linux-kernel - Re: [PATCH] serial: 8250_dw: Avoid pslverr on reading empty receiver fifo

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Date:   Wed, 13 Jul 2022 13:43:44 +0530
From:   VAMSHI GAJJELA <vamshigajjela@...gle.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Phil Edworthy <phil.edworthy@...esas.com>,
        Emil Renner Berthing <kernel@...il.dk>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Johan Hovold <johan@...nel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Manu Gautam <manugautam@...gle.com>
Subject: Re: [PATCH] serial: 8250_dw: Avoid pslverr on reading empty receiver fifo

On Tue, Jul 12, 2022 at 6:56 PM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
>
> On Tue, Jul 12, 2022 at 3:16 PM Vamshi Gajjela <vamshigajjela@...gle.com> wrote:
> >
> > From: VAMSHI GAJJELA <vamshigajjela@...gle.com>
> >
> > With PSLVERR_RESP_EN parameter set to 1, device generates an error
>
> the device
ack
>
> > response when an attempt to read empty RBR with FIFO enabled.
>
> an empty
ack
>
> > This happens when LCR writes are ignored when UART is busy.
> > dw8250_check_lcr() in retries to updateLCR, invokes dw8250_force_idle()
> > to clear and reset fifo and eventually reads UART_RX causing pslverr.
>
> fifo --> FIFO
> pslverr --> the error
ack
>
> > Avoid this by not reading RBR/UART_RX when no data is available.
>
> ...
>
> > +       unsigned int lsr;
> >         struct uart_8250_port *up = up_to_u8250p(p);
>
> Can we keep it ordered according to the reversed xmas tree?
agree.
>
> ...
>
> > +       /*
> > +        * With PSLVERR_RESP_EN parameter set to 1, device generates pslverr
>
> the device
> pslverr --> an
ack.
>
> > +        * error response when an attempt to read empty RBR with FIFO enabled
>
> Missed period.
ack
>
> > +        */
> > +       lsr = p->serial_in(p, UART_LSR);
>
> The only caller of this function already has the lsr value, why you
> can't (re)use it?
lsr is not read before, caller function (dw8250_check_lcr) reads lcr.
>
> > +       if ((up->fcr & UART_FCR_ENABLE_FIFO) && !(lsr & UART_LSR_DR))
> > +               return;
>
> --
> With Best Regards,
> Andy Shevchenko