[PATCH] ibex_uart: fix XOR-as-pow

[PATCH] ibex_uart: fix XOR-as-pow

[Paolo Bonzini]

The xor-as-pow warning in clang actually detected a genuine bug.
Fix it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/char/ibex_uart.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/char/ibex_uart.c b/hw/char/ibex_uart.c
index 3e0dd9968e..45cd724998 100644
--- a/hw/char/ibex_uart.c
+++ b/hw/char/ibex_uart.c
@@ -331,7 +331,7 @@ static void ibex_uart_write(void *opaque, hwaddr addr,
         if (value & UART_CTRL_NCO) {
             uint64_t baud = ((value & UART_CTRL_NCO) >> 16);
             baud *= 1000;
-            baud /= 2 ^ 20;
+            baud >>= 20;
 
             s->char_tx_time = (NANOSECONDS_PER_SECOND / baud) * 10;
         }
-- 
2.26.2

Re: [PATCH] ibex_uart: fix XOR-as-pow

[Eric Blake]

On 6/23/20 2:54 PM, Paolo Bonzini wrote:
> The xor-as-pow warning in clang actually detected a genuine bug.
> Fix it.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   hw/char/ibex_uart.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/char/ibex_uart.c b/hw/char/ibex_uart.c
> index 3e0dd9968e..45cd724998 100644
> --- a/hw/char/ibex_uart.c
> +++ b/hw/char/ibex_uart.c
> @@ -331,7 +331,7 @@ static void ibex_uart_write(void *opaque, hwaddr addr,
>           if (value & UART_CTRL_NCO) {
>               uint64_t baud = ((value & UART_CTRL_NCO) >> 16);
>               baud *= 1000;
> -            baud /= 2 ^ 20;
> +            baud >>= 20;

Dividing by 1M instead of 22 seems much more logical, indeed :)
It's odd that we are scaling up by 1000, down by 1024*1024, then

>   
>               s->char_tx_time = (NANOSECONDS_PER_SECOND / baud) * 10;

inverting and scaling by 10.  I don't know if there is a more succinct 
way to write the intended transformation.

>           }

But even without reading the hardware manual to see if the resulting 
scaling is correct, the typo fix is obvious enough to give:
Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Re: [PATCH] ibex_uart: fix XOR-as-pow

[Paolo Bonzini]

On 23/06/20 22:07, Eric Blake wrote:
>>
>>              uint64_t baud = ((value & UART_CTRL_NCO) >> 16);
>>               baud *= 1000;
>> -            baud /= 2 ^ 20;
>> +            baud >>= 20;
> 
> Dividing by 1M instead of 22 seems much more logical, indeed :)

Based on the spec, the "* 1000" is the clock, in other words this is a
fixed point value relative to the clock:

f_baud = NCO * f_clock / 2^20

The example in the spec (https://docs.opentitan.org/hw/ip/uart/doc/) has
f_clock = 50 MHz, while here it's only 1 kHz.  And the register is only
16 bit, so the above would only allow a baud rate up to 62 (65535 * 1000
/ 2^20).

Should the clock be a property of the device instead?

Thanks,

Paolo

> It's odd that we are scaling up by 1000, down by 1024*1024, then
> 
>>                 s->char_tx_time = (NANOSECONDS_PER_SECOND / baud) * 10; 

Re: [PATCH] ibex_uart: fix XOR-as-pow

[Philippe Mathieu-Daudé]

On 6/24/20 8:33 AM, Paolo Bonzini wrote:
> On 23/06/20 22:07, Eric Blake wrote:
>>>
>>>              uint64_t baud = ((value & UART_CTRL_NCO) >> 16);
>>>               baud *= 1000;
>>> -            baud /= 2 ^ 20;
>>> +            baud >>= 20;
>>
>> Dividing by 1M instead of 22 seems much more logical, indeed :)
> 
> Based on the spec, the "* 1000" is the clock, in other words this is a
> fixed point value relative to the clock:
> 
> f_baud = NCO * f_clock / 2^20
> 
> The example in the spec (https://docs.opentitan.org/hw/ip/uart/doc/) has
> f_clock = 50 MHz, while here it's only 1 kHz.  And the register is only
> 16 bit, so the above would only allow a baud rate up to 62 (65535 * 1000
> / 2^20).
> 
> Should the clock be a property of the device instead?

Ideally the device should use qdev_get_clock_in(),
(see docs/devel/clocks.rst):

  static void ibex_uart_init(Object *obj)
  {
     ...
     s->f_clk = qdev_get_clock_in(DEVICE(obj), "f_clock");
     ...

Then in ibex_uart_write():

     uint64_t baud = ((value & UART_CTRL_NCO) >> 16);
     baud *= clock_get_hz(dev->f_clk));
     baud >>= 20;

Devices not using the QDEV_CLOCK API use QDEV properties.

> 
> Thanks,
> 
> Paolo
> 
>> It's odd that we are scaling up by 1000, down by 1024*1024, then
>>
>>>                 s->char_tx_time = (NANOSECONDS_PER_SECOND / baud) * 10; 
> 
> 

Re: [PATCH] ibex_uart: fix XOR-as-pow

[Peter Maydell]

cc'ing Alistair as listed maintainer for the device...

On Tue, 23 Jun 2020 at 21:07, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> The xor-as-pow warning in clang actually detected a genuine bug.
> Fix it.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  hw/char/ibex_uart.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/hw/char/ibex_uart.c b/hw/char/ibex_uart.c
> index 3e0dd9968e..45cd724998 100644
> --- a/hw/char/ibex_uart.c
> +++ b/hw/char/ibex_uart.c
> @@ -331,7 +331,7 @@ static void ibex_uart_write(void *opaque, hwaddr addr,
>          if (value & UART_CTRL_NCO) {
>              uint64_t baud = ((value & UART_CTRL_NCO) >> 16);
>              baud *= 1000;
> -            baud /= 2 ^ 20;
> +            baud >>= 20;
>
>              s->char_tx_time = (NANOSECONDS_PER_SECOND / baud) * 10;
>          }
> --
> 2.26.2

Re: [PATCH] ibex_uart: fix XOR-as-pow

[Alistair Francis]

On Tue, Jun 23, 2020 at 1:07 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> The xor-as-pow warning in clang actually detected a genuine bug.
> Fix it.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  hw/char/ibex_uart.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/hw/char/ibex_uart.c b/hw/char/ibex_uart.c
> index 3e0dd9968e..45cd724998 100644
> --- a/hw/char/ibex_uart.c
> +++ b/hw/char/ibex_uart.c
> @@ -331,7 +331,7 @@ static void ibex_uart_write(void *opaque, hwaddr addr,
>          if (value & UART_CTRL_NCO) {
>              uint64_t baud = ((value & UART_CTRL_NCO) >> 16);
>              baud *= 1000;
> -            baud /= 2 ^ 20;
> +            baud >>= 20;

Whoops, that is clearly wrong. Thanks for catching this.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

Alistair

>
>              s->char_tx_time = (NANOSECONDS_PER_SECOND / baud) * 10;
>          }
> --
> 2.26.2
>
>