Re: [PATCH 3/3] tty: serial: meson: add UART driver compatible with S4 SoC on-chip

[Yu Tu <yu.tu@amlogic.com>]

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Hi Martin and Jerome，
	Thank you very much for your reply. I have learned a lot from your 
communication.

On 2021/12/28 4:04, Martin Blumenstingl wrote:
> [ EXTERNAL EMAIL ]
> 
> Hello,
> 
> On Mon, Dec 27, 2021 at 7:56 AM Yu Tu <yu.tu@amlogic.com> wrote:
> [...]
>>> Does the new S4 SoC use an external 12MHz XTAL or does it use a 24MHz XTAL?
>>> If there's still a 24MHz XTAL then I think this description is not
>>> correct - at least based on how I understand the UART controller.
>>>
>> The S4 SoC uses 12MHz(UART_EE_A_REG5[27]=0x1,the bit is set in romcode).
>> This register description is the same as the G12A and G12B you know.
> Thank you for this explanation!
> So the problem is that we're not touching bit 26 and bit 27 - and with
> the updated romcode you would not get any serial output since the
> divider is calculated off the wrong clock.
> 
> I agree with Jerome that we shouldn't put a flag in device-tree.
> 
> Also I did some experimenting with Jerome's idea to implement the
> clocks using CCF (common clock framework), see the attached patches.
> It was a bit tricky because some initial clean-ups were needed in the
> serial driver.
> Note: I have only briefly tested this on a 32-bit Meson8m2 SoC, see my
> attached patches and the clk_summary debugfs output.
> In fact, I expect that there are some issues with at least one of the
> patches as the whole bit 26 and bit 27 code is untested.
> 
> Do you see any problems with this patch?
> Could you try to implement CCF support with the idea from the attached
> patches (you don't need to re-use them, I just wrote them to make it
> clearer in our discussion what we're talking about).
> 
I couldn't agree with you more. I have verified it on a 64-bit S4 
platform. Please refer to the attachment for verification output 
information.
I will prepare the second version of patch according to the example 
ideas you provided.
> 
> Best regards,
> Martin

[-- Attachment #2: s4-clk-debug-output.txt --]
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# cat /sys/kernel/debug/clk/clk_summary 
                                 enable  prepare  protect                                duty  hardware
   clock                          count    count    count        rate   accuracy phase  cycle    enable
-------------------------------------------------------------------------------------------------------
 xtal                                 7        7        0    24000000          0     0  50000         Y
    fe07a000.serial#xtal_div2         1        1        0    12000000          0     0  50000         Y
       fe07a000.serial#xtal2_clk_sel       1        1        0    12000000          0     0  50000         Y
          fe07a000.serial#use_xtal       1        1        0    12000000          0     0  50000         Y
             fe07a000.serial#baud_div       1        1        0      923077          0     0  50000         Y
    fe07a000.serial#xtal_div3         0        0        0     8000000          0     0  50000         Y
       fe07a000.serial#xtal_clk_sel       0        0        0     8000000          0     0  50000         Y

# 
# stty -F /dev/ttyAML0  115200

# stty -F /dev/ttyAML0 
speed 115200 baud; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>;
eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R;
werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
-brkint ixoff -imaxbel