[PATCH v11 07/10] mtd: spi-nor: Add stacked memories support in spi-nor

[Tudor Ambarus]

On 15.12.2023 13:20, Mahapatra, Amit Kumar wrote:
> Hello Tudor,
> 

Hi!

>> -----Original Message-----
>> From: Tudor Ambarus <tudor.ambarus at linaro.org>
>> Sent: Friday, December 15, 2023 4:03 PM
>> To: Mahapatra, Amit Kumar <amit.kumar-mahapatra at amd.com>;
>> broonie at kernel.org; pratyush at kernel.org; miquel.raynal at bootlin.com;
>> richard at nod.at; vigneshr at ti.com; sbinding at opensource.cirrus.com;
>> lee at kernel.org; james.schulman at cirrus.com; david.rhodes at cirrus.com;
>> rf at opensource.cirrus.com; perex at perex.cz; tiwai at suse.com
>> Cc: linux-spi at vger.kernel.org; linux-kernel at vger.kernel.org; michael at walle.cc;
>> linux-mtd at lists.infradead.org; nicolas.ferre at microchip.com;
>> alexandre.belloni at bootlin.com; claudiu.beznea at tuxon.dev; Simek, Michal
>> <michal.simek at amd.com>; linux-arm-kernel at lists.infradead.org; alsa-
>> devel at alsa-project.org; patches at opensource.cirrus.com; linux-
>> sound at vger.kernel.org; git (AMD-Xilinx) <git at amd.com>;
>> amitrkcian2002 at gmail.com
>> Subject: Re: [PATCH v11 07/10] mtd: spi-nor: Add stacked memories support
>> in spi-nor
>>
>>
>>
>> On 12/15/23 10:02, Mahapatra, Amit Kumar wrote:
>>> Hello Tudor,
>>
>> Hi,
>>
>>>
>>>> -----Original Message-----
>>>> From: Tudor Ambarus <tudor.ambarus at linaro.org>
>>>> Sent: Friday, December 15, 2023 1:40 PM
>>>> To: Mahapatra, Amit Kumar <amit.kumar-mahapatra at amd.com>;
>>>> broonie at kernel.org; pratyush at kernel.org; miquel.raynal at bootlin.com;
>>>> richard at nod.at; vigneshr at ti.com; sbinding at opensource.cirrus.com;
>>>> lee at kernel.org; james.schulman at cirrus.com; david.rhodes at cirrus.com;
>>>> rf at opensource.cirrus.com; perex at perex.cz; tiwai at suse.com
>>>> Cc: linux-spi at vger.kernel.org; linux-kernel at vger.kernel.org;
>>>> michael at walle.cc; linux-mtd at lists.infradead.org;
>>>> nicolas.ferre at microchip.com; alexandre.belloni at bootlin.com;
>>>> claudiu.beznea at tuxon.dev; Simek, Michal <michal.simek at amd.com>;
>>>> linux- arm-kernel at lists.infradead.org; alsa-devel at alsa-project.org;
>>>> patches at opensource.cirrus.com; linux-sound at vger.kernel.org; git (AMD-
>>>> Xilinx) <git at amd.com>; amitrkcian2002 at gmail.com
>>>> Subject: Re: [PATCH v11 07/10] mtd: spi-nor: Add stacked memories
>>>> support in spi-nor
>>>>
>>>>
>>>>
>>>> On 15.12.2023 09:55, Mahapatra, Amit Kumar wrote:
>>>>>> Thanks! Can you share with us what flashes you used for testing in
>>>>>> the stacked and parallel configurations?
>>>>> I used SPI-NOR QSPI flashes for testing stacked and parallel.
>>>>
>>>> I got that, I wanted the flash name or device ID.
>>>
>>> N25Q00A, MX66U2G45G, IS25LP01G & W25H02JV are some of the QSPI
>> flashes
>>> on which we tested. Additionally, we conducted tests on over 30
>>> different QSPI flashes from four distinct vendors (Miron, Winbond,
>> Macronix, and ISSI).
>>>
>>
>> Great.
>>
>>>> What I'm interested is if each flash is in its own package. Are they?
>>>
>>> I'm sorry, but I don't quite understand what you mean by "if each
>>> flash in its own package."
>>>
>>
>> There are flashes that are stacked at the physical level. It's a single flash with
>> multiple dies, that are all under a single physical package.
> 
> Got it. The W25H02JV QSPI flash I mentioned earlier is a device with 
> with four dies that are stacked at the physical level.
> 
>>
>> As I understand, your stacked flash model is at logical level. You have
>> 2 flashes each in its own package. 2 different entities. Is my understanding
>> correct?
> 
> Yes, that’s correct.
> 
> I'd like to contribute to your earlier point regarding the placement of 
> the stacked layer. As you correctly highlighted, it should be in the 
> spi-mem generic layer. For instance, when a read/write operation extends 
> across multiple flashes (whether SPI-NOR or SPI-NAND), the stacked layer 
> must handle the flash crossover. This requires setting the appropriate CS 
> index in mem->spi->cs_index_mask to select the correct slave device and 
> updating the data buffer, address & data length in spi_mem_op struct 
> variable. Does this align with your understanding?
> 

This was the initial idea, yes, but we'll have to see how mtd concat
fits in. Maybe the abstraction can be made at the mtd level, which I
suspect mtd concat does. I have to read that driver, never opened it.

Something else to consider: I see that Micron has a twin quad mode:
https://media-www.micron.com/-/media/client/global/documents/products/data-sheet/nor-flash/serial-nor/mt25t/generation-b/mt25t_qljs_l_512_xba_0.pdf?rev=de70b770c5dc4da8b8ead06b57c03500

The micron's "Separate Chip-Select and Clock Signals" resembles the
AMD's dual parallel 8-bit.
Micron's "Shared Chip-Select and Clock Signals" differs from the AMD's
stacked mode, as Micron uses DQ[3:0] and DQ[7:4], whereas AMD considers
both as DQ[3:0].

I had a short chat with Michael and he highlighted that instead of the
parallel mode, one would be better of with an octal device. I wonder
whether the quad parallel is worth the effort. I see AMD can select
either quad (single/stacked/parallel) or octal (single/stacked). Is the
parallel mode considered obsolete for new IPs?

Cheers,
ta