linux-kernel - Re: [RFC PATCH 0/8] Add TI EMIF SDRAM controller driver

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Date:	Fri, 17 Feb 2012 19:26:29 +0530
From:	Aneesh V <aneesh@...com>
To:	Greg KH <greg@...ah.com>
CC:	Santosh Shilimkar <santosh.shilimkar@...com>,
	akpm@...ux-foundation.org, linux-omap@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/8] Add TI EMIF SDRAM controller driver

Greg,

On Thursday 16 February 2012 09:53 PM, Greg KH wrote:
> On Thu, Feb 16, 2012 at 04:21:11PM +0530, Santosh Shilimkar wrote:
>> Andrew, Greg,
>>
>> On Saturday 04 February 2012 05:46 PM, Aneesh V wrote:
>>> Add a driver for the EMIF SDRAM controller used in TI SoCs
>>>
>>> EMIF is an SDRAM controller that supports, based on its revision,
>>> one or more of LPDDR2/DDR2/DDR3 protocols.This driver adds support
>>> for LPDDR2.
>>>
>>> The driver supports the following features:
>>> - Calculates the DDR AC timing parameters to be set in EMIF
>>>    registers using data from the device data-sheets and based
>>>    on the DDR frequency. If data from data-sheets is not available
>>>    default timing values from the JEDEC spec are used. These
>>>    will be safe, but not necessarily optimal
>>> - API for changing timings during DVFS or at boot-up
>>> - Temperature alert configuration and handling of temperature
>>>    alerts, if any for LPDDR2 devices
>>>    * temperature alert is based on periodic polling of MR4 mode
>>>      register in DDR devices automatically performed by hardware
>>>    * timings are de-rated and brought back to nominal when
>>>      temperature raises and falls respectively
>>> - Cache of calculated register values to avoid re-calculating
>>>    them
>>>
>>> The driver will need some minor updates when it is eventually
>>> integrated with DVFS. This can not be done now as DVFS support
>>> is not available yet in mainline.
>>>
>>> Discussions with Santosh Shilimkar<santosh.shilimkar@...com>
>>> were immensely helpful in shaping up the interfaces. Vibhore Vardhan
>>> <vvardhan@...il.com>  did the initial code snippet for thermal
>>> handling.
>>>
>>> Testing:
>>> - The driver is tested on OMAP4430 SDP.
>>> - The driver in a slightly adapted form is also tested on OMAP5.
>>> - Since mainline kernel doesn't have DVFS support yet,
>>>    testing was done using a test module.
>>> - Temperature alert handling was tested with simulated interrupts
>>>    and faked temperature values as testing all cases in real-life
>>>    scenarios is difficult.
>>>
>> [...]
>>
>>>   arch/arm/mach-omap2/omap_hwmod_44xx_data.c |  110 ++
>>>   drivers/misc/Kconfig                       |   20 +
>>>   drivers/misc/Makefile                      |    2 +
>>>   drivers/misc/emif.c                        | 1522 ++++++++++++++++++++++++++++
>>>   drivers/misc/emif_regs.h                   |  461 +++++++++
>>>   drivers/misc/jedec_ddr_data.c              |  141 +++
>>>   include/linux/emif.h                       |  257 +++++
>>>   include/linux/jedec_ddr.h                  |  174 ++++
>>
>> Any suggestion on where this driver can reside. It's a memory
>> controller driver which supports standard DDR functionality
>> as per JDEC specs including thermal alert. On top of
>> that it does support DVFS using the TI PRCM IP block.
>
> I don't know what any of those TLA words mean, so I really can't suggest

This is a driver for TI's memory controller(called EMIF). The
driver is needed for adjusting the controller settings on frequency,
voltage, and temperature changes. Any suggestion as to where this
should go?

> where this code should go.  But just from this diffstat, it looks like
> you are creating a new user/kernel interface, without documenting it
> anywhere, which isn't ok.

I think you are referring to the header files added in include/linux/
They are not creating new user/kernel interface per se.

"include/linux/jedec_ddr.h" is the interface to a library that contains
data from the DDR specs. "include/linux/emif.h" has definitions for
platform data needed by the driver. Maybe these should go to some other
sub-directory within include/ or include/linux/ ?

I shall add documentation for the driver in the next revision.

Thanks,
Aneesh
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