linux-kernel - Re: [PATCH v2 7/7] x86/boot: Support nocfg parameter for earlyprintk

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Message-ID: <20190328132153.GZ9224@smile.fi.intel.com>
Date:   Thu, 28 Mar 2019 15:21:53 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 7/7] x86/boot: Support nocfg parameter for earlyprintk

On Thu, Mar 28, 2019 at 02:03:17PM +0100, Borislav Petkov wrote:
> On Tue, Mar 19, 2019 at 09:43:25PM +0300, Andy Shevchenko wrote:
> > If by BIOS or by other means the serial port is configured
> > user might want to skip reconfiguration in the boot code.
> 
> That needs more explanation: how can that happen? 

I didn't get what is 'that' here? You mean that serial interface becomes
configured before kernel even starts? See below.

> How is the user
> supposed to know whether the serial port is configured in order to use
> this cmdline param?

If the connection to the target is done via serial interface and
firmware and / or bootloader already did necessary configuration.

> Why can't the kernel detect whether the serial port is configured and
> not touch that configuration?

Because it can't at this stage. We simple can't afford to move all serial
drivers (even partially) to the _boot_ part of the kernel.

> A lot of why's and head scratching...

What other why's should be answered?

-- 
With Best Regards,
Andy Shevchenko