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 09:48:38 -0600
From:   Rob Herring <robh@...nel.org>
To:     Aswath Govindraju <a-govindraju@...com>
Cc:     devicetree@...r.kernel.org,
        "linux-kernel@...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

On Thu, Dec 17, 2020 at 7:48 AM Aswath Govindraju <a-govindraju@...com> wrote:
>
> 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.

Ah, I missed that property.

>
> > 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.

No. Genericish compatible strings are what cause the problem and this
whole discussion.

> 3) Add an additional property as proposed in this patch and use when
> required.
>
> Are there any other suggestions on solving this issue??

You need a compatible string for each vendor+model. Period.

Rob