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Date:   Tue, 22 Feb 2022 15:27:44 +0100
From:   Michael Walle <michael@walle.cc>
To:     Tudor.Ambarus@microchip.com
Cc:     p.yadav@ti.com, broonie@kernel.org, miquel.raynal@bootlin.com,
        richard@nod.at, vigneshr@ti.com, linux-mtd@lists.infradead.org,
        linux-kernel@vger.kernel.org, linux-spi@vger.kernel.org,
        Nicolas.Ferre@microchip.com, zhengxunli@mxic.com.tw,
        jaimeliao@mxic.com.tw
Subject: Re: [PATCH 0/4] spi-mem: Allow specifying the byte order in DTR mode
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Am 2022-02-22 15:23, schrieb Tudor.Ambarus@microchip.com:
> On 2/22/22 16:13, Michael Walle wrote:
>> EXTERNAL EMAIL: Do not click links or open attachments unless you know 
>> the content is safe
>> 
>> Am 2022-02-22 14:54, schrieb Tudor.Ambarus@microchip.com:
>>> On 2/21/22 09:44, Michael Walle wrote:
>>>> EXTERNAL EMAIL: Do not click links or open attachments unless you 
>>>> know
>>>> the content is safe
>>>> 
>>>> 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.

But you have to consider also 3rd parties, like an external programmer 
or
another OS. So, there has to be *one correct* way of writing/reading 
these
bytes.

-michael