[PATCH v2] ARM: pl330: Fix a race condition

[Thomas Abraham]

On 7 November 2011 16:18, Javi Merino <javi.merino at arm.com> wrote:
> On 05/11/11 19:05, Thomas Abraham wrote:
>> Hi Javi,
>>
>> On 6 October 2011 05:10, Javi Merino <javi.merino at arm.com> wrote:
>>> If two requests have been submitted and one of them is running, if you
>>> call pl330_chan_ctrl(ch_id, PL330_OP_START), there's a window of time
>>> between the spin_lock_irqsave() and the _state() check in which the
>>> running transaction may finish.  In that case, we don't receive the
>>> interrupt (because they are disabled), but _start() sees that the DMA
>>> is stopped, so it starts it.  The problem is that it sends the
>>> transaction that has just finished again, because pl330_update()
>>> hasn't mark it as done yet.
>>>
>>> This patch fixes this race condition by not calling _start() if the
>>> DMA is already executing transactions.  When interrupts are reenabled,
>>> pl330_update() will call _start().
>>>
>>> Signed-off-by: Javi Merino <javi.merino at arm.com>
>>> Acked-by: Jassi Brar <jassi.brar at samsung.com>
>>> ---
>>>  arch/arm/common/pl330.c |    2 +-
>>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/arch/arm/common/pl330.c b/arch/arm/common/pl330.c
>>> index 97912fa..7129cfb 100644
>>> --- a/arch/arm/common/pl330.c
>>> +++ b/arch/arm/common/pl330.c
>>> @@ -1546,7 +1546,7 @@ int pl330_chan_ctrl(void *ch_id, enum pl330_chan_op op)
>>>
>>>                /* Start the next */
>>>        case PL330_OP_START:
>>> -               if (!_start(thrd))
>>> +               if (!_thrd_active(thrd) && !_start(thrd))
>>>                        ret = -EIO;
>>>                break;
>>
>> On Samsung's Exynos4 platform, while testing audio playback with i2s
>> interface, the above change causes the playback to freeze. The
>> _thrd_active(thrd) call always returns '1' and hence _start(thrd) is
>> not getting called.
>
> If _thrd_active(thrd) returns '1', that means there is an active
> transfer still running or, if it has finished, you haven't called
> pl330_update() to acknowledge that.  pl330_update() calls _start() as
> soon as it can.
>
> drivers/dma/pl330.c registers the irq handler in pl330_probe(), so when
> the transaction finishes, pl330_update() should clear it and call
> _start().  If there is any outstanding transaction, it should start
> straight away. If there isn't, it would mark the channel as free, so
> _thrd_active() should return '0'.  If _thrd_active() is still '1', then
> something has gone wrong in the way.
>
> Does this shed some light?

Thanks for the detailed explanation. I will check again on the lines
you have mentioned above. Also, the i2s audio playback on exynos had
been working prior to this change. So that might imply that
pl330_update was working as you have mentioned. Anyway, I will check
again.

Thanks,
Thomas.

>
> Cheers,
> Javi
>
>