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Date:   Thu, 24 Feb 2022 10:37:25 +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
In-Reply-To: <779069a1-7389-ab70-9f03-81090d0600ba@microchip.com>
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Am 2022-02-24 07:37, schrieb Tudor.Ambarus@microchip.com:
> On 2/24/22 08:08, Tudor.Ambarus@microchip.com wrote:
>> EXTERNAL EMAIL: Do not click links or open attachments unless you know 
>> the content is safe
>> 
>> 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@microchip.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@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
>>>> 
>>>> 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.

IMHO it should just work that you can use 1S-1S-1S mode and 8D-8D-8D on 
the
same flash. After all, that is Tudor's use case. The ROM access the 
flash
in single bit mode and linux in 8D-8D-8D mode. Maybe u-boot will use 
quad
mode in between. All of these accesses should return the same flash
content.

>> How about swapping the bytes just at user request? Maybe with a 
>> Kconfig
>> option.
> 
> Michael has suggested on #irc to always swap the bytes: if the SPI 
> controller
> can't do it, to do it in software at SPI NOR level. I don't know what 
> to say
> about this, because JEDEC216 just informs the reader I guess:
> "Byte order of 16-bit words is swapped when read in 8D-8D-8D mode 
> compared to
> 1-1-1 mode.", this doesn't look like a hard request. The downside to 
> doing
> the swapping in software is performance penalty which will make 
> macronix
> users have second thoughts. I don't have a strong opinion, but I lean 
> towards
> doing the swap just at user request, regardless if I do it via the SPI
> controller
> or in software.

Just having and opt-in will be a mess in the future with flashes 
containing
byte swapped content and we can't even fix it and we will have to live 
with
that forever. IMHO right now is the best time to circumvent that 
scenario.
I don't have anything against make it user configurable, but it should 
be
an opt-out.

I haven't looked at any controllers who can do 8D-8D-8D accesses, maybe 
most
of them can do the swapping on their own? So if you don't want to 
support a
software fallback, then we should just say this mode isn't supported if
the controller can't do the byte swapping and we fall back to a slower 
mode.

-michael