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Message-ID: <68afca92-c511-4499-8143-bf6dc4a0eb02@microchip.com>
Date: Wed, 31 Aug 2022 08:49:10 +0000
From: <Sergiu.Moga@...rochip.com>
To: <richard.genoud@...il.com>, <lee@...nel.org>, <robh+dt@...nel.org>,
<krzysztof.kozlowski+dt@...aro.org>, <Nicolas.Ferre@...rochip.com>,
<alexandre.belloni@...tlin.com>, <Claudiu.Beznea@...rochip.com>,
<radu_nicolae.pirea@....ro>, <mturquette@...libre.com>,
<sboyd@...nel.org>, <gregkh@...uxfoundation.org>,
<jirislaby@...nel.org>, <admin@...iphile.com>,
<Kavyasree.Kotagiri@...rochip.com>
CC: <devicetree@...r.kernel.org>,
<linux-arm-kernel@...ts.infradead.org>,
<linux-kernel@...r.kernel.org>, <linux-spi@...r.kernel.org>,
<linux-clk@...r.kernel.org>, <linux-serial@...r.kernel.org>
Subject: Re: [PATCH 5/5] tty: serial: atmel: Make the driver aware of the
existence of GCLK
On 30.08.2022 20:29, Richard Genoud wrote:
>
> Le 17/08/2022 à 09:55, Sergiu Moga a écrit :
>> Previously, the atmel serial driver did not take into account the
>> possibility of using the more customizable generic clock as its
>> baudrate generator. Unless there is a Fractional Part available to
>> increase accuracy, there is a high chance that we may be able to
>> generate a baudrate closer to the desired one by using the GCLK as the
>> clock source. Now, depending on the error rate between
>> the desired baudrate and the actual baudrate, the serial driver will
>> fallback on the generic clock. The generic clock must be provided
>> in the DT node of the serial that may need a more flexible clock source.
>>
>> Signed-off-by: Sergiu Moga <sergiu.moga@...rochip.com>
>> ---
>> drivers/tty/serial/atmel_serial.c | 52 ++++++++++++++++++++++++++++++-
>> drivers/tty/serial/atmel_serial.h | 1 +
>> 2 files changed, 52 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/tty/serial/atmel_serial.c
>> b/drivers/tty/serial/atmel_serial.c
>> index 30ba9eef7b39..0a0b46ee0955 100644
>> --- a/drivers/tty/serial/atmel_serial.c
>> +++ b/drivers/tty/serial/atmel_serial.c
>> @@ -15,6 +15,7 @@
>> #include <linux/init.h>
>> #include <linux/serial.h>
>> #include <linux/clk.h>
>> +#include <linux/clk-provider.h>
>> #include <linux/console.h>
>> #include <linux/sysrq.h>
>> #include <linux/tty_flip.h>
>> @@ -77,6 +78,8 @@ static void atmel_stop_rx(struct uart_port *port);
>> #endif
>>
>> #define ATMEL_ISR_PASS_LIMIT 256
>> +#define ERROR_RATE(desired_value, actual_value) \
>> + ((int)(100 - ((desired_value) * 100) / (actual_value)))
>>
>> struct atmel_dma_buffer {
>> unsigned char *buf;
>> @@ -110,6 +113,7 @@ struct atmel_uart_char {
>> struct atmel_uart_port {
>> struct uart_port uart; /* uart */
>> struct clk *clk; /* uart clock */
>> + struct clk *gclk; /* uart generic clock */
>> int may_wakeup; /* cached value of
>> device_may_wakeup for times we need to disable it */
>> u32 backup_imr; /* IMR saved during
>> suspend */
>> int break_active; /* break being received */
>> @@ -2115,6 +2119,8 @@ static void atmel_serial_pm(struct uart_port
>> *port, unsigned int state,
>> * This is called on uart_close() or a suspend event.
>> */
>> clk_disable_unprepare(atmel_port->clk);
>> + if (atmel_port->gclk && __clk_is_enabled(atmel_port->gclk))
>> + clk_disable_unprepare(atmel_port->gclk);
>> break;
>> default:
>> dev_err(port->dev, "atmel_serial: unknown pm %d\n", state);
>> @@ -2129,7 +2135,8 @@ static void atmel_set_termios(struct uart_port
>> *port, struct ktermios *termios,
>> {
>> struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
>> unsigned long flags;
>> - unsigned int old_mode, mode, imr, quot, baud, div, cd, fp = 0;
>> + unsigned int old_mode, mode, imr, quot, div, cd, fp = 0;
>> + unsigned int baud, actual_baud, gclk_rate;
>>
>> /* save the current mode register */
>> mode = old_mode = atmel_uart_readl(port, ATMEL_US_MR);
>> @@ -2288,6 +2295,37 @@ static void atmel_set_termios(struct uart_port
>> *port, struct ktermios *termios,
>> cd /= 8;
>> mode |= ATMEL_US_USCLKS_MCK_DIV8;
>> }
>> +
>> + /*
>> + * If there is no Fractional Part, there is a high chance that
>> + * we may be able to generate a baudrate closer to the desired one
>> + * if we use the GCLK as the clock source driving the baudrate
>> + * generator.
