at91-sama5d2_adc crash

[<Eugen.Hristev@microchip.com>]

Hello Alex,

Sorry to disturb but we have issues again with this patch :



f3c034f61775 ("iio: at91-sama5d2_adc: adjust 
iio_triggered_buffer_{predisable,postenable} positions")

I recently discovered a crash when using buffered trigger with DMA with 
this driver:

# echo 100 > /sys/bus/iio/devices/iio\:device0/buffer/length
# echo 100 > /sys/bus/iio/devices/iio\:device0/buffer/watermark
# echo 1 > /sys/bus/iio/devices/iio\:device0/scan_elements/in_voltage4_en
# iio_generic_buffer -n fc030000.adc -t 
fc030000.adc-dev0-external_rising -c 5
iio device number being used is 0
iio trigger number being used is 0
/sys/bus/iio/devicii/iio:device0 fc030000.adc-dev0-external_risingo 
iio:device0: using dma0chan10 for rx DMA transfers

Division by zero in kernel.
CPU: 0 PID: 243 Comm: irq/182-fc03000 Not tainted 5.8.0-rc1 #1
Hardware name: Atmel SAMA5
[<c010caf0>] (unwind_backtrace) from [<c010a034>] (show_stack+0x10/0x14)
[<c010a034>] (show_stack) from [<c03a892c>] (Ldiv0+0x8/0x10)
[<c03a892c>] (Ldiv0) from [<c03a88fc>] (__aeabi_uidivmod+0x8/0x18)
[<c03a88fc>] (__aeabi_uidivmod) from [<c03592d4>] (div_s64_rem+0x3c/0xc4)
[<c03592d4>] (div_s64_rem) from [<c05ed344>] 
(at91_adc_trigger_handler+0xcc/0x494)
[<c05ed344>] (at91_adc_trigger_handler) from [<c014944c>] 
(irq_thread_fn+0x1c/0x78)
[<c014944c>] (irq_thread_fn) from [<c01496dc>] (irq_thread+0x124/0x1d0)
[<c01496dc>] (irq_thread) from [<c01325b4>] (kthread+0x138/0x140)
[<c01325b4>] (kthread) from [<c0100148>] (ret_from_fork+0x14/0x2c)
Exception stack(0xde49dfb0 to 0xde49dff8)
dfa0:                                     00000000 00000000 00000000 
00000000
dfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
00000000
dfe0: 00000000 00000000 00000000 00000000 00000013 00000000
Division by zero in kernel.
CPU: 0 PID: 243 Comm: irq/182-fc03000 Not tainted 5.8.0-rc1 #1
Hardware name: Atmel SAMA5
[<c010caf0>] (unwind_backtrace) from [<c010a034>] (show_stack+0x10/0x14)
[<c010a034>] (show_stack) from [<c03a7e24>] (Ldiv0_64+0x8/0x18)
[<c03a7e24>] (Ldiv0_64) from [<c0359344>] (div_s64_rem+0xac/0xc4)
[<c0359344>] (div_s64_rem) from [<c05ed360>] 
(at91_adc_trigger_handler+0xe8/0x494)
[<c05ed360>] (at91_adc_trigger_handler) from [<c014944c>] 
(irq_thread_fn+0x1c/0x78)
[<c014944c>] (irq_thread_fn) from [<c01496dc>] (irq_thread+0x124/0x1d0)
[<c01496dc>] (irq_thread) from [<c01325b4>] (kthread+0x138/0x140)
[<c01325b4>] (kthread) from [<c0100148>] (ret_from_fork+0x14/0x2c)
Exception stack(0xde49dfb0 to 0xde49dff8)
dfa0:                                     00000000 00000000 00000000 
00000000
dfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 
00000000
dfe0: 00000000 00000000 00000000 00000000 00000013 00000000
sched: RT throttling activated


It looks like crash is there since 5.8-rc1 introduced by that patch

I looked in the code and it looks something is zero, probably the 
received buffer size. It is likely that the DMA starts too soon before 
the buffer is properly setup ?

Can you help with fixing this ? or you know how we can do it ?
Also could you remind me why we enable and start the DMA on pre-enable 
of the buffer instead of post-enable of the buffer ?
In pre-enable, do we have everything ready inside IIO to be able to 
start the DMA? Or it's better to have it at post-enable time ?

I know you want to ditch the post-enable and pre-disable hooks, but it 
looks this driver needs them, or we need to find a way to make it work 
properly

Thanks !
Eugen