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

[Alexander Graf]

On 12.08.16 19:21, Simon Glass wrote:
> 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.

It's the same IP block on the RPi :).

> 
>>
>>>
>>>>
>>>> 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() ?

How? I guess we would need to

  a) Create the GPIO device
  b) Ask the GPIO device whether the pin is muxed correctly
  c) Create serial device based on outcome of b

Is that possible?


Alex