linux-kernel - Re: [PATCH v2 1/3] input: cygnus-update touchscreen dt node document

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Message-ID: <47ef2a75-3f97-644f-a817-c93e6b3bc658@broadcom.com>
Date:	Mon, 22 Feb 2016 11:48:47 -0800
From:	Ray Jui <ray.jui@...adcom.com>
To:	Scott Branden <scott.branden@...adcom.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Raveendra Padasalagi <raveendra.padasalagi@...adcom.com>
Cc:	Rob Herring <robh@...nel.org>,
	Russell King <linux@....linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-input@...r.kernel.org,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Jonathan Richardson <jonathar@...adcom.com>,
	Jon Mason <jonmason@...adcom.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	Anup Patel <anup.patel@...adcom.com>,
	Ray Jui <rjui@...adcom.com>,
	Scott Branden <sbranden@...adcom.com>,
	linux-kernel@...r.kernel.org, bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH v2 1/3] input: cygnus-update touchscreen dt node document



On 2/22/2016 11:41 AM, Scott Branden wrote:
> My comments below
>
> On 16-02-22 11:36 AM, Dmitry Torokhov wrote:
>> On Fri, Feb 19, 2016 at 11:43:50AM +0530, Raveendra Padasalagi wrote:
>>> On Thu, Feb 18, 2016 at 8:06 PM, Rob Herring <robh@...nel.org> wrote:
>>>> On Wed, Feb 17, 2016 at 03:13:44PM +0530, Raveendra Padasalagi wrote:
>>>>> In Cygnus SOC touch screen controller registers are shared
>>>>> with ADC and flex timer. Using readl/writel could lead to
>>>>> race condition. So touch screen driver is enhanced to support
>>>>>
>>>>> 1. If touchscreen register's are not shared. Register access
>>>>> is handled through readl/writel if "brcm,iproc-touchscreen"
>>>>> compatible is provided in touchscreen dt node. This will help
>>>>> for future SOC's if comes with dedicated touchscreen IP register's.
>>>>>
>>>>> 2. If touchscreen register's are shared with other IP's, register
>>>>> access is handled through syscon framework API's to take care of
>>>>> mutually exclusive access. This feature can be enabled by selecting
>>>>> "brcm,iproc-touchscreen-syscon" compatible string in the touchscreen
>>>>> dt node.
>>>>>
>>>>> Hence touchscreen dt node bindings document is updated to take care
>>>>> of above changes in the touchscreen driver.
>>>>>
>>>>> Signed-off-by: Raveendra Padasalagi
>>>>> <raveendra.padasalagi@...adcom.com>
>>>>> Reviewed-by: Ray Jui <ray.jui@...adcom.com>
>>>>> Reviewed-by: Scott Branden <scott.branden@...adcom.com>
>>>>> ---
>>>>>   .../input/touchscreen/brcm,iproc-touchscreen.txt   | 57
>>>>> +++++++++++++++++++---
>>>>>   1 file changed, 51 insertions(+), 6 deletions(-)
>>>>>
>>>>> diff --git
>>>>> a/Documentation/devicetree/bindings/input/touchscreen/brcm,iproc-touchscreen.txt
>>>>> b/Documentation/devicetree/bindings/input/touchscreen/brcm,iproc-touchscreen.txt
>>>>>
>>>>> index 34e3382..f530c25 100644
>>>>> ---
>>>>> a/Documentation/devicetree/bindings/input/touchscreen/brcm,iproc-touchscreen.txt
>>>>>
>>>>> +++
>>>>> b/Documentation/devicetree/bindings/input/touchscreen/brcm,iproc-touchscreen.txt
>>>>>
>>>>> @@ -1,12 +1,30 @@
>>>>>   * Broadcom's IPROC Touchscreen Controller
>>>>>
>>>>>   Required properties:
>>>>> -- compatible: must be "brcm,iproc-touchscreen"
>>>>> -- reg: physical base address of the controller and length of
>>>>> memory mapped
>>>>> -  region.
>>>>> +- compatible: should be one of
>>>>> +        "brcm,iproc-touchscreen"
>>>>> +        "brcm,iproc-touchscreen-syscon"
>>>>
>>>> More specific and this is not how you do syscon. Either the block is or
>>>> isn't. You can't have it both ways.
>>>
>>> Existing driver has support for reg, if we modify now to support only
>>> syscon
>>> then this driver will not work if some one wishes to use previous
>>> kernel version's
>>> dt and vice versa. Basically this breaks dt compatibility. Is that ok ?
>>
>> But the issue is that the driver does not actually work correctly with
>> direct register access on those systems, since the registers are
>> actually shared with other components. I am not quite sure if it is OK
>> to break DT binding in this case...
>
> The driver does work correctly with direct register access on those
> systems because the other components using those registers are not
> active in those systems - so syscon is not needed in those cases.
>
> I'm ok with not containing backwards compatibility though and always
> using syscon.  There are no deployed systems using older versions of the
> upstreamed kernel.
>>
>> Thanks.
>>
>
> Regards,
> Scott

The iproc touchscreen is currently activated in the "bcm9hmidc.dtsi" 
that represents the optional daughter card installed on reference boards 
bcm958300k and bcm958305k. While not maintaining backwards compatibility 
*might not* be a serious issue, it would be nice if we can at least make 
sure the driver change and DT are merged into the same kernel version so 
they stay in sync.

Going forward, if we are only going to support syscon based 
implementation, the existing compatible string "brcm,iproc-touchscreen" 
is preferred over "brcm,iproc-touchscreen-syscon".

Thanks,

Ray