linux-kernel - Re: [PATCH v1 0/6] x86/boot: Enable earlyprintk on MMIO (8-bit)

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Message-ID: <944ddefdc0ef3dc0f78c7957afa7f12e360a4392.camel@infradead.org>
Date: Mon, 05 May 2025 16:32:38 -0700
From: David Woodhouse <dwmw2@...radead.org>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Ashish Kalra <ashish.kalra@....com>, "Kirill A. Shutemov"
 <kirill.shutemov@...ux.intel.com>, linux-kernel@...r.kernel.org, Thomas
 Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav
 Petkov <bp@...en8.de>,  Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Denis Mukhin
 <dmukhin@...d.com>
Subject: Re: [PATCH v1 0/6] x86/boot: Enable earlyprintk on MMIO (8-bit)

On Mon, 2025-05-05 at 18:01 +0300, Andy Shevchenko wrote:
> On Mon, May 05, 2025 at 05:59:48PM +0300, Andy Shevchenko wrote:
> > On Mon, May 05, 2025 at 07:35:04AM -0700, David Woodhouse wrote:
> > > On Mon, 2025-05-05 at 17:05 +0300, Andy Shevchenko wrote:
> > > > On Fri, May 02, 2025 at 10:33:49AM -0700, David Woodhouse
> > > > wrote:
> > > > > On Fri, 2025-05-02 at 15:29 +0300, Andy Shevchenko wrote:
> > > > > > Some of the platforms may have no legacy COM ports and only
> > > > > > provide
> > > > > > an MMIO accessible UART. Add support for such to
> > > > > > earlyprintk for the
> > > > > > boot phase of the kernel.
> > > > > 
> > > > > Shiny. I had to hack QEMU's PCI serial port to do unnatural
> > > > > things, in
> > > > > order to test the mmio32 variant which was the only thing the
> > > > > earlyprintk code used to support. But I *did* so, and it
> > > > > works with the
> > > > > kexec debugging.
> > > > > 
> > > > > Can you add support for this mode to the kexec debugging too,
> > > > > please?
> > > > 
> > > > Do you mean to add MMIO 8-bit to kexec assembly code and other
> > > > parts like you
> > > > did in the below mentioned change?
> > > > 
> > > > I can try it at some point, but have no time right now for
> > > > this.
> > > > I would appreciate if you can give a try for this patch series
> > > > functionality to see if it helps for the initial messages (as
> > > > far as I understand you also want to have this in the second
> > > > kernel, right?).
> > > 
> > > I'll see if I can find the time to take a look. Got a branch I
> > > can pull
> > > your series from?
> > 
> > Yeo,
> > https://bitbucket.org/andy-shev/linux/commits/branch/topic%2Fx86%2Fboot-earlyprintk
> > .
> 
> But just FYI that with `b4` it's as easy to pull from the mailing
> list as from
> a branch:
> 
> 	b4 am
> 20250502123145.4066635-1-andriy.shevchenko@...ux.intel.com
> 	git am -C1 -s ...


Thanks. Should I be expecting this to work...?

qemu-system-x86_64 -display none -vga none  -accel kvm,kernel-irqchip=split \
  -kernel arch/x86/boot/bzImage \
  -append "console=ttyS4 root=/dev/vda1 earlyprintk=pciserial" \
  -chardev stdio,mux=on,id=char0,signal=off -mon char0 -device pci-serial,chardev=char0


I get no early output (I think it's still using the mmio32 variant),
and I only see output from when the real driver takes over:

[    0.894054] printk: legacy console [ttyS4] enabled
[    0.895498] printk: legacy bootconsole [earlyser0] disabled
...



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