Hi Andy,
On Tue, Jul 05, 2022 at 11:30:01AM +0200, Andy Shevchenko wrote:
> +Cc: Ilpo, the 8250_dw maintainer
> +Cc: Serge, who I believe is the author of the lines in 8250_port you
> cited, sorry if I'm mistaken.
Right, I was the one who got back the line with the proper max baud rate
calculation procedure in commit 7b668c064ec3 ("serial: 8250: Fix max
baud limit in generic 8250 port"). In accordance with [1, 2] the
interface baud rate is (DIV*16)-th of the reference clock frequency. So
the patch suggested by Biju will work only until he gets to the zero
divisor value. Without my fix the baud-rate search algorithm may
end up with getting unsupported baud-rates causing to have
zero-divisor, which will lead to the serial interface freeze/disable
[2].
[1] DesignWare DW_apb_uart Databook 4.03a, December 2020, p. 24
[2] DesignWare DW_apb_uart Databook 4.03a, December 2020, p. 125
>
> On Tue, Jul 5, 2022 at 8:25 AM Biju Das <biju.das.jz@xxxxxxxxxxxxxx> wrote:
> >
> > Hi Jiri and Miquel,
> >
> > While testing serial driver with RZ/N1 on 5.15 kernel, which is the backport of mainline kernel,
> > I seen performance issue with serial DMA for higher baud rates.
> >
> > The test app is taking 25 minutes finish, whereas with the below patch[1] it takes only 3 minutes to finish.
> >
> > Not sure has anyone seen this performance issue?
> >
> > [1]
> > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> > index 468d1aca5968..321430176698 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> > @@ -2680,7 +2680,7 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
> > max = (port->uartclk + tolerance) / 4;
> > } else {
> > min = port->uartclk / 16 / UART_DIV_MAX;
> > - max = (port->uartclk + tolerance) / 16;
> > + max = port->uartclk;
Are you sure uartclk is initialized with a real reference clock value?
-Sergey
> > }
> >
> > Note:-
> > I have added below change on 5.15 kernel to test on all possible use cases.
> >
> > diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> > index 7884fcd66d39..6d352981fb3e 100644
> > --- a/drivers/tty/serial/8250/8250_dw.c
> > +++ b/drivers/tty/serial/8250/8250_dw.c
> > @@ -643,6 +643,26 @@ static int dw8250_probe(struct platform_device *pdev)
> > up->dma = &data->data.dma;
> > }
> >
> > + if (data->pdata->quirks & DW_UART_QUIRK_IS_DMA_FC) {
> > + /*
> > + * When the 'char timeout' irq fires because no more data has
> > + * been received in some time, the 8250 driver stops the DMA.
> > + * However, if the DMAC has been setup to write more data to mem
> > + * than is read from the UART FIFO, the data will *not* be
> > + * written to memory.
> > + * Therefore, we limit the width of writes to mem so that it is
> > + * the same amount of data as read from the FIFO. You can use
> > + * anything less than or equal, but same size is optimal
> > + */
> > + data->data.dma.rxconf.dst_addr_width = p->fifosize / 4;
> > +
> > + /*
> > + * Unless you set the maxburst to 1, if you send only 1 char, it
> > + * doesn't get transmitted
> > + */
> > + data->data.dma.txconf.dst_maxburst = 1;
> > + }
> > +
> >
> > Cheers,
> > Biju
> >
> > > -----Original Message-----
> > > From: Biju Das
> > > Sent: 04 July 2022 08:12
> > > To: Jiri Slaby <jirislaby@xxxxxxxxxx>; Greg Kroah-Hartman
> > > <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>; Miquel Raynal
> > > <miquel.raynal@xxxxxxxxxxx>; Emil Renner Berthing <kernel@xxxxxxxx>; Phil
> > > Edworthy <phil.edworthy@xxxxxxxxxxx>; Johan Hovold <johan@xxxxxxxxxx>;
> > > linux-serial@xxxxxxxxxxxxxxx; Geert Uytterhoeven
> > > <geert+renesas@xxxxxxxxx>; Chris Paterson <Chris.Paterson2@xxxxxxxxxxx>;
> > > Biju Das <biju.das@xxxxxxxxxxxxxx>; linux-renesas-soc@xxxxxxxxxxxxxxx
> > > Subject: RE: [PATCH] serial: 8250: dw: Fix the macro
> > > RZN1_UART_xDMACR_8_WORD_BURST
> > >
> > > Hi Jiri,
> > >
> > > Thanks for the feedback.
