mmc: Driver Strength Device Property

[Adrian Hunter]

I am in the process of adding an ACPI Device Property to specify
the driver strength (aka drive strength, driver type) for use
with eMMC/SD/SDIO cards, however the ACPI Specification Workgroup
requires that Device Properties be sufficiently generic. This
raises several questions as to what is sufficiently generic. That
is covered in a Questions section below and is followed by a draft
ACPI Device Property Definition. Any comments would be greatly
appreciated.

First some background:

What are ACPI Device Properties and how do they relate to Device Tree
---------------------------------------------------------------------

ACPI Device Properties are key / value pairs that can be recorded
in the ACPI DSDT using a _DSD object with a specific UUID. Refer:
	The ACPI specification and
	http://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UUID.pdf
	http://www.uefi.org/sites/default/files/resources/web-page-v2.pdf

Linux provides a common API to read ACPI Device Properties and
Device Tree properties. Refer:
	https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/base/property.c
	http://marc.info/?l=linux-kernel&m=141354745011569&w=4

Obviously, the common API only works when the same property name
is not defined differently for the same device or class
of devices.

What is Driver Strength
-----------------------

Both the JEDEC eMMC Specification and the SD Association Physical
Layer Specification define Driver Strength. The specifications
also use the terms Drive Strength and Driver Type for the same
thing. While the specifications deal with cards, the Host
Controller can also have a Driver Strength value, for example as
specified in the SD Host Controller Specification.

For eMMC, Driver Strength is an optional setting for the HS200 and
HS400 transfer modes.

Currently JEDEC defines:

	Value		Nominal Impedance	Approx.	capability
						relative to type 0

	0		50 ohm			x1
	1		33 ohm			x1.5
	2		66 ohm			x0.75
	3		100 ohm			x0.5
	4		40 ohm			x1.2

For SD/SDIO, Driver Strength is an optional setting for the
UHS-I transfer modes.

The SD Association defines:

Driver	Value		Nominal Impedance	Approx.	capability
Type						relative to type 0

A	1		33 ohm			x1.5
B	0		50 ohm			x1
C	2		66 ohm			x0.75
D	3		100 ohm			x0.5

So the values are the same with the exception that eMMCs have the
additional value 4 (40 ohm). It is assumed that the values will
anyway not conflict because eMMC is not removable.

The specifications state that support for Driver Strength 0
(Driver Type B) is both mandatory and the default value.
In addition, cards supply a mask of supported Driver Strength
values. Therefore the Driver Strength value is validated against
the supported values.

Questions
---------

Question 1. Should there be separate Driver Strength values for
cards and host controllers?

There is no indication from the specifications that cards and
host controllers need have the same value for Driver Strength.
That suggests that separate properties for the card and host
controller should be provided for.

Originally, I was just proposing "driver-strength" for the
Driver Strength of the card, but now will separate this into
"card-driver-strength" and "host-driver-strength".

Question 2. To what devices should the Driver Strength properties
bind?

A Driver Strength property for the host controller obviously
binds to the host controller device.

Additionally it can be assumed that Driver Strength is a property
of the board (e.g. a stronger Driver Strength needed because the
eMMC is located further from the host controller), consequently
even the Driver Strength property for the card should bind against
the host controller.

Question 3. Should there be separate Driver Strength values for
different transfer modes?

Different transfer modes can have different timing requirements
which implies that there can be different Driver Strength values
for each transfer mode. To support that, there would need to be
either separate properties for each mode, or more simply two
properties that list associated transfer modes and Driver
Strengths. Transfer modes are not codified but have standard
names that could be used e.g. "HS400" or "SDR104"

Question 4. Should there be separate Driver Strength values for
different frequencies?

To be generic, another property would be needed to specify the
frequency from which the Driver Strength is needed.

Question 5. Should there be separate Driver Strength values for
different voltages?

UHS-I modes all have the same signal voltage (1.8V) however HS200
and HS400 support both 1.2V and 1.8V.
That could be handled by having having different mode strings for
the variants. i.e. "HS200", "HS200-1.2V", "HS200-1.8V", "HS400",
"HS400-1.2V", 'HS400-1.8V"

Question 6. Should there be separate Driver Strength values for
different slots?

In order to support multiple slots, the slot number can be appended
to the property names to define a separate set of properties for
each slot.

Question 7. Should the possibility of multiple cards in the same
slot be supported?

