[PATCH] dmaengine: pl330: fix the race condition in pl330 driver.

[Jassi Brar]

On Tue, Mar 31, 2015 at 9:10 AM, Scott Branden <sbranden at broadcom.com> wrote:
> Hi Vinod, Jassi,
>
> Some details on the problem encountered.
>
>
> On 15-03-30 10:25 AM, Vinod Koul wrote:
>>
>> On Mon, Mar 30, 2015 at 10:17:17PM +0530, Jassi Brar wrote:
>>>
>>> On Fri, Mar 27, 2015 at 5:25 AM, Scott Branden <sbranden at broadcom.com>
>>> wrote:
>>>>
>>>> From: ismail <ismail at broadcom.com>
>>>>
>>>> Update the thread running index before issuing the
>>>> GO command to the DMAC.
>>>>
>>>> Tested-by: Mohamed Ismail Abdul Packir Mohamed <ismail at broadcom.com>
>>>> Reviewed-by: Ray Jui <rjui at broadcom.com>
>>>> Reviewed-by: Arun Parameswaran <aparames at broadcom.com>
>>>> Reviewed-by: Scott Branden <sbranden at broadcom.com>
>>>> Signed-off-by: Scott Branden <sbranden at broadcom.com>
>>>> Signed-off-by: Mohamed Ismail Abdul Packir Mohamed <ismail at broadcom.com>
>>>> ---
>>>>   drivers/dma/pl330.c | 4 ++--
>>>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/dma/pl330.c b/drivers/dma/pl330.c
>>>> index 0e1f567..631642d 100644
>>>> --- a/drivers/dma/pl330.c
>>>> +++ b/drivers/dma/pl330.c
>>>> @@ -1072,11 +1072,11 @@ static bool _trigger(struct pl330_thread *thrd)
>>>>          /* Set to generate interrupts for SEV */
>>>>          writel(readl(regs + INTEN) | (1 << thrd->ev), regs + INTEN);
>>>>
>>>> +       thrd->req_running = idx;
>>>> +
>>>>          /* Only manager can execute GO */
>>>>          _execute_DBGINSN(thrd, insn, true);
>>>>
>>>> -       thrd->req_running = idx;
>>>> -
>>>
>>> It would help to know what the behavior looks like before and after
>>> the patch. If anything we should look at locking rather the
>>> reordering.
>>
>> Yes that ia fair request, looking at changelog it is hard to understand
>> the
>> issue seen?
>>
> We encountered this problem as we modified the driver to make SMC calls to a
> TZ handler.  This slowed down the driver to the point where DMA transactions
> easily failed.  I believe the same could be accomplished by adding a delay
> between the GOCMD and update of the req_running and running the built in
> dmatest.
>
> The DMA transaction is broken if the interrupt occurs before the
> thrd->req_running is updated.
>
> The pl330 issues a GOCMD (in _trigger function) to start a new transfer.
>
> The issue of GOCMD generates an interrupt and the IRQ handler will call the
> pl330_update function to process the interrupt.
>
> The pl330_update function will verify the thread running index and break the
> transaction, if the thread running index is not set.
>
As I suspected the locking seems screwed up. The following patch
should fix the race properly. Can you please test the attached patches
instead?

Thanks.
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