linux-kernel - Re: [PATCH v3 -next 00/11] Extensible console matching & direct earlycon

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Message-ID: <20150326171316.GA5891@kroah.com>
Date:	Thu, 26 Mar 2015 18:13:16 +0100
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Slaby <jslaby@...e.cz>, Rob Herring <robh@...nel.org>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH v3 -next 00/11]  Extensible console matching & direct
 earlycon

On Mon, Mar 09, 2015 at 04:27:11PM -0400, Peter Hurley wrote:
> Changes from v2:
> * remainder of unapplied series
> * Changed the title and commit log for
>   "serial: earlycon: Allow earlycon params with name only" to
>   "serial: earlycon: Skip parse_options() for empty string" per Rob's
>   comment.
> 
> Changes from v1:
> * rebased on and requires "console: Fix console name size mismatch"
> 
> 
> Hi Greg & Andrew,
> 
> This patch series implements:
>   1. console-definable (aka extensible) matching
>   2. generic earlycon-to-console handoff via extensible matching
>   3. arch/prom support for direct earlycon
> 
> 
> Extensible console matching
> 
> Extensible console matching enables the console itself to define the
> conditions for a "command line" match. This mimics the design of
> device matching in the driver model. Two important use-cases which this
> feature enables are generic earlycon-to-console handoff and support
> for driver migration.
> 
> Earlycon-to-console handoff was implemented in 2007. Console command
> lines of the form:
>    console=uart,io,0x3f8,115200n8
> start an earlycon and later allow the 8250 driver console to takeover.
> Unfortunately this implementation requires direct coupling between
> the earlycon and the console and is facilitated by ugly hacks like
> editing the console command line in-place.
> 
> Extensible console matching allows the 8250 driver to directly match
> that console command line instead, and enables other serial drivers
> to trivially support console handoff themselves.
> 
> In addition, extensible console matching allows a new driver to
> provide support for a different driver's console. This requirement
> stems from needing to minimize breakage when migrating serial drivers.
> Since many devices are based on the original 8250/16550 designs
> but sometimes have features incompatible with the existing 8250 driver
> support, the initial driver is often standalone. When/if the standalone
> driver is migrated to the 8250 driver, the problem of console names in
> the command line remains. Extensible console matching enables a simple
> migration path.
> 
> 
> Direct earlycon
> 
> This feature enables arches and proms to start an earlycon directly,
> rather than requiring an "earlycon=" command line parameter.
> Devicetree can already do this via the 'linux,stdout-path' property,
> but arch and prom code requires direct coupling to the serial driver.
> 
> This support is implemented by judicious refactoring and the same
> construct that devicetree and early_param use: a link table containing
> the necessary information (name and setup() function) to find and
> bind the appropriate earlycon "driver".
> 
> 
> NB: I combined these two features in this series because their
> implementations heavily overlap in the same source files.

All now queued up, thanks.

greg k-h
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