linux-kernel - Re: [PATCH RFC 1/2] Documentation: devicetree: Add property for ignoring the dummy bits sent before read transfer

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Date:   Thu, 17 Dec 2020 19:18:28 +0530
From:   Aswath Govindraju <a-govindraju@...com>
To:     Rob Herring <robh@...nel.org>
CC:     <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Vadym Kochan <vadym.kochan@...ision.eu>,
        Vignesh Raghavendra <vigneshr@...com>,
        Sekhar Nori <nsekhar@...com>
Subject: Re: [PATCH RFC 1/2] Documentation: devicetree: Add property for
 ignoring the dummy bits sent before read transfer

Hi Rob,

On 15/12/20 9:42 pm, Aswath Govindraju wrote:
> Hi Rob,
> On 15/12/20 3:53 am, Rob Herring wrote:
>> On Fri, Dec 11, 2020 at 08:34:57PM +0530, Aswath Govindraju wrote:
>>> Hi,
>>> On 11/12/20 9:03 am, Rob Herring wrote:
>>>> On Wed, Dec 09, 2020 at 11:27:07PM +0530, Aswath Govindraju wrote:
>>>>> Dummy zero bits are sent before data during a read transfer. This causes
>>>>> the data read to be shifted to the right. To fix this send zero bits after
>>>>> the address during a read transfer.
>>>>>
>>>>> Add property to send zero bits after the address during a read transfer.
>>>>
>>>> When is this necessary? Why can't it be implied by the compatible 
>>>> string which should be specific to the chip model?
>>>>
>>>
>>> This is necessary for 93AA46A/B/C, 93LC46A/B/C, 93C46A/B/C eeproms, as
>>> it can be seen in section 2.7 of [1]. We were not sure if these were the
>>> only devices supported by the driver(eeprom_93xx46.c). So, in order to
>>> apply this only to the above listed devices, we thought that it would be
>>> better to apply this change when required by introducing a DT property.
>>>
>>> May I know how has this case been handled till now ??
>>>
>>
>> No idea. From the at93c46d (which has a compatible string) datasheet it 
>> looks like it has the same thing.
>>
>>> If this is required by all the devices then we can drop the property and
>>> include the zero bit by default.
>>
>> Looks like you need a combination of compatible strings for the above  
>> devices and a property for the ORG pin state on the C devices. I assume 
>> s/w needs to know if x8 or x16?
>>
> Yes, there are separate properties for indicating different types of
> types of eeproms.
> 

Here I was saying about x8 or x16 using the data-size property. ORG pin
state is implied through data-size property and an additional property
is not required for ORG pin state.

> So, do you think that it is better to add it as a seperate property??
> 


These are the available options to my knowledge,

1) As you mentioned earlier all the eeprom's supported by the driver
send a dummy bit before the read data. This can be thought of a bug and
add this change as a fix for it. This might a problem for users who are
already using this driver and working around it using user space tools.

2) Add a special compatible string "eeprom-93xx46B", to add the extra
dummy cycle and not add an additional property.

3) Add an additional property as proposed in this patch and use when
required.

Are there any other suggestions on solving this issue??

Thanks,
Aswath