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

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Message-ID: <aBtl_zoWwuVC6Gyi@smile.fi.intel.com>
Date: Wed, 7 May 2025 16:54:07 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: David Woodhouse <dwmw2@...radead.org>
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 Tue, May 06, 2025 at 12:38:21PM -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.
> 
> Aha, I understand now... you've added this *only* for the boot code,
> and haven't added the corresponding support to the in-kernel
> earlyprintk, in arch/x86/kernel/early_printk.c

Haven't added yet. This is part 1 only.

> The latter does already support MMIO but it only supports a 32-bit
> stride, not 8-bit.
> 
> Please could you make that consistent, and ensure the earlyprintk=
> arguments function the same for both phases? I'm happy to add the
> kexec-debug parts on top of *that*.

It's not as easy as it looks like. I had done in the past the comparison table
of what we have between all early*=... for serial consoles and it all so
inconsistent to begin with. A big work needs to be fulfilled in order to put
this mess in order. Hence this is just the first step.

> It would be really helpful if we could test this in QEMU; it shouldn't
> be that hard to make it provide a 16550 on MMIO, along the lines of the
> one-line hack I posted yesterday.

I understand. But if no go for this series, I don't won't to spend time on
the next part.

In any case, thanks for looking into this.

-- 
With Best Regards,
Andy Shevchenko