>> + */
>> + if (!fp && atmel_port->gclk) {
>> + if (__clk_is_enabled(atmel_port->gclk))
>> + clk_disable_unprepare(atmel_port->gclk);
>> + clk_set_rate(atmel_port->gclk, 16 * baud);
>> + gclk_rate = clk_get_rate(atmel_port->gclk);
>> + actual_baud = clk_get_rate(atmel_port->clk) / (16 * cd);
>> + if (abs(ERROR_RATE(baud, actual_baud)) >
>> + abs(ERROR_RATE(baud, gclk_rate / 16))) {
>> + mode |= ATMEL_US_GCLK;
>> +
>> + /*
>> + * Set the Clock Divisor for GCLK to 1.
>> + * Since we were able to generate the smallest
>> + * multiple of the desired baudrate times 16,
>> + * then we surely can generate a bigger multiple
>> + * with the exact error rate for an equally
>> increased
>> + * CD. Thus no need to take into account
>> + * a higher value for CD.
>> + */
>> + cd = 1;
>> + clk_prepare_enable(atmel_port->gclk);
>> + }
>> + }
>> +
>> quot = cd | fp << ATMEL_US_FP_OFFSET;
>>
>> if (!(port->iso7816.flags & SER_ISO7816_ENABLED))
>> @@ -2883,6 +2921,16 @@ static int atmel_serial_probe(struct
>> platform_device *pdev)
>> if (ret)
>> goto err;
>>
>> + atmel_port->gclk = devm_clk_get_optional(&pdev->dev, "gclk");
>> + if (atmel_port->gclk) {
>> + ret = clk_prepare_enable(atmel_port->gclk);
>> + if (ret) {
>> + atmel_port->gclk = NULL;
>> + return ret;
>> + }
>> + clk_disable_unprepare(atmel_port->gclk);
>> + }
>> +
>> ret = atmel_init_port(atmel_port, pdev);
>> if (ret)
>> goto err_clk_disable_unprepare;
>> @@ -2929,6 +2977,8 @@ static int atmel_serial_probe(struct
>> platform_device *pdev)
>> * is used
>> */
>> clk_disable_unprepare(atmel_port->clk);
>> + if (atmel_port->gclk && __clk_is_enabled(atmel_port->gclk))
>> + clk_disable_unprepare(atmel_port->gclk);
>>
>> return 0;
>>
>> diff --git a/drivers/tty/serial/atmel_serial.h
>> b/drivers/tty/serial/atmel_serial.h
>> index 0d8a0f9cc5c3..fb718972f81a 100644
>> --- a/drivers/tty/serial/atmel_serial.h
>> +++ b/drivers/tty/serial/atmel_serial.h
>> @@ -63,6 +63,7 @@
>> #define ATMEL_US_PAR_MARK (3 << 9)
>> #define ATMEL_US_PAR_NONE (4 << 9)
>> #define ATMEL_US_PAR_MULTI_DROP (6 << 9)
>> +#define ATMEL_US_GCLK BIT(12)
>> #define ATMEL_US_NBSTOP GENMASK(13, 12) /* Number of
>> Stop Bits */
>> #define ATMEL_US_NBSTOP_1 (0 << 12)
>> #define ATMEL_US_NBSTOP_1_5 (1 << 12)
>
> Correct me if I'm wrong, but GCLK is selected by the bit 12 only in
> UART_MR, not in USART_MR.
> In USART_MR, it seems to be controlled by bits 4-5 (and bit 12 is for
> stop bits, as we can see above, and in the datasheet).
> cf
> https://ww1.microchip.com/downloads/aemDocuments/documents/MPU32/ProductDocuments/DataSheets/SAMA5D2-Series-Datasheet-DS60001476H.pdf
>
> page 1637
>
> Regards,
> Richard.
Yes, you are correct, this should have been called ATMEL_UA_GCLK
instead. I think I will add both ATMEL_UA_GCLK and ATMEL_US_GCLK bits
and an additional field in struct atmel_uart_port to hold ATMEL_UA_GCLK
for UART or ATMEL_US_GCLK for USART. I guess this field should be set in
atmel_get_ip_name(), the same place where the decision between
ATMEL_UA_RTOR and ATMEL_US_RTOR is taken.
Thanks,
Sergiu
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