From: Peter Delevoryas <pdel@fb.com>

UART5 is typically used as the default debug UART on the AST2600, but
UART1 is also designed to be a debug UART. All the AST2600 UART's have
semi-configurable clock rates through registers in the System Control
Unit (SCU), but only UART5 works out of the box with zero-initialized
values. The rest of the UART's expect a few of the registers to be
initialized to non-zero values, or else the clock rate calculation will
yield zero or undefined (due to a divide-by-zero).

For reference, the U-Boot clock rate driver here shows the calculation:

    https://github.com/facebook/openbmc-uboot/blob/main/drivers/clk/aspeed/clk_ast2600.c#L357)

To summarize, UART5 allows selection from 4 rates: 24 MHz, 192 MHz, 24 /
13 MHz, and 192 / 13 MHz. The other UART's allow selecting either the
"low" rate (UARTCLK) or the "high" rate (HUARTCLK). UARTCLK and HUARTCLK
are configurable themselves:

    UARTCLK = UXCLK * R / (N * 2)
    HUARTCLK = HUXCLK * HR / (HN * 2)

UXCLK and HUXCLK are also configurable, and depend on the APLL and/or
HPLL clock rates, which also derive from complicated calculations. Long
story short, there's lots of multiplication and division from
configurable registers, and most of these registers are zero-initialized
in QEMU, which at best is unexpected and at worst causes this clock rate
driver to hang from divide-by-zero's. This can also be difficult to
diagnose, because it may cause U-Boot to hang before serial console
initialization completes, requiring intervention from gdb.

This change just initializes all of these registers with default values
from the datasheet.

Signed-off-by: Peter Delevoryas <pdel@fb.com>
---
 hw/misc/aspeed_scu.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/hw/misc/aspeed_scu.c b/hw/misc/aspeed_scu.c
index c373e678f0..d51fe8564d 100644
--- a/hw/misc/aspeed_scu.c
+++ b/hw/misc/aspeed_scu.c
@@ -104,11 +104,16 @@
 #define AST2600_SDRAM_HANDSHAKE   TO_REG(0x100)
 #define AST2600_HPLL_PARAM        TO_REG(0x200)
 #define AST2600_HPLL_EXT          TO_REG(0x204)
+#define AST2600_APLL_PARAM        TO_REG(0x210)
 #define AST2600_MPLL_EXT          TO_REG(0x224)
 #define AST2600_EPLL_EXT          TO_REG(0x244)
 #define AST2600_CLK_SEL           TO_REG(0x300)
 #define AST2600_CLK_SEL2          TO_REG(0x304)
 #define AST2600_CLK_SEL3          TO_REG(0x308)
+#define AST2600_CLK_SEL4          TO_REG(0x310)
+#define AST2600_CLK_SEL5          TO_REG(0x314)
+#define AST2600_UARTCLK_PARAM     TO_REG(0x338)
+#define AST2600_HUARTCLK_PARAM    TO_REG(0x33C)
 #define AST2600_HW_STRAP1         TO_REG(0x500)
 #define AST2600_HW_STRAP1_CLR     TO_REG(0x504)
 #define AST2600_HW_STRAP1_PROT    TO_REG(0x508)
@@ -658,9 +663,15 @@ static const uint32_t ast2600_a1_resets[ASPEED_AST2600_SCU_NR_REGS] = {
     [AST2600_CLK_STOP_CTRL2]    = 0xFFF0FFF0,
     [AST2600_SDRAM_HANDSHAKE]   = 0x00000000,
     [AST2600_HPLL_PARAM]        = 0x1000405F,
+    [AST2600_APLL_PARAM]        = 0x1000405F,
     [AST2600_CHIP_ID0]          = 0x1234ABCD,
     [AST2600_CHIP_ID1]          = 0x88884444,
-
+    [AST2600_CLK_SEL2]          = 0x00700000,
+    [AST2600_CLK_SEL3]          = 0x00000000,
+    [AST2600_CLK_SEL4]          = 0xF3F40000,
+    [AST2600_CLK_SEL5]          = 0x30000000,
+    [AST2600_UARTCLK_PARAM]     = 0x00014506,
+    [AST2600_HUARTCLK_PARAM]    = 0x000145C0,
 };
 
