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". Laszlo -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#109144): https://edk2.groups.io/g/devel/message/109144 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] -=-=-=-=-=-=-=-=-=-=-=-
