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

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Message-ID: <6eb179ec-0b12-de30-829e-83ec6b964f21@microchip.com>
Date:   Thu, 24 Feb 2022 06:08:44 +0000
From:   <Tudor.Ambarus@...rochip.com>
To:     <p.yadav@...com>
CC:     <michael@...le.cc>, <broonie@...nel.org>,
        <miquel.raynal@...tlin.com>, <richard@....at>, <vigneshr@...com>,
        <linux-mtd@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
        <linux-spi@...r.kernel.org>, <Nicolas.Ferre@...rochip.com>,
        <zhengxunli@...c.com.tw>, <jaimeliao@...c.com.tw>
Subject: Re: [PATCH 0/4] spi-mem: Allow specifying the byte order in DTR mode

On 2/23/22 20:38, Pratyush Yadav wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> Hi Tudor,
> 
> On 22/02/22 02:43PM, Tudor.Ambarus@...rochip.com wrote:
>> On 2/22/22 16:27, Michael Walle wrote:
>>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>>
>>> Am 2022-02-22 15:23, schrieb Tudor.Ambarus@...rochip.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@...rochip.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
>>
>> Why? If you use the same mode when reading and writing, everything is fine.
>> I'm not sure what's your suggestion here.
> 
> So our stance here is that we don't care about external programs?> 
> If that is the case then why bother with all this anyway? Since the swap
> happens at both page program and read, what you write is what you read
> back. Who cares the order stored in the actual flash memory as long as
> the data read is correct?
> 
> If we do care about external programs, then what would happen if the
> external program writes data in 8D-8D-8D mode _without_ swapping the
> bytes? This would also cause data corruption. You can't control what
> they mode they use, and you can't detect it later either.
> 
> I think there is no winning here. You just have to say that external
> programs should write in 8D-8D-8D mode or it won't boot.
> 

How about swapping the bytes just at user request? Maybe with a Kconfig
option.

>>
>>> another OS. So, there has to be *one correct* way of writing/reading
>>> these
>>> bytes.
>>>
>>
>>
> 
> --
> Regards,
> Pratyush Yadav
> Texas Instruments Inc.