Date:	Tue, 17 Feb 2015 00:02:18 +0100
From:	Arend van Spriel <arend@...adcom.com>
To:	Adrian Hunter <adrian.hunter@...el.com>
CC:	Philip Rakity <prakity@...dia.com>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"rafael.j.wysocki@...el.com >> Rafael J. Wysocki" 
	<rafael.j.wysocki@...el.com>,
	"mika.westerberg@...el.com" <mika.westerberg@...el.com>,
	"dvhart@...ux.intel.com" <dvhart@...ux.intel.com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"arnd@...db.de" <arnd@...db.de>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
	"Ulf Hansson" <ulf.hansson@...aro.org>,
	"chris@...ntf.net" <chris@...ntf.net>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	Seungwon Jeon <tgih.jun@...sung.com>,
	Alex Lemberg <alex.lemberg@...disk.com>,
	"Yuliy.Izrailov@...disk.com" <Yuliy.Izrailov@...disk.com>,
	Alex Courbot <acourbot@...dia.com>,
	"s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
	"Markus.Niebel@...group.com" <Markus.Niebel@...group.com>,
	"u.kleine-koenig@...gutronix.de" <u.kleine-koenig@...gutronix.de>
Subject: Re: mmc: Driver Strength Device Property

On 02/16/15 22:47, Adrian Hunter wrote:
> On 16/02/2015 7:47 p.m., Philip Rakity wrote:
>>
>> On Feb 16, 2015, at 5:39 PM, Arend van Spriel <arend@...adcom.com> wrote:
>>
>>> On 02/16/15 15:25, Adrian Hunter wrote:
>>>> 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".
>>>
>>> Hi Adrian,
>>>
>>> I am not sure if it makes sense to have a 'card-driver-strength'
>>> specification for the host controller. I have been under the
>>> impression that the proper value for this is strongly depending
>>> on the card used in the slot. For this reason and the fact that
>>> it is programmed in our device we added brcm,drive-strength
>>> property in our device-tree bindings [1].
>
> So brcm,drive-strength is "drive strength used for SDIO pins on device
> in mA
> (default = 6)" and it is written to a brcm register rather than the SDIO
> CCCR Driver Strength register. So it is not in conflict, but an example of
> a hardware-specific way of doing things.
>
> I agree when you have a brcm value to write to a brcm register, it makes
> sense to bind it to the brcm device.

Glad to hear. I guess I was wondering how the card-driver-strength 
property would be used and by what entity, but that is not directly 
relevant for having a definition for it in ACPI/devicetree.

Regards,
Arend

> One problem with having the standard property bind to the card is that
> removable cards are not usually represented.
>
>>>
>>> Regards,
>>> Arend
>>>
>>> [1]
>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/net/wireless/brcm,bcm43xx-fmac.txt
>>>
>>
>> Adrian, Arend,
>>
>> My experience is that the value also depends very much on the board
>> design as well as the eMMC/sdio chip being used. eMMC chips have
>> some variation. A DT entry does make sense for eMMC and other wired in
>> devices but in this case the value is a property of the hardware
>> and "burnt in" as a property of the board design.
>
> So it sounds like binding to the host controller would be OK.
>
>>
>> regards,
>>
>> Philip
>>
>>>> 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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>>>
>>

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