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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH] xen/arm: debug-pl011.inc: Use macros instead of hardcoded values
On 17/11/2022 12:52, Michal Orzel wrote: On 17/11/2022 10:53, Michal Orzel wrote:Hi Julien, On 17/11/2022 10:29, Julien Grall wrote:On 17/11/2022 08:34, Michal Orzel wrote:Hi Julien, On 16/11/2022 19:37, Julien Grall wrote:Hi Michal, On 16/11/2022 18:05, Michal Orzel wrote:On 16/11/2022 16:56, Julien Grall wrote:On 16/11/2022 14:45, Michal Orzel wrote:Hi Julien,Hi Michal,and use it in the pl011-debug files (+ there is a question whether we should define WLEN_7-5 for completeness).I would not define WLEN_7-5. That said, I wonder if we really need to set the baud rate & co here? AFAICT the runtime driver never touch them. The reasoning is the firmware is responsible to configure the serial. Therefore, I would consider to drop the code (setting UARTCR might still be necessary).I do not really agree because the current behavior was done on purpose.EARLY_UART_PL011_BAUD_RATE is only used for very early debugging (this is protected by CONFIG_DEBUG and CONFIG_EXPERT). This is not a production ready code.I am fully aware of it. I just found it useful but I understand the global reasoning.At the moment early_uart_init is called only if EARLY_UART_PL011_BAUD_RATE is set to a value != 0. This is done in order to have flexibility to either stick to what firmware/bootloader configured or to change this configuration by specifying the EARLY_UART_PL011_BAUD_RATE (useful when you do not know what the firmware configured).The chances are that you want to use the baud rate that was configured by the firmware. Otherwise, you would need to change the configuration of minicom (or whatever you used) to get proper output for the firmware and then Xen. Furthermore, as I wrote before, the runtime driver doesn't configure the baud rate. This was removed in Xen 4.7 (see commit 2048e17ca9df "drivers/pl011: Don't configure baudrate") because it was buggy and this code is not simple. So it makes no sense to configure the baud rate when using early printk but not the runtime driver.Ok, so we will get rid of EARLY_UART_PL011_BAUD_RATE config and setting the bd in the early uart code. Now, what about setting "8n1"? The runtime driver sets them as well as the early code. It can also be set to a different value from the firmware (unlikely but it can happen I think). In any case, if we decide to do what the runtime driver does, I reckon setting LCR_H should be kept in early code.Good question. I think, you would end up with the same issue I mentioned above if the firmware and Xen have different line control registers (tools like minicom/screen would ask for it). So I am on the fence here. In one way, it seems pointless keep it. But on the other hand, Xen has always set it. So I have no data to prove this will be fine everywhere.If we are relying on the firmware[1] to configure the baud rate, it is not very wise not to rely on it to configure the LCR. Looking at the other serial drivers in Xen, we have a real mismatch in what is being configured. Some of the drivers (omap, imx), apart from setting 8n1 also set the baud rate explicitly to 115200 and almost all of them do set 8n1. In that case we will not benefit too much from fixing just pl011.It is not great that Xen hardcode the baud rate (I can't remember whether there was a reason), but I don't think the consistency is necessary here (see more below).On the other hand, Xen follows the zImage/Image protocols for ARM [2] which do not state that serial port initializing is something mandatory. This could indicate that the firmware does not really need to configure the serial.The firmware doesn't need to configure the serial and yes in theory Xen should configure the baud rate and parity based on the firmware table. However, this is a trade off between complexity and benefits. The patch I mentioned earlier has been removed nearly 6 years ago and I haven't seen anyone reporting any issues. Hence why I think for the PL011 it is not worth looking [3] at the baud rate and instead removing it completely in the early PL011 code as well. That said, if you feel strongly adding support for baud rate then I will be happy to review the patch.I'm not in favor of this approach either. That said, I will prepare patches to remove CONFIG_EARLY_UART_PL011_BAUD_RATE and its usage in early printk code as we agreed earlier. As for the LCR setting, I will keep it in early printk code to maintain the same behavior as runtime driver who sets them.Actually, there is one more thing to consider. early_uart_init, even though it also sets LCR apart from the baud rate, is called when CONFIG_EARLY_UART_INIT is set. The latter depends on EARLY_UART_PL011 && EARLY_UART_PL011_BAUD_RATE != 0. If we remove EARLY_UART_PL011_BAUD_RATE, we need to decide when do we want early_uart_init to be called. It is defined only for pl011 (it is also defined for meson but this is an unreachable code, as EARLY_UART_PL011 is 0 for meson), so we have the following options: Good spot. I am not sure why the function was defined for Meson if it does nothing. I would drop it but keep the comment explaining why we don't have the helper. 2. Keep CONFIG_EARLY_UART_INIT so that future drivers can use it (?) and mark it as n by default Based on the discussion with Bertrand, I would keep CONFIG_EARLY_UART_INIT in case someone wants to use a different UART for Xen and the firmware output. Also, I would like to revise my opinion from earlier. I now think we should keep the baud rate part in early PL011 code even this means inconsistency between the early and runtime driver. Cheers, -- Julien Grall
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