[yocto] [meta-raspberrypi][PATCH 3/5] rpi-config: Add UART enabler

[Theodor Gherzan]

A new config.txt setting has been introduced - enable_uart - the
purpose of which is to control the initialisation of the UART(*). The
default value varies according to the primary UART on the platform -
if the PL011 is the primary UART (that used for the console and boot
messages) then enable_uart defaults to 1, and if the mini-UART is the
primary then it defaults to 0.

The reason for the additional setting is that making use of the
mini-UART requires the core frequency to be fixed, so setting
enable_uart will limit the core to the minimum (unless force_turbo is
set, since that will fix the core frequency to the maximum, provided
the Pi is adequately powered and cooled). The previous default
behaviour, which rendered much of the output illegible, was not
useful, but we didn't want to restrict all users (even those who don't
care about the UART) to the minimum core frequency. We also wanted to
be able to remove the restriction when the PL011 is chosen on a Pi3,
hence the need for a dedicated setting indicating the reason for the
frequency limit.

On Thu, Apr 21, 2016 at 6:46 PM, Theodor Gherzan <theodor at gherzan.ro> wrote:
> Here is a full explination of this:
>
>
> On Thu, Apr 21, 2016 at 6:43 PM, Andrei Gherzan <andrei at gherzan.ro> wrote:
>> Hi,
>>
>> On Thu, Apr 21, 2016 at 06:32:11PM +0100, Theodor Gherzan wrote:
>>> From: Theodor Gherzan <theodor at resin.io>
>>>
>>> Signed-off-by: Theodor Gherzan <theodor at resin.io>
>>> ---
>>>  recipes-bsp/bootfiles/rpi-config_git.bb | 6 ++++++
>>>  1 file changed, 6 insertions(+)
>>>
>>> diff --git a/recipes-bsp/bootfiles/rpi-config_git.bb b/recipes-bsp/bootfiles/rpi-config_git.bb
>>> index 4d41723..6ddf843 100644
>>> --- a/recipes-bsp/bootfiles/rpi-config_git.bb
>>> +++ b/recipes-bsp/bootfiles/rpi-config_git.bb
>>> @@ -90,6 +90,12 @@ do_deploy() {
>>>          echo "# Enable PITFT22 display" >>${DEPLOYDIR}/bcm2835-bootfiles/config.txt
>>>          echo "dtoverlay=pitft22,rotate=270,speed=32000000,txbuflen=32768" >>${DEPLOYDIR}/bcm2835-bootfiles/config.txt
>>>      fi
>>> +
>>> +    # UART support
>>> +    if [ "${ENABLE_UART}" = "1" ]; then
>>> +        echo "# Enable UART" >>${DEPLOYDIR}/bcm2835-bootfiles/config.txt
>>> +        echo "enable_uart=1" >>${DEPLOYDIR}/bcm2835-bootfiles/config.txt
>> What is the default value of enable_uart?
>>
>> --
>> Andrei Gherzan