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, 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. thanks -- PMM -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#109146): https://edk2.groups.io/g/devel/message/109146 Mute This Topic: https://groups.io/mt/101498371/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
