linux-kernel - Re: [PATCH] mtd: spi-nor: core: replace dummy buswidth from addr to data

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Message-ID: <01160386-a65f-44b0-ac57-ad77754f01cc@linaro.org>
Date: Tue, 12 Nov 2024 06:45:06 +0000
From: Tudor Ambarus <tudor.ambarus@...aro.org>
To: Cheng Ming Lin <linchengming884@...il.com>
Cc: pratyush@...nel.org, mwalle@...nel.org, miquel.raynal@...tlin.com,
 richard@....at, vigneshr@...com, linux-mtd@...ts.infradead.org,
 linux-kernel@...r.kernel.org, alvinzhou@...c.com.tw, leoyu@...c.com.tw,
 Cheng Ming Lin <chengminglin@...c.com.tw>
Subject: Re: [PATCH] mtd: spi-nor: core: replace dummy buswidth from addr to
 data



On 11/12/24 2:42 AM, Cheng Ming Lin wrote:
> Hi Tudor,
> 
> Tudor Ambarus <tudor.ambarus@...aro.org> 於 2024年11月11日 週一 下午6:18寫道：
>>
>>
>>
>> On 11/7/24 9:30 AM, Cheng Ming Lin wrote:
>>> From: Cheng Ming Lin <chengminglin@...c.com.tw>
>>>
>>> The default dummy cycle for Macronix SPI NOR flash in Octal Output
>>> Read Mode(1-1-8) is 20.
>>>
>>> Currently, the dummy buswidth is set according to the address bus width.
>>> In the 1-1-8 mode, this means the dummy buswidth is 1. When converting
>>> dummy cycles to bytes, this results in 20 x 1 / 8 = 2 bytes, causing the
>>> host to read data 4 cycles too early.
>>>
>>> Since the protocol data buswidth is always greater than or equal to the
>>> address buswidth. Setting the dummy buswidth to match the data buswidth
>>> increases the likelihood that the dummy cycle-to-byte conversion will be
>>> divisible, preventing the host from reading data prematurely.
>>
>> This is still very wrong and the `fix` is working just by chance.
>> Consider what happens when one requires 10 dummy cycles. BTW, does this
>> fix a real problem, or it's just a theoretical fix?
> 
> In 1-1-8 mode, setting the dummy buswidth to match the data
> buswidth ensures a dummy buswidth of 8, which can accommodate
> all types of dummy cycles.
> 
> This patch resolves a significant issue in 1-1-8 mode, as described above.
> 

shall we add a fixes tag then, and a cc to stable?