[PATCH 5/5] tty: serial: atmel: Make the driver aware of the existence of GCLK
Claudiu.Beznea at microchip.com
Claudiu.Beznea at microchip.com
Wed Aug 31 23:36:05 PDT 2022
On 31.08.2022 14:32, Sergiu Moga - M68701 wrote:
> On 31.08.2022 12:46, Claudiu Beznea wrote:
>> On 17.08.2022 10:55, Sergiu Moga wrote:
>>> 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 at microchip.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))
>>
>> No need to check for atmel_port->gclk != NULL. clk APIs are already doing this.
>>
>>> + 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);
>>
>> You disabled it here, set new rate but re-enable it conditionally above, is
>> this intended? the below condition may fail.
>>
>
> Yes, it is intended. Gclk should remain disabled if the below condition
> fails.
>
>
>>> + clk_set_rate(atmel_port->gclk, 16 * baud);
>>> + gclk_rate = clk_get_rate(atmel_port->gclk);
>>
>> You should be able to use clk_round_rate() here:
>> gclk_rate = clk_round_rate(atmel_port->gclk,
>> 16 * baudrate);
>>
>> With this you can re-write all this block something like:
>>
>> gclk_rate = clk_round_rate(atmel_port->gclk,
>> 16 * baudrate);
>> actual_baud = gclk_rate / (16 * cd);
>> if (abs(ERROR_RATE(baud, actual_baud)) >
>> abd(ERROR_RATE(baud, gclk_rate / 16))) {
>> mode |= ATMEL_US_GCLK;
>> cd = 1;
>> if (__clk_is_enabled(atmel_port->gclk))
>> clk_disable_unprepare(atmel_port->gclk)
>> clk_set_rate(atmel_port->gclk, gclk_rate);
>> clk_prepare_enable(atmel_port->gclk);
>> }
>>
>>
>
> Hmm, yes, you are right, it is much better with this clk_round_rate()
> since there would be no need to disable gclk before setting the new
> rate, I did not know of this function at that point. However, in this
> case, the if(abs...) will also need an else to disable the gclk in case
> the condition failed and it was somehow enabled before.
Then you can keep the disable at the beginning of the block (as it
previously was) and use clk_round_rate() instead of:
clk_set_rate();
clk_get_rate();
And use clk_set_rate() only if the following is true:
+ if (abs(ERROR_RATE(baud, actual_baud)) >
+ abs(ERROR_RATE(baud, gclk_rate / 16))) {
Having this, in case gclk cannot be used it is just disable w/o also having
the rate changed.
>
>
>>> + 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);
>>
>> Is there a reason you enable then disable the clock here?
>>
>
>
> I think it's better to make sure in the probe method that enabling the
> gclk issues no errors, so that the error does not appear in
> set_termios(). Since the user must place the optional gclk in DT if they
> want a finer rate when missing the Fractional Part, I think they should
> know before even trying to open the port that it is first correctly
> setup in the clock tree as well by making the probe method fail from the
> very beginning in case it is not.
There is no guarantee that the disable/enable in set_termios() will not
fail. If you want to avoid failures you can just take into account the
returning code of clk_prepare_enable() in set_termios() and decide to use
generic clock for baud rate depending on this.
>
> So, I first enable it and make sure there are no errors and then disable
> it because I do not see the point of having an enabled clock that might
> not be used.
>
>
>>> + }
>>> +
>>> 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);
>>
>> Is this due to the enable in atmel_set_termios()? Is that called on probe
>> path? Also, there is no need to check for atmel_port->gclk as clk APIs are
>> already doing this.
>>
>
>
> No, I guess this is not really needed, since it is disabled once it is
> claimed from DT if enabling it succeeds. I initially placed this code
> sequence wherever the peripheral clock is disabled as well.
>
>>>
>>> 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)
>>
>> It seems there are spaces here.
>>
>>> #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)
>>
>
> Thanks,
> Sergiu
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