[GIT PULL 1/2] SOCFPGA updates for 3.18

[Ulf Hansson]

On 10 September 2014 10:33, Pavel Machek <pavel at denx.de> wrote:
> On Wed 2014-09-10 09:13:27, Ulf Hansson wrote:
>> On 9 September 2014 23:02, Pavel Machek <pavel at denx.de> wrote:
>> > On Tue 2014-09-09 17:02:34, Arnd Bergmann wrote:
>> >> On Tuesday 09 September 2014 16:17:56 Pavel Machek wrote:
>> >> > > Jaehoon Chung (1):
>> >> > >       ARM: dts: socfpga: unuse the slot-node and deprecate the supports-highspeed for dw-mmc
>> >> > >
>> >> >
>> >> > This patch is a bad idea. It removes support for two mmc cards on a
>> >> > single controller -- configuration hardware supports and configuration
>> >> > that allows using u-SD card on mcvevk board.
>> >> >
>> >>
>> >> Your objection comes too late, and to the wrong patch, since the
>> >> driver and all other users have already been changed. We had a long
>> >> discussion about this when the issue first came up, and we could
>> >> not find any example of dw-mmc actually being used in a scenario
>> >> with one controller that has multiple devices attached.
>> >
>> > Well, this is not first time I raised this. 3.17 is not yet out, so we
>> > still have chance to fix regressions without major fuss.
>> >
>> >> Apparently every user out there instead uses multiple controller
>> >> instances instead. Are you sure that the socfpga implementation
>> >> is an exception from this?
>> >
>> > Marek Vasut has the hardware. His board apprently has uSD and eMMC,
>> > and I believe it has just one controller. I'll try to get details.
>> >
>>
>> Just wanted to highlight some of the reasons to why the earlier
>> discussion we have had, came to the conclusion to remove the slot
>> node.
>>
>> 1) The mmc core don't support multiple cards attached to the same
>> host, it never has. Also, I am not aware of any requests that
>> suggested us to add it. Due to obvious reasons from performance
>> perspective it's simply a really bad idea, that's likely also why
>> there haven't been any requests for it.
>
> Well, that's Linux problem, right? (And why is it bad idea, btw? The
> bandwidth will be shared between the controllers, ok, that does not
> sound too bad.)

Bandwidth is just one issue. Latency is yet another, which is probably
far worse to handle than bandwidth.

>
>> 2) On the host level, the support for handle multiple slots in DT for
>> dw-mmc has been broken. While dw-mmc parsed the DT nodes for slots, it
>> screwed up configurations. Thus the support for slots have never
>> worked as expected from DT point of view.
>
> Well, DT is supposed to describe the hardware. From your description,
> it seems that linux does not support two slots on one controller and
> DT parsing code basically ignores the slots. (Logical, if it can't
> support two slots).
>
> So now we are breaking DT description due to Linux limitations. Which
>
> a) makes it hard for any other os not having same limitation
>
> b) makes it hard for people to fix the limitation
>
> c) does not really solve anything

Yes it does, the problem in 2) gets fixed.

>
> d) breaks backward compatibility with old dts

According to 2), it has never worked - so we don't break anything.

>
> If nothing, d) should be argument not to do this.
>                                                                         Pavel
> --
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

Kind regards
Uffe