Re: [PATCH] tty: serial: msm: Disable restoring Rx interrupts for DMA Mode

[charanya]

On 2016-05-12 10:32, Andy Gross wrote:
> On Wed, May 11, 2016 at 06:41:26PM -0700, Stephen Boyd wrote:
> > On 05/10, Abhishek Sahu wrote:
> > > From: Charanya <charanya@xxxxxxxxxxxxxx>
> >
> > Was it intentional to only have one name here?
> >
> > >
> > > The Data loss was happening with current QCOM MSM serial driver during
> > > large file transfer due to simultaneous enabling of both UART and
> > > DMA interrupt. When UART operates in DMA mode, RXLEV (Rx FIFO over
> > > watermark) or RXSTALE (stale interrupts) should not be enabled,
> > > since these conditions will be handled by DMA controller itself.
> > > If these interrupts are enabled then normal UART ISR will read some
> > > bytes of data from Rx Buffer and DMA controller will not receive
> > > these bytes of data, which will cause data loss.
> > >
> > > Now this patch removed the code for enabling of RXLEV and RXSTALE
> > > interrupt in DMA Rx completion routine.
> >
> > I'm lost, we keep both these irqs masked (well only if uartdm
> > version is 1.4 or greater) pretty much the entire time we're
> > using DMA for RX. msm_start_rx_dma() will mask them and then when
> > the callback completes we'll unmask them (the part that's deleted
> > in this patch), but then we'll go back and remask them almost
> > immediately because we call msm_start_rx_dma() from the dma
> > completion handler.
> >
> > Can you clearly describe how this is actually fixing any
> > problems? What's the sequence of events that happens to cause
> > corruption?
> >
> > This does raise the question though why we ever mask/unmask these
> > interrupts if we're always going to keep them masked while doing
> > DMA RX. Presumably if we can use DMA to RX, we can always use it
> > and set things up properly at startup time instead of later on.
>
> Thats probably the right thing to do.  We shouldn't be 
> masking/unmasking
> the unused IRQs to begin with.
Hi Stephen/Andy,

If both Tx and Rx are used simultaneously, restoring Rx interrupts in
msm_complete_rx_dma could lead to RXSTALE interrupt being triggered, 
when
the ISR execution for TXLEV interrupt is completed, since msm_port->imr 
is
rewritten to UART_IMR in msm_uart_irq. Hence, we do not have to restore
Rx interrupts since Rx is always in DMA mode once enabled.

Thanks.
Charanya.