Re: [PATCH v8 6/6] serial: 8250: retain DMA resources when termination fails

[James Hilliard]

On Fri, Oct 30, 2026 at 3:40 AM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> On Thu, Oct 29, 2026 at 11:19:59AM -0600, James Hilliard wrote:
> > On Thu, Oct 29, 2026 at 8:24 AM Andy Shevchenko
> > <andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> > > On Thu, Oct 29, 2026 at 01:37:02AM -0600, James Hilliard wrote:
>
> ...
>
> > > > +     while (rx_ret || tx_ret) {
> > > > +             dev_err_ratelimited(p->port.dev,
> > > > +                                 "DMA termination failed: RX %d, TX %d; retrying\n",
> > > > +                                 rx_ret, tx_ret);
> > >
> > > > +             msleep(100);
> > >
> > > Huh?!
> > >
> > > > +             if (rx_ret)
> > > > +                     rx_ret = dmaengine_terminate_sync(dma->rxchan);
> > > > +             if (tx_ret)
> > > > +                     tx_ret = dmaengine_terminate_sync(dma->txchan);
> > > > +     }
> > >
> > > This is unacceptable. Sleeping is okay, but value should really match the
> > > baudspeed and other parameters of the transfer.
> >
> > This sleep is backoff after dmaengine_terminate_sync() has returned
> > an error, rather than an estimate of UART transmission time. Each
> > retry asks the provider to stop the channel again; elapsed time is
> > never treated as evidence that the resources are safe to free.
> >
> > The particular 100 ms interval has no hardware-derived justification.
> > However, I don't yet see a sound way to derive the retry cadence from
> > baud rate either. RX can be waiting for input, TX can be held by flow
> > control, and callback retirement is a separate requirement from
> > moving bytes through the UART.
> >
> > Do you have a particular provider failure mode in mind where retrying
> > termination should follow the serial transfer timing? I'd like to
> > base the interval on the condition that can make termination succeed,
> > rather than substitute a wire-time estimate for an arbitrary delay.
>
> Termination of DMA sometimes require to clean up internal FIFOs and other state
> machines. That's what happens on HW side. In the software waiting for too much
> may lead to the very big undesired latencies. The value of 100ms sounds insane
> to me for the HSUART cases (megabaud range of speed). Also note, not every PCB
> design uses HW flow control, so this may be even worse for them.

The 100 ms interval can add unnecessary teardown latency if a failed
termination becomes retryable much sooner. There is no hardware-derived
justification for that particular value.

The FIFO/state-machine draining is part of the provider's stop sequence.
For example, DW performs its FIFO-empty polling inside
dwc_terminate_all(). If termination succeeds, this patch proceeds without
the retry sleep, regardless of baud rate. The sleep is only reached
after an error during DMA release, not during normal RX delivery.

Baud rate can influence how quickly a transmit FIFO drains, even without
flow control. However, the termination error does not tell 8250 whether
that drain is still progressing, how much remains, or whether the
controller needs a different recovery operation. That distinction matters
when choosing a retry interval: a shorter interval reduces the added
latency, but does not establish that retrying can recover the channel.

There is also a specific API detail here: dmaengine_terminate_sync()
returns immediately if device_terminate_all() fails, without reaching
dmaengine_synchronize(). Consequently, an error leaves callback draining
unconfirmed as well. The retry cadence can change, but resource release
still requires successful termination and synchronization.

>
> > Regardless of the retry interval, shutdown cannot release the buffers
> > until the provider establishes that DMA and its callbacks have stopped.
>
> --
> With Best Regards,
> Andy Shevchenko
>
>