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

From: charanya
Date: Thu Jun 02 2016 - 05:08:23 EST

On 2016-05-26 04:16, Stephen Boyd wrote:
On 05/13, charanya@xxxxxxxxxxxxxx wrote:
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,
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.

Ok, but what's the exact sequence of events that happens? I think
we unlock the spinlock in the dma completion handler and then the
txlev interrupt runs? At that point we may have more data to push
out and then rx stale handling runs and corrupts the fifo state?

I was hoping for some sort of CPU sequence of events like:

---- ----

<Read from FIFO and breaks>

This patch seems correct, but the commit text isn't fully
describing the sequence of events that causes this to happen, so
it's taking a while to convince myself that this patch fixes

The sequence of events is as mentioned. When the TXLEV interrupt occurs after the
spinlock is unlocked, the rx stale handling runs since the interrupts are restored
and hence it corrupts the fifo state.