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Message-ID: <0765cae5-1d07-cbaa-83bf-5553f3f8b242@linaro.org>
Date:   Tue, 9 Aug 2022 10:57:03 +0300
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Subject: Re: [PATCH 1/3] dt-bindings: spi: stm32: Add st,dual-flash property
 in st,stm32-qspi.yaml
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To:     Patrice CHOTARD <patrice.chotard@foss.st.com>,
        Mark Brown <broonie@kernel.org>,
        Alexandre Torgue <alexandre.torgue@foss.st.com>,
        robh+dt@kernel.org, krzysztof.kozlowski+dt@linaro.org
Cc:     linux-spi@vger.kernel.org,
        linux-stm32@st-md-mailman.stormreply.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        christophe.kerello@foss.st.com, devicetree@vger.kernel.org
References: <20220808074051.44736-1-patrice.chotard@foss.st.com>
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 <a4310e1d-4eb3-0081-3cd2-8fd060a28bda@foss.st.com>
From:   Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
In-Reply-To: <a4310e1d-4eb3-0081-3cd2-8fd060a28bda@foss.st.com>
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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@foss.st.com wrote:
>>>>>> From: Patrice Chotard <patrice.chotard@foss.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@foss.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.

Best regards,
Krzysztof