Re: [PATCH v2 2/2] serial: 8250_dw: Add capability to skip empty FIFO read

[Kumar, Udit]

On 9/17/2026 3:10 PM, Ilpo Järvinen wrote:
> On Wed, 16 Sep 2026, Kumar, Udit wrote:
>> On 9/16/2026 4:22 PM, Ilpo Järvinen wrote:
>>> On Wed, 16 Sep 2026, Moteen Shah wrote:
>>>
>>>> dw8250_handle_irq() does a bogus RX read on RX_TIMEOUT with no data
>>>> present, to avoid an interrupt storm. The UART core also performs
>>>> unconditional reads on the empty FIFO during startup and shutdown
>>>> of the port. On the IP version used in TDA54, that interrupt storm
>>>> no longer occurs, but reading an empty FIFO instead triggers a data
>>>> abort.
>>>>
>>>> Add a new capability to guard against the empty FIFO reads, avoiding
>>>> the data aborts.
>>>>
>>>> Signed-off-by: Moteen Shah <m-shah@xxxxxx>
>>>> ---
>>>>  drivers/tty/serial/8250/8250.h      |  1 +
>>>>  drivers/tty/serial/8250/8250_dw.c   | 16 +++++++++++++++-
>>>>  drivers/tty/serial/8250/8250_port.c | 12 +++++++++---
>>>>  3 files changed, 25 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
>>>> index 77fe0588fd6b..45e13c3a8c14 100644
>>>> --- a/drivers/tty/serial/8250/8250.h
>>>> +++ b/drivers/tty/serial/8250/8250.h
>>>> @@ -86,6 +86,7 @@ struct serial8250_config {
>>>>  					 * STOP PARITY EPAR SPAR WLEN5 WLEN6
>>>>  					 */
>>>>  #define UART_CAP_NOTEMT	BIT(18)	/* UART without interrupt on TEMT available */
>>>> +#define UART_CAP_RXFIFO_EMPTY_READ	BIT(19) /* UART needs LSR_DR check before RX read (TDA54) */
>>>
>>> IMO, this define naming contradicts with the comment because you 
>>> effectively say "capable of reading Rx while receive buffer is empty", not 
>>> that it needs DR check before issuing that read on buffer (~ named exactly 
>>> opposite of the actual meaning it is being used in the code).
>>>
>>
>> I echo your comments; the naming should reflect that the IP does not
>> allow reading an empty FIFO.
> 
> I now recalled we've also the pending RBR -> SRBR change:
> 
> https://lore.kernel.org/linux-serial/20260908141524.20302-1-yang.yicong@xxxxxxxxxxxxx/#t
> 
> I wonder if a similar thing would help in this case as well?
> 

Thanks for this patch, we are using similar IP but not same.
Let's consume above patch and revert.

Thanks


> I understand this patch covers more ground than the SRBR patch but can the 
> IP used in this case read SRBR without aborting?
> 


>> At the same time, I am thinking, reading the FIFO based on the
>> UART_LSR_DR bit being set in the serial8250_clear_interrupts and
>> serial8250_do_shutdown functions.
>> Do you see any side effects to this?
> 
> 
>