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, 10 Dec 2020 21:33:18 -0600
From:   Rob Herring <robh@...nel.org>
To:     Aswath Govindraju <a-govindraju@...com>
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

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?

> 
> Suggested-by: Vignesh Raghavendra <vigneshr@...com>
> Signed-off-by: Aswath Govindraju <a-govindraju@...com>
> ---
>  Documentation/devicetree/bindings/misc/eeprom-93xx46.txt | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/devicetree/bindings/misc/eeprom-93xx46.txt b/Documentation/devicetree/bindings/misc/eeprom-93xx46.txt
> index a8ebb4621f79..09fb1cec54f0 100644
> --- a/Documentation/devicetree/bindings/misc/eeprom-93xx46.txt
> +++ b/Documentation/devicetree/bindings/misc/eeprom-93xx46.txt
> @@ -10,7 +10,9 @@ Optional properties:
>  - read-only : parameter-less property which disables writes to the EEPROM
>  - select-gpios : if present, specifies the GPIO that will be asserted prior to
>    each access to the EEPROM (e.g. for SPI bus multiplexing)
> -
> +- read-op-dummy-cycles: If present specifies the number of dummy zero-bits to
> +  be sent after the address during a read transfer to ignore any bits sent
> +  preceding the data.
>  Property rules described in Documentation/devicetree/bindings/spi/spi-bus.txt
>  apply.  In particular, "reg" and "spi-max-frequency" properties must be given.
>  
> -- 
> 2.17.1
>