Linux does not support having multiple cards in the same slot.
Presumably, the Driver Strength for each card would bind to the
card device, which would require defining how card devices are
identified. That might be possible, but it is out of scope.

ACPI Device Property definitions:
---------------------------------

Based on my answers to the questions, the ACPI Device Property
definitions would look like this:

The following Device Properties are for general use with
any eMMC, SD card, or SDIO host controller.

---------------------------------------------------------------------------
Property:	card-driver-strength-0
Value:		Package of Integers
Description:
     If present, defines a list of the driver strength values
     for a card in slot 0. The driver strength is an optional
     setting defined by the JEDEC eMMC specification or the
     SD Association Physical Layer specification, for use with
     HS200/HS400 or UHS-I transfer modes. In absence of this
     property, the default value of 0 is used.

     Example values:

	Value		Nominal Impedance	Approx.	capability
						relative to type 0

	0		50 ohm			x1
	1		33 ohm			x1.5
	2		66 ohm			x0.75
	3		100 ohm			x0.5
	4		40 ohm			x1.2

     The driver strength values associate one-to-one
     with the values of properties card-driver-strength-mode-0 and
     card-driver-strength-freq-0. Transfer modes or frequencies not
     specified will have the default driver strength of 0.
     Neither card-driver-strength-mode-0 nor card-driver-strength-freq-0
     need be present in which case there must be only one driver
     strength value which will be used with all relevant transfer
     modes and frequencies.

     For host controllers that define multiple slots,
     the value applies to the first slot (slot 0), and
     each subsequent slot requires another property of
     the form card-driver-strength-n when n is the slot
     number (e.g. for the 2nd slot card-driver-strength-1)

Example for a single slot host controller with a single value:
     Package (2) { "card-driver-strength-0",
                   Package (1) { 1 }
     }

Example for a multiple slot host controller with 3 slots all with
a single value:
     Package (2) { "card-driver-strength-0",
                   Package (1) { 1 }
     }
     Package (2) { "card-driver-strength-1",
                   Package (1) { 3 }
     }
     Package (2) { "card-driver-strength-2",
                   Package (1) { 0 }
     }

---------------------------------------------------------------------------
Property:	card-driver-strength-mode-0
Value:		Package of Strings
Description:
    If present, defines the transfer modes to which the values
    of card-driver-strength-0 apply. The transfer modes are
    identified by name and where necessary voltage. i.e.

    eMMC transfer modes:

        "HS200",
        "HS200-1.2V",
        "HS200-1.8V",
        "HS400",
        "HS400-1.2V",
        "HS400-1.8V",

    UHS-I transfer modes:

        "SDR12",
        "SDR25",
        "SDR50",
        "SDR104",
        "DDR50",

    The empty string ("") can be used to mean all modes not
    otherwise specified. The names of new modes should be
    inferred from the relevant specifications.

Example for a single slot host controller with a single value for
HS400 mode:
     Package (2) { "card-driver-strength-0",
                   Package (1) { 1 }
     }
     Package (2) { "card-driver-strength-mode-0",
                   Package (1) { "HS400" }
     }

---------------------------------------------------------------------------
Property:	card-driver-strength-freq-0
Value:		Package of Integers
Description:
    If present, defines the starting frequencies to which the values
    of card-driver-strength-0 apply. The frequency is in Hz.

Example for a single slot host controller with Driver Strength 4
for HS400 starting at 50MHz changing to Driver Strength 1 at 100MHz.
     Package (2) { "card-driver-strength-0",
                   Package (1) { 4, 1 }
     }
     Package (2) { "card-driver-strength-mode-0",
                   Package (1) { "HS400", "HS400" }
     }
     Package (2) { "card-driver-strength-freq-0",
                   Package (1) { 50000000, 100000000 }
     }

---------------------------------------------------------------------------
Property:	host-driver-strength-0
Value:		Package of Integers
Description:
    Same as card-driver-strength-0 but values are used for the
    host controller not the card.

---------------------------------------------------------------------------
Property:	host-driver-strength-mode-0
Value:		Package of Strings
Description:
    Same as card-driver-strength-mode-0 but values are used for the
    host controller not the card.

---------------------------------------------------------------------------
Property:	host-driver-strength-freq-0
Value:		Package of Integers
    Same as card-driver-strength-freq-0 but values are used for the
    host controller not the card.



Regards
Adrian

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