comment

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lgtm, thanks a lot!

@HaoboGu HaoboGu merged commit e2f2be6 into HaoboGu:main May 10, 2026
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@scottlamb @HaoboGu