On Thu, 2016-05-05 at 13:10 +0200, Arnd Bergmann wrote:

On Thursday 05 May 2016 09:41:32 Yangbo Lu wrote:

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From: Arnd Bergmann [mailto:arnd-r2nGTMty4D4@public.gmane.org]
Sent: Thursday, May 05, 2016 4:32 PM
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Santosh Shilimkar; Yang-Leo Li; Scott Wood; Rob Herring; Claudiu Manoil;
Kumar Gala; Xiaobo Xie; Qiang Zhao
Subject: Re: [v10, 7/7] mmc: sdhci-of-esdhc: fix host version for T4240-
R1.0-R2.0

On Thursday 05 May 2016 11:12:30 Yangbo Lu wrote:

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IIRC, it is the same IP block as i.MX and Arnd's point is this won't
even compile on !PPC. It is things like this that prevent sharing the
driver.

The whole point of using the MMIO SVR instead of the PPC SPR is so that
it will work on ARM...  The guts driver should build on any platform as
long as OF is enabled, and if it doesn't find a node to bind to it will
return 0 for SVR, and the eSDHC driver will continue (after printing an
error that should be removed) without the ability to test for errata
based on SVR.

It feels like a bad design to have to come up with a different
method for each SoC type here when they all do the same thing
and want to identify some variant of the chip to do device
specific quirks.

As far as I'm concerned, every driver in drivers/soc that needs to
export a symbol to be used by a device driver is an indication that
we don't have the right set of abstractions yet. There are cases
that are not worth abstracting because the functionality is rather
obscure and only a couple of drivers for one particular chip
ever need it.

Finding out the version of the SoC does not look like this case.

I'm open to new ways of abstracting this, but can that please be discussed
after these patches are merged?  This patchset is fixing a problem, the
existing abstraction is unappealing and not widely adopted, a new abstraction
is not ready, and we're only touching code for our hardware.

Oh, and the existing abstraction isn't even "existing".  I don't see any
examples where soc_device is being used like this -- or even any way for a
driver (the one consuming the information, not the soc "driver") to get a
reference to the soc_device that's been registered short of searching for the
device object by name -- and you're asking for new functionality in
drivers/base/soc.c.

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I think the first four patches take care of building for ARM,
but the problem remains if you want to enable COMPILE_TEST as
we need for certain automated checking.

What specific problem is there with COMPILE_TEST?

COMPILE_TEST is solvable here and the way it is implemented in this
case (selecting FSL_GUTS from the driver) indeed looks like it works
correctly, but it's still awkward that this means building the
SoC specific ID stuff into the vmlinux binary for any driver that
uses something like that for a particular SoC.

Please keep in mind that this is a Freescale-specific driver... it's not as if
we're attaching this dependency to common SDHCI code.

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Dealing with Si revs is a common problem. We should have a
common solution. There is soc_device for this purpose.

Exactly. The last time this came up, I think we agreed to implement a
helper using glob_match() on the soc_device strings. Unfortunately
this hasn't happened then, but I'd still prefer that over yet another
vendor-specific way of dealing with the generic issue.

soc_device would require encoding the SVR as a string and then decoding
the string, which is more complicated and error prone than having
platform-specific code test a platform-specific number. 

You already need to encode it as a string to register the soc_device,

No we don't, because we don't already register a soc_device on arm64 or ppc
(and it looks like whatever does get registered on at least some relevant
arm32 chips is not particularly useful).

and the driver just needs to pass a glob string, so the only part that
is missing is the generic function that takes the string from the
driver and passes that to glob_match for the soc_device.

"just"

And what would the glob look like?

I'd rather not write kernel code as if it were a shell/Perl script.

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And when would it get registered on arm64, which doesn't have
platform code?

Whenever the soc driver is loaded, as is the case now. The match
function can return -EPROBE_DEFER if no SoC device is registered
yet.

That's too late for some places where we need access to SVR, e.g. clock
drivers (which use CLK_OF_DECLARE and are initialized very early, not as part
of the driver model and thus can't defer).  Currently we have an #ifdef
CONFIG_PPC for this in drivers/clk/clk-qoriq.c... Maybe we should have done
that here as well, and saved some grief. :-)  At least until an erratum pops
up on an ARM-based chip.

And what happens if we're running on arm32, and thus the arch code already
registered an soc_device with a different (and less useful) encoding?

-Scott