linux-kernel - Re: dw_mmc: Does anyone use multiple slots?

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Date:	Fri, 9 Aug 2013 08:51:57 -0700
From:	Doug Anderson <dianders@...gle.com>
To:	Seungwon Jeon <tgih.jun@...sung.com>
Cc:	Chris Ball <cjb@...top.org>, Olof Johansson <olof@...om.net>,
	James Hogan <james.hogan@...tec.com>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	Grant Grundler <grundler@...omium.org>,
	Alim Akhtar <alim.akhtar@...sung.com>,
	Abhilash Kesavan <a.kesavan@...sung.com>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Sonny Rao <sonnyrao@...omium.org>
Subject: Re: dw_mmc: Does anyone use multiple slots?

Hi,

On Thu, Aug 8, 2013 at 8:18 PM, Seungwon Jeon <tgih.jun@...sung.com> wrote:
> On Fri, August 09, 2013, Chris Ball wrote:
>> On Fri, Aug 09 2013, Olof Johansson wrote:
>> > On Thu, Aug 8, 2013 at 5:16 PM, Doug Anderson <dianders@...gle.com> wrote:
>> >
>> >> I guess my overall question is: if there are no actual implementations
>> >> of multislot, shouldn't we kill it and simplify the code a whole lot?
>> >> If someone out there has a real multislot device they can step back in
>> >> and do it more correctly?
>> >>
>> >> Of course we need to find someone to actually go through and do the
>> >> killing of multislot, but finding that person might be easier if there
>> >> was some agreement that it was good to do.
>> >
>> > There clearly seems to be no in-tree users of multislot. If someone
>> > new comes in, we have the code in the history and can revert the
>> > removal (or at least use it as reference for re-introduction).
>> >
>> > I vote for removing it. It adds really annoying complexity for
>> > something that nobody uses.
>>
>> I agree with Olof, for what it's worth.  (The maintainers of the
>> driver are Jaehoon and Seungwon, though.)
>
> I feel like there is no actual use case for that though origin Synopsys IP supports.
> Multi-slot  might be not useful in terms of performance because shared bus should be allowed.
> (At least this is the way I see it, though)
> As Exynos's host does so, other hosts which are introduced in Linux seems use one card per host.
> If it's really not found now, I could agree on this topic.

This all sounds very promising.  Certainly we should wait a little
longer to see if others find / respond to this thread, but otherwise
we can go ahead?

It's possible to do this in somewhat small steps.  I think the first
step is to remove num_slots and remove all loops over num_slots.  That
actually sounds pretty easy/small, though it will touch a lot of code.
 After that we can try to move things out of the separate slot
structure, I think.  That might be a bit of a bigger change.  I can
keep that as a back burner task, but I wouldn't object at all to
someone else doing it!  ;)

The big question, though, is what to do about device tree bindings
(cringe).  Really bus-width, wp-gpios, and disable-wp ought to be
promoted up and we should remove the "slot" subnode.  ...but that of
course breaks the stable API.

-Doug
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