Re: [PATCH v2] serial: uart: add hw flow control support configuration

From: Murali Karicheri
Date: Sat Aug 09 2014 - 07:30:02 EST

On 08/08/2014 06:59 PM, Murali Karicheri wrote:
On 08/08/2014 06:09 PM, Peter Hurley wrote:
On 08/08/2014 05:02 PM, Murali Karicheri wrote:
On 08/08/2014 04:44 PM, Peter Hurley wrote:
On 08/08/2014 03:36 PM, Murali Karicheri wrote:
On 08/07/2014 07:03 PM, Peter Hurley wrote:


But I realize now that a different question needs asking:
Is the MSR read showing delta CTS set when AFE is on, ever?

Unfortunately this was tested on a customer board that I don't have
access to and can't check this out right away. I am trying to
findout if I can get some hardware to test the patch to address the
issue being discussed. Customer board is currently using RTS and
CTS lines and the same works fine for them with this patch.


Because serial8250_modem_status() assumes the answer is no for
_all_ AFE-capable devices, and if yes, would mean that
is broken if AFE is on.

As per Keystone UART spec

bit 0 in MSR: DCTS: Change in CTS indicator bit. DCTS indicates
that the CTS input has changed state since the last time it was
read by the CPU. When DCTS is set (autoflow control is not enabled
and the modem status interrupt is enabled), a modem status
interrupt is generated. When autoflow control is enabled, no
interrupt is generated

So based on this, there shouldn't be any CTS change if AFE is
enabled and will indicate change if AFE is disabled. Probably add
WARN_ON_ONCE() as you suggested to detect any offending h/w.

That's identical wording to the 16750 datasheet.

But notice that it only says "no interrupt is generated" when AFE is
It doesn't say if the MSR is read, that DCTS won't be set. And that's
an important difference for how serial8250_modem_status() works.


uart_throttle() and uart_unthrottle() are used indirectly by
line disciplines
for high-level rx flow control, such as when a read buffer fills
up because
there is no userspace reader. The 8250 core doesn't define a
method because writing MCR to drop RTS is sufficient to disable

As per spec. hardware has rx threshold levels set to trigger an
RTS level change to tell
the remote from sending more bytes. So if h/w flow control is
enabled, then not sure why
uart_throttle() is to be doing anything when h/w flow control is
supported? A dummy
function required to satisfy the line discipline?

I understand how auto-RTS works.

Let's pretend for a moment that uart_throttle() does nothing when
auto-RTS is enabled:

1. tty buffers start to fill up because no process is reading the
2. The throttle threshold is reached.
3. uart_throttle() is called but does nothing.
4. more data arrives and the DR interrupt is triggered
5. serial8250_rx_chars() reads in the new data.
6. tty buffers keep filling up even though the driver was told to
7. eventually the tty buffers will cap at about 64KB and start
buf_overrun errors


Couple of observation on the AFE implementation in 8250 driver
prior to my patch.

From the discussion so far, AFE is actually hardware assisted
hardware flow control. Auto CTS is sw assisted hardware flow control
where sw uses RTS line for recieve side flow control and I assume
it uses MCR RTS bit for this where AFE does this automatically. From
the 16550 or Keystone UART spec, I can't find how RTS line can be
asserted to do this through sw instead of hardware doing it
automatically. Spec says

MCR RTS bit: RTS control. When AFE = 1, the RTS bit determines th
e autoflow control enabled. Note that all UARTs do not support this
feature. See the device-specific data manual for supported
features. If this feature is not available, this bit is reserved
and should be cleared to 0.
0 = UARTn_RTS is disabled, only UARTn_CTS is enabled.
1 = UARTn_RTS and UARTn_CTS are enabled.

Then since AFE was already supported before my patch for FIFO size
32bytes or higher, I am wondering why there was no implementation
of throttle()/unthrottle() to begin with and why UPF_HARD_FLOW flag
is not set at all if AFE implemented in 8250 driver is hw assisted,
hw flow control. Also what do these API supposed to do?

uart_throttle() does _not_ call ops->throttle() unless either
UPF_SOFT_FLOW and/or UPF_HARD_FLOW is set in uport->flags.

Not based on port flag. Here is the actual code in serial_core.c as I
see it.

