RE: [PATCH] tty: serial: fsl_lpuart: Clear the error flags by writing 1 for lpuart32 platforms

[Sherry Sun]

> -----Original Message-----
> From: Jiri Slaby <jirislaby@xxxxxxxxxx>
> Sent: 2023年7月31日 13:24
> To: Sherry Sun <sherry.sun@xxxxxxx>; gregkh@xxxxxxxxxxxxxxxxxxx;
> tomonori.sakita@xxxxxxxxxx; atsushi.nemoto@xxxxxxxxxx
> Cc: linux-serial@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; dl-linux-imx
> <linux-imx@xxxxxxx>
> Subject: Re: [PATCH] tty: serial: fsl_lpuart: Clear the error flags by writing 1
> for lpuart32 platforms
> 
> On 31. 07. 23, 3:50, Sherry Sun wrote:
> > Do not read the data register to clear the error flags for lpuart32
> > platforms, the additional read may cause the receive FIFO underflow
> > since the DMA has already read the data register.
> > Now all lpuart32 platforms support write 1 to clear those error bits,
> 
> What does this "Now" mean? Will this change break some older platforms?

Hi Jiri,

Sorry for the confusion, maybe the "Now" should be removed here. I can send a V2 to improve the commit message if needed.
I checked the Reference Manuals corresponding to the following compatible platforms, all can support W1C for those error bits. So I don't think it will break the supported platforms.

    { .compatible = "fsl,ls1021a-lpuart",   .data = &ls1021a_data, },
    { .compatible = "fsl,ls1028a-lpuart",   .data = &ls1028a_data, },
    { .compatible = "fsl,imx7ulp-lpuart",   .data = &imx7ulp_data, },
    { .compatible = "fsl,imx8ulp-lpuart",   .data = &imx8ulp_data, },
    { .compatible = "fsl,imx8qxp-lpuart",   .data = &imx8qxp_data, },
    { .compatible = "fsl,imxrt1050-lpuart", .data = &imxrt1050_data},

Best Regards
Sherry

> 
> > let's use this method to better clear the error flags.
> >
> > Fixes: 42b68768e51b ("serial: fsl_lpuart: DMA support for 32-bit
> > variant")
> > Signed-off-by: Sherry Sun <sherry.sun@xxxxxxx>
> > ---
> >   drivers/tty/serial/fsl_lpuart.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/tty/serial/fsl_lpuart.c
> > b/drivers/tty/serial/fsl_lpuart.c index f6644c5989d3..f72e1340b47d
> > 100644
> > --- a/drivers/tty/serial/fsl_lpuart.c
> > +++ b/drivers/tty/serial/fsl_lpuart.c
> > @@ -1120,8 +1120,8 @@ static void lpuart_copy_rx_to_tty(struct
> lpuart_port *sport)
> >   		unsigned long sr = lpuart32_read(&sport->port, UARTSTAT);
> >
> >   		if (sr & (UARTSTAT_PE | UARTSTAT_FE)) {
> > -			/* Read DR to clear the error flags */
> > -			lpuart32_read(&sport->port, UARTDATA);
> > +			/* Clear the error flags */
> > +			lpuart32_write(&sport->port, sr, UARTSTAT);
> >
> >   			if (sr & UARTSTAT_PE)
> >   				sport->port.icount.parity++;
> 
> thanks,
> --
> js
> suse labs