Re: [PATCH] arm64: dts: rockchip: decrease rising edge time of UART2

[Robin Murphy <robin.murphy@arm.com>]

On 2019-03-03 6:45 pm, Tony McKahan wrote:
> Hello Katsushiro,
> 
> On Sun, Mar 3, 2019 at 12:31 PM Katsuhiro Suzuki
> <katsuhiro@katsuster.net> wrote:
>>
>> Hello Tony,
>>
>> On 2019/03/04 0:13, Tony McKahan wrote:
>>> On Sun, Mar 3, 2019 at 9:04 AM Katsuhiro Suzuki <katsuhiro@katsuster.net> wrote:
>>>>
>>>> Hello Heiko,
>>>>
>>>> Thank you for comments.
>>>>
>>>> On 2019/03/03 22:19, Heiko Stuebner wrote:
>>>>> Hi,
>>>>>
>>>>> Am Sonntag, 3. März 2019, 13:27:05 CET schrieb Katsuhiro Suzuki:
>>>>>> This patch increases drive strength of UART2 from 3mA to 12mA for
>>>>>> getting more faster rising edge.
>>>>>>
>>>>>> RockPro64 is using a very high speed rate (1.5Mbps) for UART2. In
>>>>>> this setting, a bit width of UART is about 667ns.
>>>>>>
>>>>>> In my environment (RockPro64 UART2 with FTDI FT232RL UART-USB
>>>>>> converter), falling time of RockPro64 UART2 is 40ns, but riging time
>>>>>> is over 650ns. So UART receiver will get wrong data, because receiver
>>>>>> read intermediate data of rising edge.
>>>>>>
>>>>>> Rising time becomes 300ns from 650ns if apply this patch. This is not
>>>>>> perfect solution but better than now.
>>>>>>
>>>>>> Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>
>>>>>> ---
>>>>>>     arch/arm64/boot/dts/rockchip/rk3399.dtsi | 9 +++++++--
>>>>>>     1 file changed, 7 insertions(+), 2 deletions(-)
>>>>>
>>>>> your changing a core rk3399 property here, so I'd really like to get
>>>>> input from other board stakeholders on this before applying a core
>>>>> change.
>>>>>
>>>>> Could you either include the submitters of other rk3399-boards in the
>>>>> recipient list so that they're aware or limit the change to rockpro64 for
>>>>> the time being (aka overriding the property in the board-dts) please?
>>>>>
>>>>
>>>> OK, I'm adding other boards members.
>>>> by ./scripts/get_maintainer.pl arch/arm64/boot/dts/rockchip/rk3399-*.dts
>>>>
>>>>
>>>> RockPro64 directly connect UART2 pins of RK3399 to external connector.
>>>> I think maybe other RK3399 boards are facing same problem, but I cannot
>>>> check it because I have RockPro64 only...
>>>>
>>>> I'm happy if someone tell me other boards situation.
>>>
>>> I'm pulling out other rockchip boards momentarily to see what kind of
>>> population we have.
>>>
>>> Note these are not all running 5.x kernels, however none of them have
>>> the UART2 drive levels modified to my knowledge, and regardless, none
>>> show over 100 ns.
>>>
>>> board:    rise/fall
>>>
>>> rk3399-roc-pc: 90ns/90ns
>>> rk3399-rockpro64 V2.0:  90ns/45ns
>>> rk3399-rockpro64 V2.1:  40ns/41ns

I'm having to eyeball off a 20MHz analog scope (thank goodness for "yes" 
being able to generate a nice periodic signal), but for my NanoPC-T4 
with a cheap eBay FT232R adapter both rise and fall times are certainly 
<100ns. FWIW I've not noticed any issues with letting the Bluetooth 
interface on UART0 run flat-out at 4Mbaud either.

>>>
>>> Please make sure there's not a large amount of flux or something
>>> around the terminals on your board, that seems excessively high.
>>>
>>
>> Thank you for valuable information. For more deeply discussion,
>> I tried other conditions and watch the rise/fall times.
>>
>> 1) Not connect
>> The rise/fall times are 40ns/5ns when nothing connect (impedance is
>> very high) to external pin of RockPro64.
>>
>> What UART device are you using with RockPro64? If you use some device
>> with RockPro64 and board shows rise/fall times = 90ns/45ns, my device
>> is not suitable for RockPro64 by some reason. So it's better to drop
>> my patch.
> 
> The adapter is an FTDI FT232RL breakout board, attached with some
> generic Dupont connector jumpers.
> Interesting your RockPro is showing this symptom, perhaps there is a
> cold solder joint somewhere introducing resistance?
> 
>>
>> 2) Other SoC
>> I have other SoC board rk3328-rock64, Rock64 shows rise/fall times =
>> 90ns/80ns when same UART-USB device is connected to UART pin.
> 
> I measured similar on my Rock64 as well.
> 
>>
>> I think it shows rk3399's (or RockPro64's?) drive strength is a little
>> weak. So it's better to increase the drive strength of UART of rk3399.
> 
> I do not think this is a bad idea generally, it simply allows for more
> available current from the interface.  I'll let others be the judge of
> that, however.

There may be RK3399 users who would care about the possible EMI impact, 
so it's probably still best to limit such a change to boards which 
definitely need it.

Robin.

> 
>>
>> Best Regards,
>> Katsuhiro Suzuki
>>
>>>>
>>>> Best Regards,
>>>> Katsuhiro Suzuki
>>>>
>>>>
>>>>> Thanks
>>>>> Heiko
>>>>>
>>>>>
>>>>>
>>>>>> diff --git a/arch/arm64/boot/dts/rockchip/rk3399.dtsi b/arch/arm64/boot/dts/rockchip/rk3399.dtsi
>>>>>> index beaa92744a64..e3c8f91ead50 100644
>>>>>> --- a/arch/arm64/boot/dts/rockchip/rk3399.dtsi
>>>>>> +++ b/arch/arm64/boot/dts/rockchip/rk3399.dtsi
>>>>>> @@ -2000,6 +2000,11 @@
>>>>>>                        drive-strength = <8>;
>>>>>>                };
>>>>>>
>>>>>> +            pcfg_pull_up_12ma: pcfg-pull-up-12ma {
>>>>>> +                    bias-pull-up;
>>>>>> +                    drive-strength = <12>;
>>>>>> +            };
>>>>>> +
>>>>>>                pcfg_pull_up_18ma: pcfg-pull-up-18ma {
>>>>>>                        bias-pull-up;
>>>>>>                        drive-strength = <18>;
>>>>>> @@ -2521,8 +2526,8 @@
>>>>>>                uart2c {
>>>>>>                        uart2c_xfer: uart2c-xfer {
>>>>>>                                rockchip,pins =
>>>>>> -                                    <4 RK_PC3 RK_FUNC_1 &pcfg_pull_up>,
>>>>>> -                                    <4 RK_PC4 RK_FUNC_1 &pcfg_pull_none>;
>>>>>> +                                    <4 RK_PC3 RK_FUNC_1 &pcfg_pull_up_12ma>,
>>>>>> +                                    <4 RK_PC4 RK_FUNC_1 &pcfg_pull_none_12ma>;
>>>>>>                        };
>>>>>>                };
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>>
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>>>
>>>
>>