On 9/28/23 13:50, Laszlo Ersek wrote: > On 9/28/23 13:24, Peter Maydell wrote: >> On Thu, 28 Sept 2023 at 08:54, Laszlo Ersek <ler...@redhat.com> wrote: >>> >>> On 9/21/23 14:02, ardb at kernel.org (Ard Biesheuvel) wrote: >>>> EDK2's DEBUG output is extremely noisy, so being able to redirect this >>>> output to a different UART would be very useful. >>>> >>>> The stdout-path is the intended console, and so we should honour that. >>>> This also means that we should parse aliases. But the console is >>>> actually configurable [persistenly] via the UEFI menu, and so it would >>>> be nice if we could take advantage of this flexibility. This means in >>>> principle that the UARTs should be represented via different device >>>> paths (which would include the base address so they are >>>> distinguishable) with perhaps a magical alias which is the default and >>>> is tied to whatever stdout-path points to. This way, all the logic we >>>> introduce is spec compliant and reusable on physical platforms with >>>> multiple UARTs. >> >>>> What we might do is use stdout-path as well, unless a certain DT alias >>>> exist perhaps? We should probably align here with other projects, >>>> although this a distinction of the same nature may not exist there. >>>> >>> >>> Alias parsing in edk2 would be a bit too complicated for my taste. :) >>> >>> I see the following two problems with the current state (based on >>> Peter's captures, using the original UART order in the DTB, i.e., >>> <https://people.linaro.org/~peter.maydell/uart0.txt> and >>> <https://people.linaro.org/~peter.maydell/uart1.txt>): >>> >>> (1) The DEBUG output switches from one UART to the other when we reach >>> the DXE_CORE (in this case, from UART0 to UART1, but the precise numbers >>> aren't the problem, the switchover is), >>> >>> (2) The UEFI console (which is used by the setup browser, the UEFI >>> shell, grub, etc) is on UART1, while the kernel stuff is on UART0. >>> >>> Here's what I'd propose: >>> >>> - if there is only one UART in the DTB, no change >>> >>> - otherwise, direct all DEBUG messages to the UART found *second* via >>> forward traversal in the DTB (let's call this UART1), and include the >>> UART found *first* via forward traversal in the DTB (let's call this >>> UART0) in the UEFI console. Furthermore, do not expose UART1 in the UEFI >>> protocol database *at all* (don't install devpath protocol / SerialIo >>> protocol); make it effectively hidden hardware (similarly how the x86 >>> QEMU debug console, IO Port 0x402, is not exposed at all). Let the >>> system think there is only one UART (UART0), and treat UART1 as a >>> "bespoke", custom debug device only. This also ensures that existent >>> higher level products such as libvirt, which may only handle UART0 at >>> the moment, will expose the interactive console (UEFI and Linux) to the >>> user, and at worst the firmware debug log will not be captured. >> >> The 16550 version of the QEMU-specific EDK uart-location code (used when >> running it under kvmtool) already honours stdout-path, > > Right -- I wasn't aware. I can see GetSerialConsolePortAddress() in > "ArmVirtPkg/Library/Fdt16550SerialPortHookLib/EarlyFdt16550SerialPortHookLib.c", > which is used in "ArmVirtPkg/ArmVirtKvmTool.dsc". > >> so I'm not sure >> why we wouldn't want to be consistent with that. I'm not really a fan of >> anything that depends on ordering of nodes in the DTB -- it is pretty >> fragile in my experience. The DTB spec provides a mechanism to >> correctly identify which UART to use, so I think that there would >> need to be a really strong reason not to do it that way. > > OK -- I think the proposal might still work if we replaced "UART found > second" with "first non-stdout-path UART", and "UART found first" with > "stdout-path UART".
I'm working on this. Laszlo -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#109239): https://edk2.groups.io/g/devel/message/109239 Mute This Topic: https://groups.io/mt/101498371/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/leave/9847357/21656/1706620634/xyzzy [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
