Re: [PATCH] serial: 8250_dw: Revert: Do not reclock if already at correct rate

Peter Collingbourne
2024-03-29T02:36:00Z

On Thu, Mar 28, 2024 at 5:35 AM Hans de Goede <hdegoede@redhat.com> wrote:
>
> Hi,
>
> On 3/28/24 8:10 AM, Hans de Goede wrote:
> > Hi,
> >
> > On 3/18/24 7:52 PM, Peter Collingbourne wrote:
> >> On Mon, Mar 18, 2024 at 3:36 AM Andy Shevchenko
> >> <andriy.shevchenko@linux.intel.com> wrote:
> >>>
> >>> On Sun, Mar 17, 2024 at 10:41:23PM +0100, Hans de Goede wrote:
> >>>> Commit e5d6bd25f93d ("serial: 8250_dw: Do not reclock if already at
> >>>> correct rate") breaks the dw UARTs on Intel Bay Trail (BYT) and
> >>>> Cherry Trail (CHT) SoCs.
> >>>>
> >>>> Before this change the RTL8732BS Bluetooth HCI which is found
> >>>> connected over the dw UART on both BYT and CHT boards works properly:
> >>>>
> >>>> Bluetooth: hci0: RTL: examining hci_ver=06 hci_rev=000b lmp_ver=06 lmp_subver=8723
> >>>> Bluetooth: hci0: RTL: rom_version status=0 version=1
> >>>> Bluetooth: hci0: RTL: loading rtl_bt/rtl8723bs_fw.bin
> >>>> Bluetooth: hci0: RTL: loading rtl_bt/rtl8723bs_config-OBDA8723.bin
> >>>> Bluetooth: hci0: RTL: cfg_sz 64, total sz 24508
> >>>> Bluetooth: hci0: RTL: fw version 0x365d462e
> >>>>
> >>>> where as after this change probing it fails:
> >>>>
> >>>> Bluetooth: hci0: RTL: examining hci_ver=06 hci_rev=000b lmp_ver=06 lmp_subver=8723
> >>>> Bluetooth: hci0: RTL: rom_version status=0 version=1
> >>>> Bluetooth: hci0: RTL: loading rtl_bt/rtl8723bs_fw.bin
> >>>> Bluetooth: hci0: RTL: loading rtl_bt/rtl8723bs_config-OBDA8723.bin
> >>>> Bluetooth: hci0: RTL: cfg_sz 64, total sz 24508
> >>>> Bluetooth: hci0: command 0xfc20 tx timeout
> >>>> Bluetooth: hci0: RTL: download fw command failed (-110)
> >>>>
> >>>> Revert the changes to fix this regression.
> >>>
> >>> Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> >>>
> >>>> Note it is not entirely clear to me why this commit is causing
> >>>> this issue. Maybe probe() needs to explicitly set the clk rate
> >>>> which it just got (that feels like a clk driver issue) or maybe
> >>>> the issue is that unless setup before hand by firmware /
> >>>> the bootloader serial8250_update_uartclk() needs to be called
> >>>> at least once to setup things ? Note that probe() does not call
> >>>> serial8250_update_uartclk(), this is only called from the
> >>>> dw8250_clk_notifier_cb()
> >>>>
> >>>> This requires more debugging which is why I'm proposing
> >>>> a straight revert to fix the regression ASAP and then this
> >>>> can be investigated further.
> >>>
> >>> Yep. When I reviewed the original submission I was got puzzled with
> >>> the CLK APIs. Now I might remember that ->set_rate() can't be called
> >>> on prepared/enabled clocks and it's possible the same limitation
> >>> is applied to ->round_rate().
> >>>
> >>> I also tried to find documentation about the requirements for those
> >>> APIs, but failed (maybe was not pursuing enough, dunno). If you happen
> >>> to know the one, can you point on it?
> >>
> >> To me it seems to be unlikely to be related to round_rate(). It seems
> >> more likely that my patch causes us to never actually set the clock
> >> rate (e.g. because uartclk was initialized to the intended clock rate
> >> instead of the current actual clock rate).
> >
> > I agree that the likely cause is that we never set the clk-rate. I'm not
> > sure if the issue is us never actually calling clk_set_rate() or if
> > the issue is that by never calling clk_set_rate() dw8250_clk_notifier_cb()
> > never gets called and thus we never call serial8250_update_uartclk()
> >
> >> It should be possible to
> >> confirm by checking the behavior with my patch with `&& p->uartclk !=
> >> rate` removed, which I would expect to unbreak Hans's scenario. If my
> >> hypothesis is correct, the fix might involve querying the clock with
> >> clk_get_rate() in the if instead of reading from uartclk.
> >
> > Querying the clk with clk_get_rate() instead of reading it from
> > uartclk will not help as uartclk gets initialized with clk_get_rate()
> > in dw8250_probe(). So I believe that in my scenario clk_get_rate()
> > already returns the desired rate causing us to never call clk_set_rate()