> > >
> > > > Subject: Re: [PATCH] serial: 8250: dw: Fix the macro
> > > > RZN1_UART_xDMACR_8_WORD_BURST
> > > >
> > > > On 30. 06. 22, 10:39, Biju Das wrote:
> > > > > As per RZ/N1 peripheral user
> > > > > manual(r01uh0752ej0100-rzn1-peripheral.pdf)
> > > > > rev 1.0.0 Mar,2019,
> > > >
> > > > Is this public anywhere?
> > >
> > > Yes, It is available here[1] see page 72 and 73.
> > >
> > > [1] https://www.renesas.com/us/en/document/mah/rzn1d-group-rzn1s-group-
> > > rzn1l-group-users-manual-peripherals?language=en&r=1054561
> > >
> > >
> > > >
> > > > > the value for 8_WORD_BURST is 4(b2,b1=2’b10).
> > > > >
> > > > > This patch fixes the macro as per the user manual.
> > > >
> > > > I'm curious, is the bottom bit from "3" ignored by the HW or does this
> > > > fix a real problem in behavior? Stating that might help backporters to
> > > > decide if to take the patch or not.
> > >
> > > See page 72 and 73.
> > >
> > > Yes, it fixes a real problem as by using a value of 8 , you are wrongly
> > > configuring DMA_BURST_SIZE of 1 instead of DMA_BURST_SIZE of 8.
> > >
> > > b2, b1 bUart_DEST_BURST
> > > _SIZE
> > > DEST_BURST_SIZE
> > > Destination Burst Transaction Size in Transmit FIFO.
> > > UART is the flow controller. Thus, the user must write this field before
> > > or at the same time the DMA mode is enabled. Number of data byte, to be
> > > written to the Transmit FIFO every time a transmit burst transaction
> > > request are made on DMA request.
> > > 2’b00 = 1 byte
> > > 2’b01 = 4 bytes
> > > 2’b10 = 8 bytes
> > > 2’b11 = Reserved, not used
> > >
> > > Cheers,
> > > Biju
> > >
> > >
> > > >
> > > > > Fixes: aa63d786cea2 ("serial: 8250: dw: Add support for DMA flow
> > > > > controlling devices")
> > > > > Signed-off-by: Biju Das <biju.das.jz@xxxxxxxxxxxxxx>
> > > > > ---
> > > > > drivers/tty/serial/8250/8250_dw.c | 2 +-
> > > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/drivers/tty/serial/8250/8250_dw.c
> > > > > b/drivers/tty/serial/8250/8250_dw.c
> > > > > index f57bbd32ef11..931490b27d6b 100644
> > > > > --- a/drivers/tty/serial/8250/8250_dw.c
> > > > > +++ b/drivers/tty/serial/8250/8250_dw.c
> > > > > @@ -47,7 +47,7 @@
> > > > > #define RZN1_UART_xDMACR_DMA_EN BIT(0)
> > > > > #define RZN1_UART_xDMACR_1_WORD_BURST (0 << 1)
> > > > > #define RZN1_UART_xDMACR_4_WORD_BURST (1 << 1)
> > > > > -#define RZN1_UART_xDMACR_8_WORD_BURST (3 << 1)
> > > > > +#define RZN1_UART_xDMACR_8_WORD_BURST (2 << 1)
> > > > > #define RZN1_UART_xDMACR_BLK_SZ(x) ((x) << 3)
> > > > >
> > > > > /* Quirks */
> > > >
> > > > thanks,
> > > > --
> > > > js
>
>
>
> --
> With Best Regards,
> Andy Shevchenko
