[PATCH 09/16] tty: serial: 8250_dma: Add a TX trigger workaround for AM33xx

[frans.klaver@xsens.com (Frans Klaver)]

On Thu, Sep 11, 2014 at 02:50:50PM +0200, Sebastian Andrzej Siewior wrote:
> On 09/11/2014 02:32 PM, Peter Hurley wrote:
> > On 09/11/2014 07:42 AM, Sebastian Andrzej Siewior wrote:
> >> I also need a watchdog timer for TX since it seems that on omap3 the
> >> DMA engine suddenly forgets to continue with DMA?
> > 
> > One difference I noticed between the omap driver and the 8250 driver is
> > the way modem status interrupts are handled.
> > 
> > The omap driver only checks modem status for the UART_IIR_MSI interrupt type.
> > The 8250 driver checks modem status at every interrupt (other than NO_INT).
> > 
> > I think the UART_MSR_DCTS bit always reflects that the CTS input has changed
> > between reads of the MSR _even if auto CTS is on_. So perhaps the hardware
> > is being stopped by uart_handle_cts_change() when auto CTS is on?
> 
> I doubt that. What I see from a timer debug is that the TX-FIFO level
> is at 0, the DMA transfer for say 1024 bytes start.
> The FIFO is filled to 64bytes and refilled so it doesn't drop below 50.
> At the time of the stall I see that the DMA engine has outstanding
> bytes which it should transfer and the TX FIFO is empty. If hardware
> flow control stops the transfer, I would expect that the DMA engine
> still fills the TX-FIFO until 64 and then waits. But it doesn't.
> Writing bytes into the FIFO leads to bytes beeing sent (and I see them
> on the other side) but the DMA transfer is still on hold. Canceling the
> DMA transfer and re-programming the remaining bytes transfers the
> remaining bytes.
> 
> The odd thing is that I only triggered it with "less file". It doesn't
> happen on regular console interaction or "cat large-file". And it only
> triggers on beagle board xm (omap34xx) and I wasn't able to reproduce
> it on am335x or dra7. The latter shares the same DMA engine as beagle
> board xm.

I can still reproduce it on am335x. I can get out of it as soon as
something else gets written to the console though.

# echo "<3>something" >/dev/kmsg

Frans


> 
> I remember also that I disabled the HW/SW float control just to make
> sure it is not it.
> 
> > 
> > Regards,
> > Peter Hurley
> > 
> > [The UPF_HARD_FLOW thing was pretty much just done for omap even though
> > 8250 already had auto CTS/auto RTS. Serial core hardware flow control support
> > needs a redo as drivers have pretty much tacked stuff on randomly.]
> 
> Sebastian
> 
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