> > at all which leaves 2 possible root causes for the regressions:
> >
> > 1. The clk generator has non readable registers and the returned
> > rate from clk_get_rate() is a default rate and the actual hw is
> > programmed differently, iow we need to call clk_set_rate() at
> > least once on this hw to ensure that the clk generator is prggrammed
> > properly.
> >
> > 2. The 8250 code is not working as it should because
> > serial8250_update_uartclk() has never been called.
>
> Ok, so it looks like this actually is an issue with how clk_round_rate()
> works on this hw (atm, maybe the clk driver needs fixing).
>
> I have added the following to debug this:
>
> diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
> index a3acbf0f5da1..3152872e50b2 100644
> --- a/drivers/tty/serial/8250/8250_dw.c
> +++ b/drivers/tty/serial/8250/8250_dw.c
> @@ -306,6 +306,8 @@ static void dw8250_clk_work_cb(struct work_struct *work)
> if (rate <= 0)
> return;
>
> + pr_info("uartclk work_cb clk_get_rate() returns: %ld\n", rate);
> +
> up = serial8250_get_port(d->data.line);
>
> serial8250_update_uartclk(&up->port, rate);
> @@ -353,11 +355,15 @@ static void dw8250_set_termios(struct uart_port *p, struct ktermios *termios,
> {
> unsigned long newrate = tty_termios_baud_rate(termios) * 16;
> struct dw8250_data *d = to_dw8250_data(p->private_data);
> + unsigned long currentrate = clk_get_rate(d->clk);
> long rate;
> int ret;
>
> +
> rate = clk_round_rate(d->clk, newrate);
> - if (rate > 0 && p->uartclk != rate) {
> + pr_info("uartclk set_termios new: %ld new-rounded: %ld current %ld cached %d\n",
> + newrate, rate, currentrate, p->uartclk);
> + if (rate > 0) {
> clk_disable_unprepare(d->clk);
> /*
> * Note that any clock-notifer worker will block in
> @@ -593,6 +599,8 @@ static int dw8250_probe(struct platform_device *pdev)
> if (!p->uartclk)
> return dev_err_probe(dev, -EINVAL, "clock rate not defined\n");
>
> + pr_info("uartclk initial cached %d\n", p->uartclk);
> +
> data->pclk = devm_clk_get_optional_enabled(dev, "apb_pclk");
> if (IS_ERR(data->pclk))
> return PTR_ERR(data->pclk);
>
> And then I get the following output:
>
> [ 3.119182] uartclk initial cached 44236800
> [ 3.139923] uartclk work_cb clk_get_rate() returns: 44236800
> [ 3.152469] uartclk initial cached 44236800
> [ 3.172165] uartclk work_cb clk_get_rate() returns: 44236800
> [ 34.128257] uartclk set_termios new: 153600 new-rounded: 44236800 current 44236800 cached 44236800
> [ 34.130039] uartclk work_cb clk_get_rate() returns: 153600
> [ 34.131975] uartclk set_termios new: 153600 new-rounded: 153600 current 153600 cached 153600
> [ 34.132091] uartclk set_termios new: 153600 new-rounded: 153600 current 153600 cached 153600
> [ 34.132140] uartclk set_termios new: 153600 new-rounded: 153600 current 153600 cached 153600
> [ 34.132187] uartclk set_termios new: 1843200 new-rounded: 153600 current 153600 cached 153600
> [ 34.133536] uartclk work_cb clk_get_rate() returns: 1843200
>
> Notice how the new-rounded just returns the current rate of the clk,
> rather then a rounded value of new.
>
> I'm not familiar enough with the clk framework to debug this further.
>
> Peter, IMHO we really must revert your commit since it is completely
> breaking UARTs on many different Intel boards. Can you please give your
> ack for reverting this for now ?

That's fine with me. I will try to dig into the code soon to figure
out what is going on unless someone gets there first.

Acked-by: Peter Collingbourne <pcc@google.com>

Peter

>
> Regards,
>
> Hans
>
>
> p.s.
>
> For anyone who wants to dive into the clk_round_rate() issue deeper,
> the code registering the involved clks is here:
>
> drivers/acpi/acpi_lpss.c: register_device_clock()
>
> And for the clocks in question fixed_clk_rate is 0 and both
> the LPSS_CLK_GATE and LPSS_CLK_DIVIDER flags are set, so
> for a single UART I get:
>
> [root@fedora ~]# ls -d /sys/kernel/debug/clk/80860F0A:01*
> /sys/kernel/debug/clk/80860F0A:01 /sys/kernel/debug/clk/80860F0A:01-update
> /sys/kernel/debug/clk/80860F0A:01-div
>
> With the 80860F0A:01-update clk being the clk which is
> actually used / controlled by the 8250_dw.c code.
>