[PATCH V2 2/4] mmc: pwrseq: Document DT bindings for the simple MMC power sequence

On 15 January 2015 at 19:53, NeilBrown [off-list ref] wrote:

On Thu, 15 Jan 2015 16:58:26 +0000 Mark Rutland [off-list ref] wrote:

quoted

Hi Ulf,

On Wed, Jan 14, 2015 at 01:02:08PM +0000, Ulf Hansson wrote:

quoted

The simple MMC power sequence provider, intends to supports a set of
common properties between SOC designs. It thus enables us to re-use the
same provider for several SOCs.

In this initial step, let's add the top level description of the MMC
power sequence and describe the compatible string for the simple MMC
power sequence provider.

Following patches will step by step add support for new properties to
the simple MMC power sequence provider.

Signed-off-by: Ulf Hansson <redacted>
---

Changes in v2:
    - None.

---
 .../devicetree/bindings/mmc/mmc,pwrseq-simple.txt      | 18 ++++++++++++++++++
 Documentation/devicetree/bindings/mmc/mmc.txt          |  5 +++++
 2 files changed, 23 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/mmc/mmc,pwrseq-simple.txt

diff --git a/Documentation/devicetree/bindings/mmc/mmc,pwrseq-simple.txt b/Documentation/devicetree/bindings/mmc/mmc,pwrseq-simple.txt
new file mode 100644
index 0000000..e1b7f9c
--- /dev/null
+++ b/Documentation/devicetree/bindings/mmc/mmc,pwrseq-simple.txt

@@ -0,0 +1,18 @@
+* The simple MMC power sequence provider
+
+System on chip designs may specify a specific MMC power sequence. To
+successfully detect an (e)MMC/SD/SDIO card, that power sequence must be
+maintained while initializing the card.
+
+The simple MMC power sequence provider, intends to supports a set of common
+properties between SOC designs. It thus enables us to re-use the same provider
+for several SOC designs.
+
+Required properties:
+- compatible : contains "mmc,pwrseq-simple".

Nit: "mmc" is not a vendor prefix.

quoted

+
+Example:
+
+   sdhci0_pwrseq {
+           compatible = "mmc,pwrseq-simple";
+   }

I'm a little confused here. What specific sequence is described by this
node? We don't appear to have referred to any resources used as part of
that sequence, and the description above only mentions that there could
be a specific sequence, not what that sequence is.

So I don't think this makes sense on its own, and should probably be
folded with patches adding the initial support for the resources used as
part of the sequence (e.g. the GPIOs added in a later patch).

I guess I assumed that this "simple" referred to the current behaviour, which
includes some of vmmc-supply, vqmmc-supply, vmmc_aux-supply and pbias-supply
being switched at "appropriate" times.

Would it make sense to bring all of these explicitly under the "pwrseq"
umbrella, leaving the current behaviour only when no pwrseq node is provided?

In principle there is nothing that prevents us from following your
suggestion. But why should we?

Moreover, I don't think it's required at this point, let's first get
the basic mmc-pwrseq support in place. Then we can evolve it. :-)

Also, it isn't clear to me whether the need for power/reset is a function of
the mmc-host, or a function of the device attached to the host.  I suspect
some are needed by one, some by the other.  Any by both?

To me the the power is coupled with the host and thus controlled by
the mmc layer. But I guess we can discuss this in a forever long loop.
:-)

They may be other resources which is coupled with an SDIO function /
eMMC,SDIO card, such as irqs. Those shall be described in the child
node of the host.
I have queued the below patchset from Hans, which I believe should address this:

https://www.mail-archive.com/linux-mmc at vger.kernel.org/msg27241.html

Should the two needs be kept separate?

Yes, I think so.

Kind regards
Uffe