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

From: Peter Hurley
Date: Fri Aug 08 2014 - 16:45:25 EST

> Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
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 serial8250_modem_status()
>> 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 on.
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 throttle/unthrottle
>>>> method because writing MCR to drop RTS is sufficient to disable auto-RTS.
>>> 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 data.
>> 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 throttle
>> 7. eventually the tty buffers will cap at about 64KB and start counting
>> buf_overrun errors
> Ok.
> 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.

So with AFE, UPF_HARD_FLOW is not set (even though AFE is hw flow control),
because no special throttle/unthrottle is required; the default action of
uart_throttle() suffices.

With AFE, as long as the MCR RTS bit is set by software, auto-RTS is functioning;
ie., the receiver FIFO level is controlling the state of the RTS output.
If uart_throttle() is called, the MCR RTS bit is cleared, which overrides the
auto-RTS control and deasserts the RTS output.

When uart_unthrottle() is called, the MCR RTS bit is set, which re-enables the
auto-RTS function.

> in serial_core.c, uart_throttle() function
> if CRTSCTS is set
> port->ops->throttle(port);
not called in 8250 driver because UPF_SOFT_FLOW and UPF_HARD_FLOW are both unset
> Also call at the end
> uart_clear_mctrl(port, TIOCM_RTS);
> and this go an clear the MCR RTS bit.
which turns off auto-RTS

When uart_unthrottle() reenables MCR RTS, then auto-RTS kicks back on.

> So what does uart_throttle() API expected to do since MCR is updated using uart_clear_mctrl().
> I searched who sets the UPF_HARD_FLOW in port->flags and only driver that does set this flag is omap-serial.c. The check was introduced by commit from Ruessel King to support h/w assisted h/w flow control.
> ======================================================================
> commit dba05832cbe4f305dfd998fb26d7c685d91fbbd8
> Author: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
> Date: Tue Apr 17 16:41:10 2012 +0100
> SERIAL: core: add hardware assisted h/w flow control support
> Ports which are handling h/w flow control in hardware must not have
> their RTS state altered depending on the tty's hardware-stopped state.
> Avoid this additional logic when setting the termios state.
> Acked-by: Alan Cox <alan@xxxxxxxxxxxxxxx>

> ======================================================================
> This flag is checked in uart_set_termios() and the reason is in the commit log above.
> So shouldn't AFE support in 8250 make use of this flag?

No. Hopefully my explanation of how the 8250 driver uses AFE clears that up.

> My patch uses DT attribute to set this flag so above uart_set_termios() function behave as expected. But as you have rightly pointed out throttle()/unthrottle() is missing that needs to be added. Only driver that I can find that has implemented this is omap_serial.c. It does disable RDI and RLSI as part of throttle() interrupt and re-enable it as part of unthrottle(). Probably I can add this in this driver as well if this is what is expected. I will post a patch for this anyways, with some basic testing, but testing on customer h/w for this was initialially implemented has to wait until my return from vacation (on vacation from 08/11-08/16).

Which is why we should revert the previous patch, and restart the effort with
a patch that adds a way to disable the fifo level check and see if that just
works with your customer hardware (with UART_CAP_AFE set in the firmware).

>>> Also 8250.c support other port types that doesn't have AFE. So shoudl this
>>> be driving RTS line to stop the sender and resume when uart_unthrottle() is called?
>> Yes, because that's how sw-assisted CTS flow control works, and the
>> default behavior of uart_throttle/uart_unthrottle.
>> IOW, with no extra code or special-casing, AFE just works.
> 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?

>>> 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

Peter Hurley
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at