linux-kernel - Re: [PATCH 1/3] dt-bindings: spi: stm32: Add st,dual-flash property in st,stm32-qspi.yaml

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Date:   Tue, 9 Aug 2022 10:00:36 +0200
From:   Patrice CHOTARD <patrice.chotard@...s.st.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        <robh+dt@...nel.org>, <krzysztof.kozlowski+dt@...aro.org>
CC:     <linux-spi@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <christophe.kerello@...s.st.com>,
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH 1/3] dt-bindings: spi: stm32: Add st,dual-flash property
 in st,stm32-qspi.yaml



On 8/9/22 09:57, Krzysztof Kozlowski wrote:
> On 09/08/2022 10:52, Patrice CHOTARD wrote:
>> Hi Krzystof
>>
>> On 8/9/22 07:29, Krzysztof Kozlowski wrote:
>>> On 09/08/2022 07:18, Krzysztof Kozlowski wrote:
>>>> On 08/08/2022 19:08, Patrice CHOTARD wrote:
>>>>> Hi Krzystof
>>>>>
>>>>> On 8/8/22 11:01, Krzysztof Kozlowski wrote:
>>>>>> On 08/08/2022 10:40, patrice.chotard@...s.st.com wrote:
>>>>>>> From: Patrice Chotard <patrice.chotard@...s.st.com>
>>>>>>>
>>>>>>> Add new property st,dual-flash which allows to use the QSPI interface as a
>>>>>>> communication channel using up to 8 qspi line.
>>>>>>> This mode can only be used if cs-gpios property is defined.
>>>>>>>
>>>>>>> Signed-off-by: Patrice Chotard <patrice.chotard@...s.st.com>
>>>>>>> ---
>>>>>>>  Documentation/devicetree/bindings/spi/st,stm32-qspi.yaml | 8 ++++++++
>>>>>>>  1 file changed, 8 insertions(+)
>>>>>>>
>>>>>>> diff --git a/Documentation/devicetree/bindings/spi/st,stm32-qspi.yaml b/Documentation/devicetree/bindings/spi/st,stm32-qspi.yaml
>>>>>>> index 6ec6f556182f..5e4f9109799e 100644
>>>>>>> --- a/Documentation/devicetree/bindings/spi/st,stm32-qspi.yaml
>>>>>>> +++ b/Documentation/devicetree/bindings/spi/st,stm32-qspi.yaml
>>>>>>> @@ -46,6 +46,14 @@ properties:
>>>>>>>        - const: tx
>>>>>>>        - const: rx
>>>>>>>  
>>>>>>> +  st,dual-flash:
>>>>>>> +    type: boolean
>>>>>>> +    description:
>>>>>>> +      Allows to use 8 data lines in case cs-gpios property is defined.
>>>>>>
>>>>>> It's named dual-flash, but what if you want to use QSPI to connect for
>>>>>> example to FPGA?
>>>>>>
>>>>>> Also how is this related to parallel-memories property?
>>>>>
>>>>> I called it "dual-flash" simply because it enable the dual flash feature of the QSPI block (bit CR_DFM : Dual Flash Mode)
>>>>> which allows to use the 8 lines simultaneously of our dual QSPI block.
>>>>
>>>> And how is it related to existing parallel-memories property?
>>>
>>> Maybe I was not specific enough, so let me rephrase - we have already
>>> parallel-memories property. How this one is different (to justify the
>>> new property)? Is just one memory connected in your case to QSPI over 8
>>> data lines?
>>
>> Our QSPI block is a dual Quad-SPI memory interface, it can managed 2 QSPI flashes independently.
>> There is a specific mode, dual flash mode, where the 2 QSPI flashes can be accessed simultaneously 
>> using 8 data lines. In this case, both throughput and capacity are two fold with this mode.
>>
>> To illustrate, you can have a look at STM32MP157 reference manual (chapter 27.3.1 figure 172) available here:
>> https://www.st.com/resource/en/reference_manual/rm0436-stm32mp157-advanced-armbased-32bit-mpus-stmicroelectronics.pdf 
>>
>> As you mentioned above, the goal is to connect a FPGA to this 8 lines bus.
>>
>> Hope it clarifies enough ;-)
> 
> Only partially. Based on the explanation above, it looks like you want
> to use existing parallel-memories property. Please use that one, instead
> of adding a new property for the same feature.

Okay, got it, i didn't notice the parallel-memories property recently added. 

Thanks for pointing it.
Patrice

> 
> Best regards,
> Krzysztof