Re: [PATCH] serial: qcom-geni: Show '@' characters if we have a FIFO underrun

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Jul 10, 2024 at 10:47:16AM -0700, Doug Anderson wrote:
> Hi,
> 
> On Wed, Jul 10, 2024 at 10:28 AM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Wed, Jul 10, 2024 at 09:01:10AM -0700, Doug Anderson wrote:
> > > Hi,
> > >
> > > On Tue, Jul 9, 2024 at 10:35 PM Greg Kroah-Hartman
> > > <gregkh@linuxfoundation.org> wrote:
> > > >
> > > > On Tue, Jul 09, 2024 at 04:28:45PM -0700, Douglas Anderson wrote:
> > > > > As of commit 2ac33975abda ("serial: qcom-geni: do not kill the machine
> > > > > on fifo underrun") a FIFO underrun will no longer hard lockup the
> > > > > machine. Instead, a FIFO underrun will cause the UART to output a
> > > > > bunch of '\0' characters. The '\0' characters don't seem to show up on
> > > > > most terminal programs and this hides the fact that we had an
> > > > > underrun. An underrun is aq sign of problems in the driver and
> > > > > should be obvious / debugged.
> > > > >
> > > > > Change the driver to put '@' characters in the case of an underrun
> > > > > which should make it much more obvious.
> > > > >
> > > > > Adding this extra initialization doesn't add any real overhead. In
> > > > > fact, this patch reduces code size because the code was calling
> > > > > memset() to init 4 bytes of data. Disassembling the new code shows
> > > > > that early in the function w22 is setup to hold the '@@@@' constant:
> > > > >   mov     w22, #0x40404040
> > > > >
> > > > > Each time through the loop w22 is simply stored:
> > > > >   str     w22, [sp, #4]
> > > > >
> > > > > Cc: Johan Hovold <johan@kernel.org>
> > > > > Signed-off-by: Douglas Anderson <dianders@chromium.org>
> > > > > ---
> > > > >
> > > > >  drivers/tty/serial/qcom_geni_serial.c | 4 ++--
> > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> > > > > index 69a632fefc41..332eaa2faa2b 100644
> > > > > --- a/drivers/tty/serial/qcom_geni_serial.c
> > > > > +++ b/drivers/tty/serial/qcom_geni_serial.c
> > > > > @@ -872,10 +872,10 @@ static void qcom_geni_serial_send_chunk_fifo(struct uart_port *uport,
> > > > >  {
> > > > >       struct qcom_geni_serial_port *port = to_dev_port(uport);
> > > > >       unsigned int tx_bytes, remaining = chunk;
> > > > > -     u8 buf[BYTES_PER_FIFO_WORD];
> > > > >
> > > > >       while (remaining) {
> > > > > -             memset(buf, 0, sizeof(buf));
> > > > > +             u8 buf[BYTES_PER_FIFO_WORD] = { '@', '@', '@', '@' };
> > > >
> > > > Why is '@' a valid character for an underrun?  Why would any characters
> > > > be ok?  Where is this now documented?
> > >
> > > '@' is arbitrary. If you have a different character suggestion then
> > > I'm happy to change it. I'm mostly looking for something other than
> > > '\0' to be printed out in the case of underruns, which is what happens
> > > now. Printing out '\0' is much harder to notice but could still end up
> > > causing problems with file transfers / automated programs trying to
> > > work with serial data.
> >
> > Any character is "wrong", so picking this one feels odd.
> >
> > Do we know when an underrun happens?  If so, handle that error.  If not,
> > well, something else is really wrong with this uart then
> 
> It no longer happens. Johan's recent patches fixed it. Quick history:
> 
> 1. Pre-kfifo, we used to output stale characters (ones that had been
> dropped) in the FIFO underrun case. Nobody noticed for years.
> 
> 2. After kfifo we got a hard lockup.
> 
> 3. Johan's early patches to fix the hard lockup caused us to output
> '\0' characters upon FIFO underrun. It was not obvious that the '\0'
> characters were being output. To make it easier to debug / see, both
> he and I locally made it output some other character which was more
> obvious.
> 
> 4. Johan fixed the FIFO underrun.
> 
> 5. Johan added a patch such that if we ever get another FIFO underrun
> in the future we'll output '\0' characters in the FIFO instead of
> getting a hard lockup.
> 
> If we're really confident that we can't get a FIFO underun we could
> just revert commit 2ac33975abda ("serial: qcom-geni: do not kill the
> machine on fifo underrun") and we'll get a hard lockup if we ever
> underrun. IMO, though, it's better to output _something_ in this case
> to make it more obvious. If you hate this patch, though, fine. Let's
> drop it and we'll hope that either we never introduce a bug causing a
> FIFO underrun in the future or that someone notices the '\0'
> characters.

Let's just drop this one, if \0 are seen, that's a good enough character
as any to send when something bad happens.

thanks,

greg k-h