Re: [PATCH] serial: 8250: Fix clearing FIFOs in RS485 mode again

From: Marek Vasut
Date: Thu Dec 13 2018 - 10:25:55 EST

On 12/13/2018 06:11 AM, Paul Burton wrote:
> Hi Marek,


> On Thu, Dec 13, 2018 at 05:18:19AM +0100, Marek Vasut wrote:
>>>>> I wonder whether the issue may be the JZ4780 UART not automatically
>>>>> resetting the FIFOs when FCR[0] changes. This is a guess, but the manual
>>>>> doesn't explicitly say it'll happen & the programming example it gives
>>>>> says to explicitly clear the FIFOs using FCR[2:1]. Since this is what
>>>>> the kernel has been doing for at least the whole git era I wouldn't be
>>>>> surprised if other devices are bitten by the change as people start
>>>>> trying 4.20 on them.
>>>> The patch you're complaining about is doing exactly that -- it sets
>>>> UART_FCR_CLEAR_RCVR | UART_FCR_CLEAR_XMIT in FCR , and then clears it.
>>>> Those two bits are exactly FCR[2:1]. It also explicitly does not touch
>>>> any other bits in FCR.
>>> No - your patch does that *only* if the FIFO enable bit is set, and
>>> presumes that clearing FIFOs is a no-op when they're disabled. I don't
>>> believe that's true on my system. I also believe that not touching the
>>> FIFO enable bit is problematic, since some callers clearly expect that
>>> to be cleared.
>> Why would you disable FIFO to clear it ? This doesn't make sense to me,
>> the FIFO must be operational for it to be cleared. Moreover, it
>> contradicts your previous statement, "the programming example it gives
>> says to explicitly clear the FIFOs using FCR[2:1]" .
> No, no, not at all... I'm saying that my theory is:
> - The JZ4780 requires that the FIFO be reset using FCR[2:1] in order
> to not read garbage.

Which we do

> - Prior to your patch serial8250_clear_fifos() did this,
> unconditionally.

btw originally, this also cleared the UME bit. Could this be what made
the difference on the JZ4780 ?

Can you try this patch on the JZ4780 , just out of curiosity ?

diff --git a/drivers/tty/serial/8250/8250_port.c
index c39482b961110..3dce99f4c6802 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -553,7 +553,7 @@ static unsigned int serial_icr_read(struct
uart_8250_port *up, int offset)
static void serial8250_clear_fifos(struct uart_8250_port *p)
unsigned char fcr;
- unsigned char clr_mask = UART_FCR_CLEAR_RCVR | UART_FCR_CLEAR_XMIT;
+ unsigned char clr_mask = UART_FCR_CLEAR_RCVR |

if (p->capabilities & UART_CAP_FIFO) {

> - After your patch serial8250_clear_fifos() only clears the FIFOs if
> the FIFO is already enabled.

I'd suppose this is OK, since clearing a disabled FIFO makes no sense ?

> - When called from serial8250_do_startup() during boot, the FIFO may
> not be enabled - for example if the bootloader didn't use the FIFO,
> or if the UART is used with 8250_early which zeroes FCR.

If it's not enabled, do you need to clear it ?

> - Therefore after your patch the FIFO reset bits are never set if the
> UART was used with 8250_early, or if it wasn't but the bootloader
> didn't enable the FIFO.

Hence my question above, do you need to clear the FIFO even if it was
not enabled ?

> I suspect that you get away with that because according to the PC16550D
> documentation the FIFOs should reset when the FIFO enable bit changes,
> so when the FIFO is later enabled it gets reset. I suspect that in the
> JZ4780 this does not happen, and the FIFO contains garbage. Our previous
> behaviour (always set FCR[2:1] to reset the FIFO) has been around for a
> long time, so I would not be surprised to find other devices relying
> upon it.

It is well possible, but I'd like to understand what really happens
here, not just guess.

> I'm also saying that if the FIFOs are enabled during boot then your
> patch will leave them enabled after serial8250_do_startup() calls
> serial8250_clear_fifos(), which a comment in serial8250_do_startup()
> clearly states is not the expected behaviour:

In that case, that needs to be fixed. But serial8250_clear_fifos()
should only do what the name says -- clear FIFOs -- not disable them.

>> Clear the FIFO buffers and disable them.
>> (they will be reenabled in set_termios())
> (From
> And further, with your patch serial8250_do_shutdown() will leave the
> FIFO enabled which again does not match what comments suggest it expects
> ("Disable break condition and FIFOs").
>> What exactly does your programming example for the JZ4780 say about the
>> FIFO clearing ? And does it work in that example ?
> It says to set FCR[2:1] to reset the FIFO after enabling it, which as
> far as I can tell from the PC16550D documentation would not be necessary
> there because when you enable the FIFO it would automatically be reset.
> Linux did this until your patch.

Linux sets the FCR[2:1] if the FIFO is enabled even now.

>>>>> @@ -558,25 +558,26 @@ static void serial8250_clear_fifos(struct uart_8250_port *p)
>>>>> if (p->capabilities & UART_CAP_FIFO) {
>>>>> /*
>>>>> * Make sure to avoid changing FCR[7:3] and ENABLE_FIFO bits.
>>>>> - * In case ENABLE_FIFO is not set, there is nothing to flush
>>>>> - * so just return. Furthermore, on certain implementations of
>>>>> - * the 8250 core, the FCR[7:3] bits may only be changed under
>>>>> - * specific conditions and changing them if those conditions
>>>>> - * are not met can have nasty side effects. One such core is
>>>>> - * the 8250-omap present in TI AM335x.
>>>>> + * On certain implementations of the 8250 core, the FCR[7:3]
>>>>> + * bits may only be changed under specific conditions and
>>>>> + * changing them if those conditions are not met can have nasty
>>>>> + * side effects. One such core is the 8250-omap present in TI
>>>>> + * AM335x.
>>>>> */
>>>>> fcr = serial_in(p, UART_FCR);
>>>>> + serial_out(p, UART_FCR, fcr | clr_mask);
>>>>> + serial_out(p, UART_FCR, fcr & ~clr);
>>>> Note that, if I understand the patch correctly, this will _not_ restore
>>>> the original (broken) behavior. The original behavior cleared the entire
>>>> FCR at the end, which cleared even bits that were not supposed to be
>>>> cleared .
>>> It will restore the original behaviour with regards to disabling the
>>> FIFOs, so long as callers that expect that use
>>> serial8250_clear_and_disable_fifos().
>> Does your platform need the FIFO to be explicitly disabled for it to be
>> cleared, can you confirm that ? And if so, does this apply to other
>> platforms with 8250 UART or is this specific to JZ4780 implementation of
>> the UART block ?
>> I just remembered U-Boot has this patch for JZ4780 UART [1], which
>> touches the FCR UME bit, so the FCR behavior on JZ4780 does differ from
>> the generic 8250 core. Could it be that this is what you're hitting ?
> No - we already set the UME bit & this has all worked fine until your
> patch changed the FIFO reset behaviour.

The UME bit is in the FCR, serial8250_clear_fifos() originally cleared
it, cfr:
- serial_out(p, UART_FCR, 0);
in f6aa5beb45be27968a4df90176ca36dfc4363d37 . So the code was originally
completely disabling the UART on JZ4780 .

Best regards,
Marek Vasut