Re: [PATCH v3 06/13] mmc: tmio-mmc: define device-tree bindings

On Thu, 14 Feb 2013, Magnus Damm wrote:

On Thu, Feb 14, 2013 at 10:59 AM, Simon Horman [off-list ref] wrote:

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On Thu, Feb 14, 2013 at 10:42:21AM +0900, Magnus Damm wrote:

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On Thu, Feb 14, 2013 at 12:59 AM, Guennadi Liakhovetski
[off-list ref] wrote:

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On Thu, 7 Feb 2013, Simon Horman wrote:

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On Wed, Feb 06, 2013 at 10:24:20PM +0000, Arnd Bergmann wrote:

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On Wednesday 06 February 2013, Guennadi Liakhovetski wrote:

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+* Toshiba Mobile IO SD/MMC controller
+
+The tmio-mmc driver doesn't probe its devices actively, instead its binding to
+devices is managed by either MFD drivers or by the sh_mobile_sdhi platform
+driver. Those drivers supply the tmio-mmc driver with platform data, that either
+describe hardware capabilities, known to them, or are obtained by them from
+their own platform data or from their DT information. In the latter case all
+compulsory and any optional properties, common to all SD/MMC drivers, as
+described in mmc.txt, should or can be used. Additionally the following optional
+bindings can be used. They set respective TMIO_MMC_* flags.
+
+Optional properties:
+- toshiba,mmc-wrprotect-disable        : set TMIO_MMC_WRPROTECT_DISABLE flag
+- toshiba,mmc-blksz-2bytes     : set TMIO_MMC_BLKSZ_2BYTES
+- toshiba,mmc-has-idle-wait    : set TMIO_MMC_HAS_IDLE_WAIT

Please write the binding in a way that does not refer to a specific
implementation in Linux: The binding should describe the hardware
independent of details in the driver. In particular, I think you
should not refer to the TMIO_MMC_BLKSZ_2BYTES etc macros but describe
in text what the flags are about.

Regarding the toshiba,mmc-wrprotect-disable property, would it be
enough to just check the presence of the wp-gpios property?

No, normally WP would be implemented, using the native dedicated SDHI WP 
pin. So, toshiba,mmc-wrprotect-disable is used when no such pin is 
available on this interface, or it's not routed to the socket and no GPIO 
is used for that either. So, this flag is the only way to know, whether a 
WP pin is used, if no wp-gpios is used.

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TMIO_MMC_BLKSZ_2BYTES seems to be set unconditionally in
sh_mobile_sdhi_probe and nowhere else, so I'd assume we don't
actually need to provide this here, but can keep that knowledge
implicit based on whether we're talking to sh_mobile_sdhi
or another tmio_mmc variant.

Can do that, yes.

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For the other last one, is that actually board specific, or just
a feature of a given chip? If we can tell by the SoC, then I'd
suggest using separate "compatible" properties instead, and
put a bitmask of features into the .data field of the of match
table. For all I can tell, SH7372 does not set it, while SH73A0,
R8A7740 and R8A7779 always do.

My understanding is that TMIO_MMC_HAS_IDLE_WAIT can be set based
on the SoC in use.

So far TMIO_MMC_HAS_IDLE_WAIT is set on

board-kzm9g.c (sh73a0 / AG5)
board-ag5evm.c (sh73a0 / AG5)
board-armadillo800eva.c (r8a7740 / A1)
board-kota2.c (sh73a0 / AG5)
board-marzen.c (r8a7779 / H1)

and isn't set on

board-ap4evb.c (sh7372 / ap4)
board-bonito.c (r8a7740 / a1, SDHI isn't used)
board-mackerel.c (sh7372 / ap4)

So, shall we use a compatible property for this and drop this property? We
can add later at any time, if needed, which is better, than defining
something redundant. OTOH I seem to remember, that using SoC-version from
the "compatible" property was considered by someone inappropriate. Magnus,
what do you think?

I prefer you to use a hardware-block version compatible suffix instead
of SoC suffix.

This since we have more SoCs than actual hardware block
configurations. Using the list above, how many configurations would we
have?

Actually, forcing the drivers to be updated for each new SoC sounds
like a pretty terrible idea. Wouldn't that be against one of the
merits of using DT? Also, don't you have enough interesting work piled
up already? =)

Basically, I can't see any point in adding an extra unnecessary need
for updating the drivers when there is no real functional change.

My understanding is that the discussion is about the details of
bindings that are required for SDHI to function when brought
up using DT on a variety of boards. Not exciting new work.

In particular how to set TMIO_MMC_HAS_IDLE_WAIT via DT.

No, not exciting new work. More describing certain versions of the
SDHI hardware. This is a SDHI-specific configuration which may end up
being used on a certain SoC. There are also some board specific
details that need to be taken into consideration. I believe it is
important to understand the difference between hardware-block
configuration (SDHI in this particular case), SoC and board.

So the way I see it we have 3 ways to deal with it:

1) Use the "toshiba,mmc-has-idle-wait" property proposed by Guennadi
or
2) Use a SoC suffix in the compatible string and deal with
TMIO_MMC_HAS_IDLE_WAIT in the driver
or
3) Use a SDHI-specific version suffix in the compatible string and
deal with TMIO_MMC_HAS_IDLE_WAIT in the driver

I am fine with 1) or 3) but I don't want to go down the route of 2)
because it will just lead to more pointless driver changes than are
actually needed. And the TMIO_MMC_HAS_IDLE_WAIT flag is not
SoC-specific so using a SoC suffix seems incorrect to me.

My take on this is the following: having N optionally available on 
different IP-versions features, I'd rather have N DT properties, than up 
to 2^N abstract vX versions. Yes, I realise, that in practice we'll never 
have 2^N, rather just a bit more than N, still, my main problem with those 
versions, is that they are purely abstracted. I'd be happy if that was a 
real hardware revision string, that you could look up in a datasheet. But 
if you have to look in some text file... That just seems much less 
user-friendly to me, than selecting single properties. Just imagine, what 
would you prefer, either specifying

	feature-A;
	feature-B;

in your .dts or going through a list of

v1: has no features
v2: feature A only
v3: feature B only
v4: features A and B
...

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/