ACPI [SPCR] (Serial Port Console Redirection Table) specifies which console should be used by system. 'ARM Server Base Boot Requirements' [SBBR] mentions SPCR as a mandatory ACPI table. Support for this table for Linux kernel ([SPCR v10]) was merged for 4.9 (patches 1/4-3/4) and 4.10 (patch 4/4). It turns out that this approach does not work for all the existing hardware. There are two problems with AppliedMicro X-Gene based boards ([discussion], [v1]): 1. Their console is a 16550 port that requires 32-bit access. Now SPCR does not have any method to specify this. 2. Some of the boards don't use the "standard" 16550 clock rate so supplying a baud rate makes it change to a random baud rate. Patch 1/4 uses 'Register Bit Width' field of the ACPI Generic Address Structure that specifies the address of the UART registers to decide if the driver should use "mmio32" access instead of "mmio". This fixes problem 1 for existing hardware/firmware. To fix problem 2, I suggest to introduce a new value '0' for the "Baud Rate" field of SPCR (now this value is reserved). I would like to discuss if this could be added to SPCR spec and will fix the problem. Patch 2/4 introduces a check for this value. In this case the code does not emit baud rate specification for initialization of the console. This fixes problem 2. It was also suggested ([v1]) to add a new Microsoft Debug Port Table 2 ([DBG2]) (the table used to enumerate the various subtypes of serial port covered by the SPCR) 16550 UART subtype that may be needed for some additional platforms, such as those based upon AppliedMicro X-Gene ARMv8 SoCs. This new subtype would be 16550-compatible with 32-bit access. There already exists 32-bit variant ACPI_DBG2_ARM_SBSA_32BIT of SBSA console ACPI_DBG2_ARM_SBSA_GENERIC. Patch 3/4 introduces this value for DBG2 table. Patch 4/4 uses it. RFC v2: - Add a fix for consoles with non-standard baud rate. - Introduce a new type of console that is 16550 with 32-bit access - Check for that type of console. v1: [v1] [SPCR] https://msdn.microsoft.com/en-us/library/windows/hardware/dn639132(v=vs.85).aspx [DBG2] http://go.microsoft.com/fwlink/p/?LinkId=234837 [SBBR] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0044a/index.html [SPCR v10] https://lkml.kernel.org/r/20160905123617.18775-1-aleksey.makarov@xxxxxxxxxx [discussion] https://lkml.kernel.org/r/7fa523de-3fbb-1566-f521-927143f73d1e@xxxxxxxxxx [v1] https://lkml.kernel.org/r/20161205130534.11080-1-aleksey.makarov@xxxxxxxxxx Aleksey Makarov (4): ACPI: SPCR: check bit width for the 16550 UART ACPI: SPCR: don't initialize baud rate ACPI: DBG2: add 16550 UART with 32-bit access ACPI: SPCR: support 16550 UART with 32-bit access drivers/acpi/spcr.c | 29 ++++++++++++++++++++++------- include/acpi/actbl2.h | 1 + 2 files changed, 23 insertions(+), 7 deletions(-) -- 2.10.2 -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
