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

[Scott Branden]

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