linux-kernel - Re: [PATCH 0/3] Deterministic UART numbering on Samsung SoCs

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Date:	Tue, 8 Jul 2014 09:32:02 +0100
From:	Daniel Drake <drake@...lessm.com>
To:	Tomasz Figa <t.figa@...sung.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org, devicetree@...r.kernel.org,
	Kukjin Kim <kgene.kim@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Rob Herring <robh+dt@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Tomasz Figa <tomasz.figa@...il.com>
Subject: Re: [PATCH 0/3] Deterministic UART numbering on Samsung SoCs

Hi,

How can we move forward here?

On Thu, Jun 26, 2014 at 1:02 PM, Tomasz Figa <t.figa@...sung.com> wrote:
> - basically Samsung UART already has its own namespace (ttySAC) and the
> order inside it is well-defined - instance ID shall be the hardware
> instance number as specified by documentation. The ports vary in certain
> aspects and the ID is important knowledge of the driver. The problem
> here was broken implementation of assigning IDs based on probe order,
> which worked only because on all Exynos platforms all ports have been
> always registered (which we want to change now and keep unused ones
> "disabled" in DT),

Yes, the kernel help text documents this quite well:

config SERIAL_SAMSUNG
    tristate "Samsung SoC serial support"
    depends on PLAT_SAMSUNG
    select SERIAL_CORE
    help
      Support for the on-chip UARTs on the Samsung S3C24XX series CPUs,
      providing /dev/ttySAC0, 1 and 2 (note, some machines may not
      provide all of these ports, depending on how the serial port
      pins are configured.

> - we already have a lot of userspace depending on the aforementioned
> ttySAC namespace and proper ordering of instances there. While I believe
> the proper solution as of today would be to go back to standard ttyS
> namespace and make userspace use a smarter way of identifying the
> instances (e.g. by path or id, as you suggested), I don't think this
> will make anyone's life easier with current assumptions,

I like the sound of going to the standard ttyS notation and only
providing ports for ones that exist, but is this userspace-visible
naming change OK? You could argue that userspace that relies on fixed
device paths is a bit broken, but that argument would be a bit cloudy
given the kernel documentation for the ttySAC devices above.

> - correct me if I'm wrong, but I don't think the
> /dev/serial/by-{path,id} would be handled in kernel's console= parameter.

That's right, that problem is left to the user, but at least we'd be
consistent with other SoCs (and open to generic solutions to that
inconvenience).

Daniel
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