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Date:   Tue, 9 Aug 2022 09:52:43 +0200
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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:     Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
        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>
 <20220808074051.44736-2-patrice.chotard@foss.st.com>
 <9ad4b4a8-988e-f185-f80c-6f15f341ce8c@linaro.org>
 <79fd7e19-ceef-14fb-5a83-603740735f8f@foss.st.com>
 <38c3977a-0196-1832-ff94-317064cbc439@linaro.org>
 <b15184d6-c9e7-d042-621b-ef4ccd3c87ba@linaro.org>
From:   Patrice CHOTARD <patrice.chotard@foss.st.com>
In-Reply-To: <b15184d6-c9e7-d042-621b-ef4ccd3c87ba@linaro.org>
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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 ;-)

Thanks
Patrice


> 
> Best regards,
> Krzysztof