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

[Stephen Warren]

On 08/14/2015 05:57 PM, Bin Meng wrote:
> Hi,
> 
> On Sat, Aug 15, 2015 at 7:18 AM, Simon Glass <sjg at chromium.org> wrote:
>> Hi,
>>
>> On 14 August 2015 at 16:51, Stephen Warren <swarren at wwwdotorg.org> wrote:
>>> 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.
>>
>> I'm not a big fan of weak functions either. In fact I think with
>> driver model we should avoid them. If we can't call a uclass to get
>> the info then perhaps we should wait until we can.
>>
>> Pragmatically I wonder if a UART clock frequency would not be a useful
>> compromise? Some bindings have it, some do not. Maybe we should just
>> add it?
>>
> 
> I am not sure which bindings you are looking at,

The binding for nvidia,tegra20-uart. In the kernel tree, that's at
Documentation/devicetree/bindings/serial/nvidia,tegra20-hsuart.txt.

> but I checked the
> Power.org ePAPR spec v1.1, the clock-frequency is there specially
> listed for NS16550 compatible nodes.
>
> clock-frequency R <u32> Specifies the frequency (in Hz) of the baud
> rate generator’s input clock
> 
> If you don't have such clock-frequency in your device tree, I would
> say you don't follow the spec.

The binding for Tegra UARTs doesn't inherit from the generic NS16550
binding, so there's that binding isn't relevant.