 static void aspeed_ast2600_scu_reset(DeviceState *dev)
-- 
2.30.2

On 8/27/21 11:04 PM, pdel@fb.com wrote:
> From: Peter Delevoryas <pdel@fb.com>
> 
> UART5 is typically used as the default debug UART on the AST2600, but
> UART1 is also designed to be a debug UART. All the AST2600 UART's have
> semi-configurable clock rates through registers in the System Control
> Unit (SCU), but only UART5 works out of the box with zero-initialized
> values. The rest of the UART's expect a few of the registers to be
> initialized to non-zero values, or else the clock rate calculation will
> yield zero or undefined (due to a divide-by-zero).
>
> For reference, the U-Boot clock rate driver here shows the calculation:
> 
>     https://github.com/facebook/openbmc-uboot/blob/main/drivers/clk/aspeed/clk_ast2600.c#L357)
> 
> To summarize, UART5 allows selection from 4 rates: 24 MHz, 192 MHz, 24 /
> 13 MHz, and 192 / 13 MHz. The other UART's allow selecting either the
> "low" rate (UARTCLK) or the "high" rate (HUARTCLK). UARTCLK and HUARTCLK
> are configurable themselves:
> 
>     UARTCLK = UXCLK * R / (N * 2)
>     HUARTCLK = HUXCLK * HR / (HN * 2)
> 
> UXCLK and HUXCLK are also configurable, and depend on the APLL and/or
> HPLL clock rates, which also derive from complicated calculations. Long
> story short, there's lots of multiplication and division from
> configurable registers, and most of these registers are zero-initialized
> in QEMU, which at best is unexpected and at worst causes this clock rate
> driver to hang from divide-by-zero's. This can also be difficult to
> diagnose, because it may cause U-Boot to hang before serial console
> initialization completes, requiring intervention from gdb.
> 
> This change just initializes all of these registers with default values
> from the datasheet.
> 
> Signed-off-by: Peter Delevoryas <pdel@fb.com>
> ---
>  hw/misc/aspeed_scu.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/misc/aspeed_scu.c b/hw/misc/aspeed_scu.c
> index c373e678f0..d51fe8564d 100644
> --- a/hw/misc/aspeed_scu.c
> +++ b/hw/misc/aspeed_scu.c
> @@ -104,11 +104,16 @@
>  #define AST2600_SDRAM_HANDSHAKE   TO_REG(0x100)
>  #define AST2600_HPLL_PARAM        TO_REG(0x200)
>  #define AST2600_HPLL_EXT          TO_REG(0x204)
> +#define AST2600_APLL_PARAM        TO_REG(0x210)
>  #define AST2600_MPLL_EXT          TO_REG(0x224)
>  #define AST2600_EPLL_EXT          TO_REG(0x244)
>  #define AST2600_CLK_SEL           TO_REG(0x300)
>  #define AST2600_CLK_SEL2          TO_REG(0x304)
>  #define AST2600_CLK_SEL3          TO_REG(0x308)
> +#define AST2600_CLK_SEL4          TO_REG(0x310)
> +#define AST2600_CLK_SEL5          TO_REG(0x314)
> +#define AST2600_UARTCLK_PARAM     TO_REG(0x338)
> +#define AST2600_HUARTCLK_PARAM    TO_REG(0x33C)
>  #define AST2600_HW_STRAP1         TO_REG(0x500)
>  #define AST2600_HW_STRAP1_CLR     TO_REG(0x504)
>  #define AST2600_HW_STRAP1_PROT    TO_REG(0x508)
> @@ -658,9 +663,15 @@ static const uint32_t ast2600_a1_resets[ASPEED_AST2600_SCU_NR_REGS] = {
>      [AST2600_CLK_STOP_CTRL2]    = 0xFFF0FFF0,
>      [AST2600_SDRAM_HANDSHAKE]   = 0x00000000,
>      [AST2600_HPLL_PARAM]        = 0x1000405F,
> +    [AST2600_APLL_PARAM]        = 0x1000405F,
>      [AST2600_CHIP_ID0]          = 0x1234ABCD,
>      [AST2600_CHIP_ID1]          = 0x88884444,
> -
> +    [AST2600_CLK_SEL2]          = 0x00700000,
> +    [AST2600_CLK_SEL3]          = 0x00000000,
> +    [AST2600_CLK_SEL4]          = 0xF3F40000,
> +    [AST2600_CLK_SEL5]          = 0x30000000,
> +    [AST2600_UARTCLK_PARAM]     = 0x00014506,
> +    [AST2600_HUARTCLK_PARAM]    = 0x000145C0,
>  };
>  
>  static void aspeed_ast2600_scu_reset(DeviceState *dev)
> 

Some parts have been already covered by the A3 emulation changes 
provided by Joel. I will merge the UART registers only.

Reviewed-by: Cédric Le Goater <clg@kaod.org>

Thanks,

C.