[U-Boot] [RFC] Merge all ns16550 dm serial drivers into one

[Stephen Warren]

On 08/14/2015 04:40 PM, Bin Meng wrote:
> On Sat, Aug 15, 2015 at 12:59 AM, Simon Glass <sjg at chromium.org> wrote:
>> Hi Stephen,
>>
>> On 14 August 2015 at 10:58, Stephen Warren <swarren at wwwdotorg.org> wrote:
>>> On 08/14/2015 10:50 AM, Simon Glass wrote:
>>>>
>>>> Hi Bin,
>>>>
>>>> On 14 August 2015 at 03:18, Bin Meng <bmeng.cn at gmail.com> wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> Currently there are 5 dm serial drivers, all of which are ns16550
>>>>> compatible drivers. They are:
>>>>>
>>>>>     serial_omap.c
>>>>>     serial_dw.c
>>>>>     serial_tegra.c
>>>>>     serial_x86.c
>>>>>     serial_ppc.c
>>>>>
>>>>> All these drivers are pretty much similar. I think we can justmerge
>>>>> these into one ns16550 driver.
>>>>>
>>>>> If you think this is necessary, I will send a patch series to do this.
>>>>
>>>>
>>>> The tegra one is there because it needs an input clock and Stephen
>>>> didn't want to add this to the device tree binding (the kernel has a
>>>> clock framework which gets around this problem).
>>>>
>>>> After that I followed the same pattern. I would support updating the
>>>> binding to support an input clock. Even with the new clock framework
>>>> in U-Boot it might be painful to fit it into SPL in some cases.
>>>
>>>
>>> The clock is already in the DT, in both Linux and U-Boot's copy, at least
>>> for Tegra DTs:
>>>
>>>          uarta: serial at 0,70006000 {
>>>                  compatible = "nvidia,tegra124-uart", ...
>>> ...
>>>                  clocks = <&tegra_car TEGRA124_CLK_UARTA>;
>>>
>>
>> I mean the clock-frequency property. However if there is a plan to
>> implement the clock framework in U-Boot that would be good too.
>>
>
> The clock-frequency is a fixed value on x86 super i/o chipset, and
> fixed on the PCI bus too. But for ARM and PPC, it might get
> dynamically calculated due to different PLL settings. We can implement
> a _weak function like the one in serial_ppc.c get_serial_clock() to
> initialize plat->clock with its return value. The _weak function gets
> clock-frequency from device tree. If there is not, platform codes
> which uses the ns16550 driver should provide the implementation of
> get_serial_clock(). Thoughts?

There is no clock-frequency property in DT, at least for the Tegra DT 
binding. It looks like some other bindings have it. To obtain the clock 
frequency from DT for Tegra, you'd need to parse the clocks property, 
find the clock driver associated with the phandle in DT, and go and ask 
that clock driver what the clock frequency is.

I'd prefer not to have a weak function that parses clock-frequency, 
since it's too easy to accidentally use it on systems where parsing that 
property is incorrect.

Certainly, a generic UART driver can call out to a platform-supplied 
function to retrieve the clock, and we can provide driver-specific 
implementations for x86 super IO and PCI, and generic implementations 
that appropriate drivers can call to parse the clocks or clock-frequency 
property from DT, and finally for Tegra if we can't parse the clocks 
property right now, call the Tegra clock driver directly to look up the 
value.