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

[Andy Gross]

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.