i2c-xiic: An error occurs while handling RX irq

[Jeaho Hwang]

Hello xilinx and arm linux experts.
We are testing periodic send & recv i2c msgs on zynq7000 based target
board. It shows -5(EIO) errors rarely and works fine again from the
next period.
We use the Linux kernel 5.4.0 provided by Petalinux 2021.2, with
RT-patch. Investigating the code I think the RT patch would not affect
this symptom. We will also try to reproduce it with non-preempt-RT
configuration.
The debug log is attached below.

[   95.115539] i2c-dev: i2c-1 reading 2 bytes.
[   95.115552] xiic-i2c 40700000.i2c: xiic_xfer entry SR: 0xc0
[   95.115562] xiic-i2c 40700000.i2c: xiic_setclk entry,
i2c->input_clk: 100000000, i2c->i2c_clk: 0
[   95.115576] xiic-i2c 40700000.i2c: __xiic_start_xfer entry, msg:
f14e832a, fifos space: 15
[   95.115588] xiic-i2c 40700000.i2c: xiic_start_recv entry, ISR: 0xd0, CR: 0x1
[   95.115605] xiic-i2c 40700000.i2c: xiic_isr entry
[   95.115674] xiic-i2c 40700000.i2c: xiic_process: IER: 0x1b, ISR:
0xc2, pend: 0x2

                           ~~~~~~~~
[   95.115685] xiic-i2c 40700000.i2c: xiic_process: SR: 0x8c, msg:
f14e832a, nmsgs: 1
[   95.115698] xiic-i2c 40700000.i2c: xiic_process, ISR: 0xca, CR: 0x1

         ~~~~~~~~
[   95.115706] xiic-i2c 40700000.i2c: xiic_process error

BC pend calculated was 0x2(XIIC_INTR_TX_ERROR_MASK without
RX_FULL_MASK) this pended value is handled as an error. But the latter
read value of ISR was 0xca, which means normal RX completion. It seems
that reading ISR is too earlier or updating RXFULL bit is delayed.

I found the second ISR print is added from commit
a6183c1688142adcf55b11baff95222b47792644, by Raviteja. I wonder this
or similar cases have been considered before. I also wonder if we
should add the second chance of reading ISR in similar cases inside
the irq handler (e.g. drivers/i2c/busses/i2c-xiic.c:635)

Best regards.
Jeaho Hwang.
-- 
황재호, Jay Hwang, linux team manager of RTst