linux-kernel - Re: [PATCH v2 0/4] Patches to allow consistent mmc / mmcblk numbering w/ device tree

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Message-ID: <CAD=FV=U5jnJJ3v=nFGFecoKB5KGv1ufC+aKbHx2iCqBTWpDTww@mail.gmail.com>
Date:	Fri, 29 Apr 2016 16:01:58 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Ulf Hansson <ulf.hansson@...aro.org>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	Shawn Lin <shawn.lin@...k-chips.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Stefan Agner <stefan@...er.ch>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	Brian Norris <computersforpeace@...il.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Heiko Stuebner <heiko@...ech.de>,
	Jisheng Zhang <jszhang@...vell.com>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	devicetree-spec@...r.kernel.org,
	Mark Rutland <mark.rutland@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Venu Byravarasu <vbyravarasu@...dia.com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Jon Hunter <jonathanh@...dia.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Grant Grundler <grundler@...omium.org>,
	Kumar Gala <galak@...eaurora.org>,
	"Luca Porzio (lporzio)" <lporzio@...ron.com>,
	Rob Herring <robh+dt@...nel.org>,
	Chaotian Jing <chaotian.jing@...iatek.com>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	Sudeep Holla <sudeep.holla@....com>,
	zhonghui.fu@...ux.intel.com, kirill.shutemov@...ux.intel.com
Subject: Re: [PATCH v2 0/4] Patches to allow consistent mmc / mmcblk numbering
 w/ device tree

Hi,

On Fri, Apr 29, 2016 at 3:44 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> My reply would be... why should MMC have special handling that no
> other subsystem has?

No other subsystem?

* i2c allows numbering devices by alias
* rtc allows numbering devices by alias.
* serial allows numbering devices by alias.
* spi allows numbering devices by alias.
* watchdog allows numbering devices by alias.

...at least that's my impression doing a grep for of_alias_get_id(),
which I suggested earlier in this thread but apparently wasn't done.

> Here's another example.  Plug in several USB serial adapters.  Which
> USB serial /dev/ttyUSB* device corresponds to which adapter?  The
> answer is... it depends on the order you plug them in, which could
> well be different from the order in which they are found if the
> machine reboots.  That's a very real problem.

USB is, by definition, hotplug and probable and there is no ordering.
For peripherals built-in to a SoC there is a sane ordering.  Thus
hotplug peripherals and builtin peripherals shouldn't have the same
requirements.

Quite honestly, it _would_ be quite convenient that if you are on a
SoC and you know it has builtin USB controllers to have the root hubs
numbered in a sane and consistent manner.  An area for a future patch,
maybe.

> However, the answer to this problem is not to find some way to bind
> a particular /dev/ttyUSB* node to a particular USB device, but to
> use the solution(s) already provided - iow, /dev/serial/by-id or
> /dev/serial/by-path trees which give a stable view of these devices.
>
> Now, while that allows the appropriate /dev/ttyUSB* device to be
> found, it doesn't solve the "which ttyUSB* entry in the kernel
> message log corresponds with which adapter" (which is the basis
> of your point #1.)  That's not solved, and isn't purposely isn't
> solved for any kernel subsystem.
>
> Another example - I have boards here with multiple RTCs... guess
> what happens with those?  Yep, same problem.  They get dynamic
> /dev/rtc* assignment.

Are you quite certain about that?  See above.

> Another example - I have a board with three ethernet devices...
> yep, same problem again, the order depends on the driver probe
> order, and they get dynamically assigned eth* names.

Ethernet is often provided by USB and thus hotplug and probable.
Quite honestly if there was a builtin Ethernet adapter provided on a
SoC (not connected over USB), it would be super handy if it was forced
to be "eth0" (and if there were more than one if they could be ordered
in a way that made sense for that SoC).  Dynamic ordering could come
after.

> The list of subsystems that have this property is large, because
> it's all dependent on the device/driver probing order.

Sure.  For hotplug, there is no sane device ordering.

For things built in to an SoC, there is sane device ordering.  Dynamic
ordering should come after static.

> So, please answer this question: why should _only_ MMC be treated
> as a special case?  Please give a _technical_ reason rather than a
> _personal_ reason.

Because technically it makes it easier for people to understand their
system to have a sane ordering for builtin peripherals.

-Doug