2016-08-18 2:00 GMT+03:00 Viresh Kumar <viresh.kumar@xxxxxxxxxx>:
> On 17-08-16, 18:35, Matwey V. Kornilov wrote:
>> Hello,
>>
>> Does anybody know whether it is possible to implement fast_switch()
>> for cpufreq module used on am335x (like BeagleBone)?
>> I would like to use fast_switch feature in my software driver rs485
>> duplex control implementation in tty/serial/8250/8250_port.c
>> Recently I've faced an timing issue with it. Depending on current CPU
>> on BeagleBone Black it takes from 40 usec to 200 usec to handle UART
>> IRQ.
>> Consequently, it takes 40-200 usec to switch transfer direction after
>> the last char transmitted.
>> It appeared that there are peripheral which start transmission after
>> about 60 usec after last char received (When BeagleBone is not quick
>> enough to switch direction to that time, the data go to nowhere).
>> In order to avoid input data loss, I have to set CPU governor to
>> `performance' on each boot.
>> I suppose it would be great to use CPU frequency fast switching in
>> UART IRQ handling when delay_rts_after_send is set to 0 in order to
>> avoid similar issues.
>
> You mean that you want to update frequency of the cpu from your tty driver? I am
> not sure if its a good idea really.
I mean something like the following:
static irqreturn_t omap8250_irq(int irq, void *dev_id)
{
struct uart_port *port = dev_id;
struct uart_8250_port *up = up_to_u8250p(port);
unsigned int iir;
int ret;
// fast_switch to the highest rate here
#ifdef CONFIG_SERIAL_8250_DMA
if (up->dma) {
ret = omap_8250_dma_handle_irq(port);
return IRQ_RETVAL(ret);
}
#endif
serial8250_rpm_get(up);
iir = serial_port_in(port, UART_IIR);
ret = serial8250_handle_irq(port, iir);
serial8250_rpm_put(up);
// switch back here
return IRQ_RETVAL(ret);
}
Probably, I've incorrectly understood the purpose of cpu freq fast switch.
>
> CPU frequency is governed by cpufreq governors, which evaluate the load at run
> time and based on that take a decision to increase/decrease the frequency.
>
> --
> viresh
>
--
With best regards,
Matwey V. Kornilov.
Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
119991, Moscow, Universitetsky pr-k 13, +7 (495) 9392382
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