U-Boot fails to use MMC interface on Rock 5B/5B+

I see this with our current U-Boot branch:

Rock 5B

DDR ff1a08bde6 typ 25/03/13-15:39:39,fwver: v1.19
ch0 ttot10
ch1 ttot10
ch2 ttot10
ch3 ttot10
ch0 ttot18
LPDDR4X, 2112MHz
channel[0] BW=16 Col=10 Bk=8 CS0 Row=16 CS=1 Die BW=16 Size=1024MB
ch1 ttot18
channel[1] BW=16 Col=10 Bk=8 CS0 Row=16 CS=1 Die BW=16 Size=1024MB
ch2 ttot16
channel[2] BW=16 Col=10 Bk=8 CS0 Row=16 CS=1 Die BW=16 Size=1024MB
ch3 ttot16
channel[3] BW=16 Col=10 Bk=8 CS0 Row=16 CS=1 Die BW=16 Size=1024MB
Manufacturer ID:0xff
DQS rds:h5,h4 
CH0 RX Vref:27.1%, TX Vref:21.8%,0.0%
DQ rds:h2 l0 h3 h1 h2 h1 h3 h2, h1 l0 l0 h2 l0 h1 h6 h3 

DQS rds:h1,h1 
CH1 RX Vref:28.9%, TX Vref:22.8%,0.0%
DQ rds:h3 h2 h5 h5 h2 h3 h4 h3, h1 h4 h3 h7 h2 h2 h3 l0 

DQS rds:h1,l0 
CH2 RX Vref:29.3%, TX Vref:20.8%,0.0%
DQ rds:h1 h1 h2 h1 h1 l0 h3 h2, h1 h2 h1 l0 l0 h1 h1 h1 

DQS rds:l0,h1 
CH3 RX Vref:30.1%, TX Vref:20.8%,0.0%
DQ rds:l0 l0 h2 h1 l0 h2 l0 h1, h3 h3 h1 h6 h3 l0 l0 h3 

stride=0x2, ddr_config=0x0
hash ch_mask0-1 0x20 0x40, bank_mask0-3 0xa00 0x1400 0x2800 0x0, rank_mask0 0x0
change to F1: 528MHz
ch0 ttot10
ch1 ttot10
ch2 ttot10
ch3 ttot10
change to F2: 1068MHz
ch0 ttot12
ch1 ttot12
ch2 ttot12
ch3 ttot12
change to F3: 1560MHz
ch0 ttot16
ch1 ttot14
ch2 ttot14
ch3 ttot14
change to F0: 2112MHz
ch0 ttot18
ch1 ttot18
ch2 ttot16
ch3 ttot18
out

U-Boot SPL 2025.10-g79dc2821375c (Oct 07 2025 - 13:05:09 +0000)
Trying to boot from MMC1
mmc_load_image_raw_sector: mmc block read error
Error: -38
SPL: failed to boot from all boot devices
### ERROR ### Please RESET the board ###

Rock 5B+

DDR ff1a08bde6 typ 25/03/13-15:39:39,fwver: v1.19
ch0 ttot6
ch1 ttot6
ch2 ttot6
ch3 ttot6
ch0 ttot7
LPDDR5, 2400MHz
channel[0] BW=16 Col=10 Bk=16 CS0 Row=16 CS=1 Die BW=16 Size=2048MB
ch1 ttot7
channel[1] BW=16 Col=10 Bk=16 CS0 Row=16 CS=1 Die BW=16 Size=2048MB
ch2 ttot7
channel[2] BW=16 Col=10 Bk=16 CS0 Row=16 CS=1 Die BW=16 Size=2048MB
ch3 ttot7
channel[3] BW=16 Col=10 Bk=16 CS0 Row=16 CS=1 Die BW=16 Size=2048MB
Manufacturer ID:0x6
DQS rds:l0,h1 
CH0 RX Vref:30.1%, TX Vref:21.0%,0.0%
DQ rds:h1 h2 h1 h3 h3 h2 h2 h1, h1 h1 h3 h1 h1 h1 h2 l0 

DQS rds:l0,l0 
CH1 RX Vref:31.0%, TX Vref:19.0%,0.0%
DQ rds:h2 h3 h1 h1 h1 h1 l1 h1, l0 l0 l0 h1 h1 h1 h1 h1 

DQS rds:l0,l0 
CH2 RX Vref:30.1%, TX Vref:20.0%,0.0%
DQ rds:h4 h2 h1 l0 h1 h1 h1 h2, h3 h4 h3 l1 h1 h3 h1 h1 

DQS rds:l0,l0 
CH3 RX Vref:31.0%, TX Vref:20.0%,0.0%
DQ rds:h1 h2 l0 l0 h1 h1 h2 h2, h2 h1 h1 h1 h1 h1 h2 h3 

stride=0x2, ddr_config=0x2
hash ch_mask0-1 0x20 0x40, bank_mask0-3 0x0 0x2400 0x44800 0x89000, rank_mask0 0x0
change to F1: 534MHz
ch0 ttot6
ch1 ttot6
ch2 ttot6
ch3 ttot6
change to F2: 1320MHz
ch0 ttot8
ch1 ttot8
ch2 ttot8
ch3 ttot8
change to F3: 1968MHz
ch0 ttot6
ch1 ttot6
ch2 ttot6
ch3 ttot6
change to F0: 2400MHz
ch0 ttot7
ch1 ttot7
ch2 ttot7
ch3 ttot7
out

U-Boot SPL 2025.10-g79dc2821375c (Oct 07 2025 - 13:05:09 +0000)
Trying to boot from MMC1
Card did not respond to voltage select! : -110
spl: mmc init failed with error: -95
Error: -95
SPL: failed to boot from all boot devices
### ERROR ### Please RESET the board ###

I'm still looking for the root cause, but currently suspect issues with the regulator code.