linux-kernel - Re: [PATCH] dmaengine: pl330: fix the race condition in pl330 driver.

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Date:	Mon, 30 Mar 2015 20:40:43 -0700
From:	Scott Branden <sbranden@...adcom.com>
To:	Vinod Koul <vinod.koul@...el.com>,
	Jassi Brar <jassisinghbrar@...il.com>
CC:	Dan Williams <dan.j.williams@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	ismail <ismail@...adcom.com>, Anatol Pomazao <anatol@...gle.com>,
	Dmitry Torokhov <dtor@...gle.com>,
	<bcm-kernel-feedback-list@...adcom.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] dmaengine: pl330: fix the race condition in pl330 driver.

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@...adcom.com> wrote:
>>> From: ismail <ismail@...adcom.com>
>>>
>>> Update the thread running index before issuing the
>>> GO command to the DMAC.
>>>
>>> Tested-by: Mohamed Ismail Abdul Packir Mohamed <ismail@...adcom.com>
>>> Reviewed-by: Ray Jui <rjui@...adcom.com>
>>> Reviewed-by: Arun Parameswaran <aparames@...adcom.com>
>>> Reviewed-by: Scott Branden <sbranden@...adcom.com>
>>> Signed-off-by: Scott Branden <sbranden@...adcom.com>
>>> Signed-off-by: Mohamed Ismail Abdul Packir Mohamed <ismail@...adcom.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.

Regards,
  Scott


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