Bug 207751

Summary:	RS485 RTS(TXEN) inactivated too late, resulting in received frame distortion
Product:	Drivers	Reporter:	YuDong Zhang (mtwget)
Component:	Serial	Assignee:	Russell King (rmk)
Status:	NEW ---
Severity:	high	CC:	slaveze, uwe+kernel
Priority:	P1
Hardware:	All
OS:	Linux
Kernel Version:	5.4.40	Subsystem:
Regression:	No	Bisected commit-id:
Attachments:	RTS delay oscilloscope

Description YuDong Zhang 2020-05-15 09:22:55 UTC

the hardware platform is NXP IMX6UL.
the serial port driver is imx-uart.
serial rs485conf is:
  SER_RS485_RTS_ON_SEND=high
  SER_RS485_RTS_AFTER_SEND=low
  delay_rts_before_send=0
  delay_rts_after_send=0

the oscilloscope shows that after tx completes 2.5ms-5ms, the RTS pin is pulled down, but the opposite end returns data after tx completes 2ms

Comment 1 slaveze 2023-05-04 06:57:05 UTC

Hi

Comment 2 slaveze 2023-05-04 07:08:37 UTC

Hi,

I also confirm RTS signal delay when transmitting. (same imx serial IP, on i.MX6ULL)
In rs485, using an external chip (UM3483EESA) it corrupts received frame. (see oscilloscope capture)
This is not consistent and only happens every 10-20 frames in my case.

Thanks,
Sebastien

Comment 3 slaveze 2023-05-04 07:10:05 UTC

Created attachment 304210 [details]
RTS delay oscilloscope

Comment 4 slaveze 2023-05-04 07:34:17 UTC

Tested on 6.1.14 and 5.10.162

Comment 5 YuDong Zhang 2023-05-17 08:01:03 UTC

(In reply to slaveze from comment #4)
> Tested on 6.1.14 and 5.10.162
According to my test, this problem only occurs when power management is turned on.

Comment 6 slaveze 2023-05-17 08:18:35 UTC

(In reply to YuDong Zhang from comment #5)
> (In reply to slaveze from comment #4)
> > Tested on 6.1.14 and 5.10.162
> According to my test, this problem only occurs when power management is
> turned on.

Tried disabling runtime pm:
$ echo on > /sys/class/tty/ttymxc2/power/control

But the behavior is still the same

Comment 7 YuDong Zhang 2023-05-23 01:55:55 UTC

(In reply to slaveze from comment #6)
> (In reply to YuDong Zhang from comment #5)
> > (In reply to slaveze from comment #4)
> > > Tested on 6.1.14 and 5.10.162
> > According to my test, this problem only occurs when power management is
> > turned on.
> 
> Tried disabling runtime pm:
> $ echo on > /sys/class/tty/ttymxc2/power/control
> 
> But the behavior is still the same

I recompile the kernel after turning off CONFIG_PM.

Comment 8 YuDong Zhang 2023-05-23 01:58:04 UTC

(In reply to slaveze from comment #6)
> (In reply to YuDong Zhang from comment #5)
> > (In reply to slaveze from comment #4)
> > > Tested on 6.1.14 and 5.10.162
> > According to my test, this problem only occurs when power management is
> > turned on.
> 
> Tried disabling runtime pm:
> $ echo on > /sys/class/tty/ttymxc2/power/control
> 
> But the behavior is still the same

It may be caused by other reasons, but I didn't investigate deeply.