[U-Boot] [linux-sunxi] [PATCH v4 00/26] sunxi: Allwinner A64: SPL support

[jonsmirl at gmail.com]

On Tue, Jan 3, 2017 at 5:41 AM, Jagan Teki <jagannadh.teki at gmail.com> wrote:
> On Tue, Jan 3, 2017 at 3:38 AM, jonsmirl at gmail.com <jonsmirl at gmail.com> wrote:
>> I recently ran into a probably with the UARTs on the A64. Many
>> Bluetooth modules (like Ampak) use the UART. The data rate of EDR BT
>> is 3Mb/s with about 2.1Mb/s though put. To handle this most systems
>> set the speed of the BT UART to 3Mb/s.
>>
>> By default the Allwinner UART clock input is OSC24. When using OSC24
>> the maximum speed the UART can be set to is 1.5Mb/s. The clock input
>> (apb2) can be changed over to PERIPH0x2 (1.2ghz) via the device tree
>> and 3Mb/s is then supported.
>>
>> But... there's a problem, UART0 (the console) is using the same master
>> clock source. So when you change the clock input over to PERIPH0x2 the
>> console stops working. There is no mechanism in Linux to handle this
>> clock source change and adjust the dividers on active uarts. So it
>> would be best if this master clock was set very early in u-boot and
>> then the console is adjusted to use it.
>>
>> Are there any downsides to making this change in u-boot?
>
> I don't understand, did you find this behaviour with these SPL changes
> or general sunxi u-boot?

Previously the boot console uart0 was getting setup in the SPL code. I
have not been closely following these changes, is that still true?

Changing the clock parent needs to be done before uart0 is
initialized. Changing this parent should have no other impact on
u-boot other than changing the clock divisor uart0 is using.

Once Linux is up, the Linux uart code will see the changed clock
parent and allow higher baud rates to be set.

This clock parent also impacts the I2C clocks, but I don't believe I2C
is enabled in A64 uboot.


>
> thanks!
> --
> Jagan Teki
> Free Software Engineer | www.openedev.com
> U-Boot, Linux | Upstream Maintainer
> Hyderabad, India.



-- 
Jon Smirl
jonsmirl at gmail.com