Re: [PATCH] xen/arm: debug-pl011: Use 32-bit accessors for broader compatibility

[Michal Orzel <michal.orzel@xxxxxxx>]

Hi Bertrand,

On 01/06/2023 12:19, Bertrand Marquis wrote:
> 
> 
> Hi Michal,
> 
>> On 1 Jun 2023, at 10:50, Michal Orzel <michal.orzel@xxxxxxx> wrote:
>>
>> There are implementations of the PL011 that can only handle 32-bit
>> accesses (i.e. no 16-bit or 8-bit), usually advertised by 'reg-io-width'
>> dt property set to 4. On such UARTs, the current early printk code for
>> arm64 does not work. To fix this issue, make all the accesses to be 32-bit
>> by using ldr, str without a size field. This makes it possible to use
>> early printk on such platforms, while all the other implementations should
>> generally cope with 32-bit accesses. In case they do not, they would
>> already fail as we explicitly use writel/readl in the runtime driver to
>> maintain broader compatibility and to be SBSAv2 compliant. Therefore, this
>> change makes the runtime/early handling consistent (also it matches the
>> arm32 debug-pl011 code).
>>
>> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
>> ---
>> xen/arch/arm/arm64/debug-pl011.inc | 8 ++++----
>> 1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/xen/arch/arm/arm64/debug-pl011.inc 
>> b/xen/arch/arm/arm64/debug-pl011.inc
>> index 6d60e78c8ba3..80eb8fdc1ec7 100644
>> --- a/xen/arch/arm/arm64/debug-pl011.inc
>> +++ b/xen/arch/arm/arm64/debug-pl011.inc
>> @@ -25,9 +25,9 @@
>>  */
>> .macro early_uart_init xb, c
>>         mov   x\c, #(7372800 / CONFIG_EARLY_UART_PL011_BAUD_RATE % 16)
>> -        strh  w\c, [\xb, #FBRD]      /* -> UARTFBRD (Baud divisor fraction) 
>> */
>> +        str   w\c, [\xb, #FBRD]      /* -> UARTFBRD (Baud divisor fraction) 
>> */
>>         mov   x\c, #(7372800 / CONFIG_EARLY_UART_PL011_BAUD_RATE / 16)
>> -        strh  w\c, [\xb, #IBRD]      /* -> UARTIBRD (Baud divisor integer) 
>> */
>> +        str   w\c, [\xb, #IBRD]      /* -> UARTIBRD (Baud divisor integer) 
>> */
>>         mov   x\c, #WLEN_8           /* 8n1 */
>>         str   w\c, [\xb, #LCR_H]     /* -> UARTLCR_H (Line control) */
>>         ldr   x\c, =(RXE | TXE | UARTEN)
>> @@ -41,7 +41,7 @@
>>  */
>> .macro early_uart_ready xb, c
>> 1:
>> -        ldrh  w\c, [\xb, #FR]        /* <- UARTFR (Flag register) */
>> +        ldr   w\c, [\xb, #FR]        /* <- UARTFR (Flag register) */
>>         tst   w\c, #BUSY             /* Check BUSY bit */
>>         b.ne  1b                     /* Wait for the UART to be ready */
>> .endm
>> @@ -52,7 +52,7 @@
>>  * wt: register which contains the character to transmit
>>  */
>> .macro early_uart_transmit xb, wt
>> -        strb  \wt, [\xb, #DR]        /* -> UARTDR (Data Register) */
>> +        str   \wt, [\xb, #DR]        /* -> UARTDR (Data Register) */
> 
> Is it really ok to drop the 8bit access here ?
It is not only ok, it is necessary. Otherwise it won't work on the above 
mentioned UARTs (they can only perform 32-bit access).
And following to what I wrote in commit msg:
- we use str already in arm32 which results in 32-bit access
- we use reald/writel that end up as str/ldr in runtime driver
- we are down to SBSAv2 spec that runtime driver follows (meaning we can use 
early printk for SBSA too)
- this way we support broader list of PL011s consistently (i.e. both early and 
runtime driver works as oppose to only runtime)

Also, before every early_uart_transmit we use ldrb (to convert to char) which 
means that the rest of the \wt register (8:31) is zero extended.

~Michal