You're misreading the code.

static void uart_throttle(struct tty_struct *tty)
struct uart_state *state = tty->driver_data;
struct uart_port *port = state->uart_port;
uint32_t mask = 0;

if (I_IXOFF(tty))
mask |= UPF_SOFT_FLOW;
if (tty->termios.c_cflag& CRTSCTS)
mask |= UPF_HARD_FLOW;


port->flags does not have UPF_HARD_FLOW set so

(port->flags& mask) == false

Ok. My bad.

if (port->flags& mask) {
mask&= ~port->flags;

if (mask& UPF_SOFT_FLOW)
uart_send_xchar(tty, STOP_CHAR(tty));

if (mask& UPF_HARD_FLOW)
uart_clear_mctrl(port, TIOCM_RTS);


Based on my above discussion, there are few things required to be
done on top of AFE and some of it is done by my patch and the
remaining thing to be addressed in another patch.

Assuming that AFE, as already implemented in the 8250 driver, works
as expected,
the fifo level check seems to be the only hurdle, right?

Also how uart_set_termios() expect to work without my patch? that is
needed as well.

That looks buggy, even if UPF_HARD_FLOW is set.

But that's my point: the most general cases should be fixed, if
Then, a trivial change to override the fifo size check from firmware
is all you'll need

But then it seems like UPF_HARD_FLOW flag was introduced by
dba05832cbe4f305dfd998fb26d7c685d91fbbd8 SERIAL: core: add hardware
assisted h/w flow control support and I worked my patch around this.
This is misleading.

Assume we don't use the UPF_HARD_FLOW anymore. Then in
uart_set_termios(), how does it know if the port has hw assisted hw flow
control? What is the other bug you mentioned?

I want to work to fix this rather than revert this change as our
customer is already using this feature.

3.16 was released 4 days ago.

As I said, I will work to address this with priority.

My point was that I'm not understanding how your customer could be
using this
feature when it came out 4 days ago, but yet now you can't even test
on the

This fix was back ported to v3.13 that the customer is using.

Ok, so your customer is running a custom kernel. Then I don't see the
problem with backing
this change out, rather than building on top of it.

Customer will soon be switching to newer kernel and this become an
issue. So this must be addressed even if it requires a different fix.
At this point, I still think a fix is workable if we can make use of
existing UPF_HARD_FLOW flag that is meant for this scenario.

Assuming we re-use auto-flow-control instead of the has-hw-flow-control,
and discard UPF_HARD_FLOW, we need to fix

1. limit to 32 bytes for fifo size as we have 16 bytes for keystone uart
2. uart_prt_startup() support for the hw flow control
3. uart_set_termios(), avoid stopping the hardware if port has hw flow

For 1) no idea why 32 byte limit is required and for hw flow control
this is not needed. For 2) and 3, how does the serial core driver knows
if the uart port has the AFE capability with out using the flag.


I want to add one more piece of information related to my original patch that I had forgotten to mention so that right decision can be taken on this.

The patch was added for one more use case with a different customer. The use case was running BT over uart and this required hw flow control. In their testing they have never encountered any issue w.r.t throttle when they had run their performance test. So it makes me believe throttle is in fact may not be needed for keystone UART wih h/w flow control. So we might as well add a check in serial-core.c to check if throttle()/unthrottle() is implemented and then invoke it. This should address your concern. Also in your description of AFE, default behavior is good enough for AFE.

Regarding the second issue, the change was added for the BT use case. As I don't have access to this customer's hardware, I wouldn't be able to verify if this use case indeed causes call to uart_handle_cts_change() due to a hardware bug since as per spec below, it is not supposed to generate interrupt or CTS change.

DCTS - Change in CTS indicator bit. DCTS indicates that the CTS input has changed state since the last time it was read by the CPU. When DCTS is set (autoflow control is not enabled and the modem status interrupt is enabled), a modem status interrupt is generated. When autoflow control is enabled, no interrupt is generated.

I believe this check indeed can be moved to the 8250 function that make call to this and also increment the cts count as done in this function so that we could verify if this indeed increases for the AFE casee. I might be able to query the customer for the CTS count ever increase with BT use case, then if it doesn't this may be removed later or keep it to address the hardware issue.

As this patch was added to support 2 different use cases, one for a virtual serial port and another for BT over uart, I would strongly defer from reverting this patch and add a fix as described above. Do you know if there is any bug report because of this commit or you raised it as part of reviewing the code? If latter, I could send out a patch to fix it as described above.

Hope this will not get reverted and I will have an opportunity to send a fix once I am back from my vacation.

Thanks and regards,


I will restart this thread after my vacation. Meanwhile if you have
suggestions as to how to deal with 1-3, please respond so that I can
work on a patch based on it.

Thanks and regards,


Peter Hurley

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