[U-Boot] [PATCH v2] serial: bcm283x_mu: Detect disabled serial device

[Simon Glass]

Hi Alex,

On 11 August 2016 at 23:27, Alexander Graf <agraf at suse.de> wrote:
>
>
>> Am 12.08.2016 um 00:38 schrieb Simon Glass <sjg at chromium.org>:
>>
>> Hi Alex,
>>
>>> On 11 August 2016 at 05:33, Alexander Graf <agraf at suse.de> wrote:
>>>
>>>
>>>> On 09.08.16 06:28, Stephen Warren wrote:
>>>>> On 08/04/2016 05:15 PM, Alexander Graf wrote:
>>>>>
>>>>>> On 04 Aug 2016, at 20:11, Stephen Warren <swarren at wwwdotorg.org> wrote:
>>>>>>
>>>>>> On 08/04/2016 01:11 AM, Alexander Graf wrote:
>>>>>>> On the raspberry pi, you can disable the serial port to gain dynamic
>>>>>>> frequency
>>>>>>> scaling which can get handy at times.
>>>>>>>
>>>>>>> However, in such a configuration the serial controller gets its rx
>>>>>>> queue filled
>>>>>>> up with zero bytes which then happily get transmitted on to whoever
>>>>>>> calls
>>>>>>> getc() today.
>>>>>>>
>>>>>>> This patch adds detection logic for that case by checking whether
>>>>>>> the RX pin is
>>>>>>> mapped to GPIO15 and disables the mini uart if it is not mapped
>>>>>>> properly.
>>>>>>>
>>>>>>> That way we can leave the driver enabled in the tree and can
>>>>>>> determine during
>>>>>>> runtime whether serial is usable or not, having a single binary that
>>>>>>> allows for
>>>>>>> uart and non-uart operation.
>>>>>>
>>>>>>> diff --git a/drivers/serial/serial_bcm283x_mu.c
>>>>>>> b/drivers/serial/serial_bcm283x_mu.c
>>>>>>
>>>>>>> @@ -72,9 +87,18 @@ static int bcm283x_mu_serial_probe(struct udevice
>>>>>>> *dev)
>>>>>>> {
>>>>>>>    struct bcm283x_mu_serial_platdata *plat = dev_get_platdata(dev);
>>>>>>>    struct bcm283x_mu_priv *priv = dev_get_priv(dev);
>>>>>>> +    struct bcm283x_gpio_regs *gpio = (struct bcm283x_gpio_regs
>>>>>>> *)plat->gpio;
>>>>>>>
>>>>>>>    priv->regs = (struct bcm283x_mu_regs *)plat->base;
>>>>>>>
>>>>>>> +    /*
>>>>>>> +     * The RPi3 disables the mini uart by default. The easiest way
>>>>>>> to find
>>>>>>> +     * out whether it is available is to check if the pin is muxed.
>>>>>>> +     */
>>>>>>> +    if (((readl(&gpio->gpfsel1) >> BCM283X_GPIO_GPFSEL1_F15_SHIFT) &
>>>>>>> +        BCM283X_GPIO_ALTFUNC_MASK) != BCM283X_GPIO_ALTFUNC_5)
>>>>>>> +        priv->disabled = true;
>>>>>>> +
>>>>>>>    return 0;
>>>>>>
>>>>>> Comment on the current implementation: Can't probe() return an error
>>>>>> if the device should be disabled? That would avoid the need to check
>>>>>> priv->disabled in all the other functions.
>>>>>
>>>>> I guess I should’ve put that in a comment somewhere. Unfortunately we
>>>>> can’t. If I just return an error on probe, U-Boot will panic because
>>>>> we tell it in a CONFIG define that we require a serial port (grep for
>>>>> CONFIG_REQUIRE_SERIAL_CONSOLE).
>>>>>
>>>>> We could maybe try to unset that define instead?
>>>>
>>>> Yes, assuming that U-Boot runs just fine with HDMI console only, I think
>>>> it's fine to unset CONFIG_REQUIRE_SERIAL_CONSOLE.
>>>>
>>>>>> Overall comment: I'd rather not put this logic into the UART driver
>>>>>> itself; it is system-specific rather than device-specific. I'd also
>>>>>> rather not have the UART driver touching GPIO registers; that's not
>>>>>> very modular, and could cause problems if the Pi is converted to use
>>>>>> DT to instantiate devices.
>>>>>>
>>>>>> Instead, can we put the logic into board/raspberrypi/rpi/rpi.c? I.e.
>>>>>> have some early function come along and enable/disable the
>>>>>> bcm2837_serials device object as appropriate? That way it isolates
>>>>>> the code to the Pi specifically, and not any other bcm283x board.
>>>>>> We'd want to wrap that code in #ifdef CONFIG_PL01X_SERIAL.
>>>>>
>>>>> We can do that if we can fail at probe time. If we absolutely must
>>>>> have a serial driver to work in the first place, that doesn’t work. I
>>>>> can try to poke at it, but it’ll be a few days I think :).
>>>
>>> So I couldn't find a sane way to fail probing based on something defined
>>> in the board file, reusing the existing gpio device.
>>
>> Would it be possible to move this code into the serial driver?
>
> You mean like in v2 which Stephen nacked? :)

Yes :-(

Well you can put what you like in the board code, and if this is only
on the rpi, then it makes sense.

Really though, this is a pinctrl thing, so if there were a pinctrl
driver you could just use it. The GPIO driver should not deal with pin
muxing.

>
>>
>>>
>>> However, there's an easy alternative. We can make the console code just
>>> ignore our serial device if we set its pointer to NULL. That way we
>>> still have the device, but can contain all logic to disable usage of the
>>> mini uart to the board file.
>>
>> I'm not very keen on that - feels like a hack.  What is stopping
>> Stephen's idea from working? I could perhaps help with dm plumbing is
>> that is the issue...
>
> The problem is that we need the gpio device to determine whether the pin is muxed. There is no temporal control that I could see that would allow me to be in a place where the gpio device exists, the serial device does not exist, and where I could then not spawn the serial device based on board logic.

Can you use board_early_init_f() ?

Regards,
Simon