Hi Bertrand, On 01/06/2023 12:19, Bertrand Marquis wrote: > > > Hi Michal, > >> On 1 Jun 2023, at 10:50, Michal Orzel <michal.or...@amd.com> 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.or...@amd.com> >> --- >> 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
