Re: [PATCH 0/4] spi-mem: Allow specifying the byte order in DTR mode

[<Tudor.Ambarus@microchip.com>]

On 2/22/22 16:13, Michael Walle wrote:
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> Am 2022-02-22 14:54, schrieb Tudor.Ambarus@microchip.com:
>> On 2/21/22 09:44, Michael Walle wrote:
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>>> Am 2022-02-18 15:58, schrieb Tudor Ambarus:
>>>> Fortunately there are controllers
>>>> that can swap back the bytes at runtime, fixing the endiannesses.
>>>> Provide
>>>> a way for the upper layers to specify the byte order in DTR mode.
>>>
>>> Are there any patches for the atmel-quadspi yet? What happens if
>>
>> not public, but will publish them these days.
>>
>>> the controller doesn't support it? Will there be a software fallback?
>>
>> no need for a fallback, the controller can ignore op->data.dtr_bswap16
>> if
>> it can't swap bytes.
> 
> I don't understand. If the controller doesn't swap the 16bit values,
> you will read the wrong content, no?
> 

In linux no, because macronix swaps bytes on a 2 byte boundary both on
reads and on page program. The problem is when you mix 8D-8D-8D mode and
1-1-1 mode along the boot stages. Let's assume you write all boot binaries
in 1-1-1 mode. When reaching u-boot if you enable 8D-8D-8D mode, when u-boot
will try to get the kernel it will fail, as the flash swaps the bytes compared
to what was written with 1-1-1 mode. You write D0 D1 D2 D3 in 1-1-1 mode and
when reaching u-boot you will read D1 D0 D3 D2 and it will mess the kernel image.