Date:   Tue, 27 Jun 2017 12:32:26 -0700 (PDT)
From:   matthew.gerlach@...ux.intel.com
To:     Marek Vasut <marek.vasut@...il.com>
cc:     vndao@...era.com, dwmw2@...radead.org, computersforpeace@...il.com,
        boris.brezillon@...e-electrons.com, richard@....at,
        cyrille.pitchen@...ev4u.fr, robh+dt@...nel.org,
        mark.rutland@....com, linux-mtd@...ts.infradead.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        gregkh@...uxfoundation.org, davem@...emloft.net, mchehab@...nel.org
Subject: Re: [PATCH 1/3] ARM: dts: Bindings for Altera Quadspi Controller
 Version 2



On Tue, 27 Jun 2017, Marek Vasut wrote:

> On 06/27/2017 07:18 PM, matthew.gerlach@...ux.intel.com wrote:
>>
>>
>> On Tue, 27 Jun 2017, Marek Vasut wrote:
>>
>>> On 06/27/2017 05:57 PM, matthew.gerlach@...ux.intel.com wrote:
>>>>
>>>>
>>>> On Tue, 27 Jun 2017, Marek Vasut wrote:
>>>>
>>>>> On 06/27/2017 04:32 PM, matthew.gerlach@...ux.intel.com wrote:
>>>>>>
>>>>>>
>>>>>> On Tue, 27 Jun 2017, Marek Vasut wrote:
>>>>>>
>>>>>> Hi Marek,
>>>>>>
>>>>>> Thanks for the feedback.  See my comments below.
>>>>>>
>>>>>> Matthew Gerlach
>>>>>>
>>>>>>> On 06/26/2017 06:13 PM, matthew.gerlach@...ux.intel.com wrote:
>>>>>>>> From: Matthew Gerlach <matthew.gerlach@...ux.intel.com>
>>>>>>>>
>>>>>>>> Device Tree bindings for Version 2 of the Altera Quadspi Controller
>>>>>>>> that can be optionally paired with a windowed bridge.
>>>>>>>>
>>>>>>>> Signed-off-by: Matthew Gerlach <matthew.gerlach@...ux.intel.com>
>>>>>>>> ---
>>>>>>>>  .../devicetree/bindings/mtd/altera-quadspi-v2.txt  | 37
>>>>>>>> ++++++++++++++++++++++
>>>>>>>>  1 file changed, 37 insertions(+)
>>>>>>>>  create mode 100644
>>>>>>>> Documentation/devicetree/bindings/mtd/altera-quadspi-v2.txt
>>>>>>>>
>>>>>>>> diff --git
>>>>>>>> a/Documentation/devicetree/bindings/mtd/altera-quadspi-v2.txt
>>>>>>>> b/Documentation/devicetree/bindings/mtd/altera-quadspi-v2.txt
>>>>>>>> new file mode 100644
>>>>>>>> index 0000000..8ba63d7
>>>>>>>> --- /dev/null
>>>>>>>> +++ b/Documentation/devicetree/bindings/mtd/altera-quadspi-v2.txt
>>>>>>>> @@ -0,0 +1,37 @@
>>>>>>>> +* Altera Quad SPI Controller Version 2
>>>>>>>> +
>>>>>>>> +Required properties:
>>>>>>>> +- compatible : Should be "altr,quadspi-v2".
>>>>>>>> +- reg : Contains at least two entries, and possibly three entries,
>>>>>>>> each of
>>>>>>>> +    which is a tuple consisting of a physical address and length.
>>>>>>>> +- reg-names : Should contain the names "avl_csr" and "avl_mem"
>>>>>>>> corresponding
>>>>>>>> +          to the control and status registers and qspi memory,
>>>>>>>> respectively.
>>>>>>>> +
>>>>>>>> +
>>>>>>>> +The Altera Quad SPI Controller Version 2 can be paired with a
>>>>>>>> windowed bridge
>>>>>>>> +in order to reduce the footprint of the memory interface.  When a
>>>>>>>> windowed
>>>>>>>> +bridge is used, reads and writes of data must be 32 bits wide.
>>>>>>>> +
>>>>>>>> +Optional properties:
>>>>>>>> +- reg-names : Should contain the name "avl_window", if the windowed
>>>>>>>> bridge
>>>>>>>> +          is used.  This name corresponds to the register space
>>>>>>>> that
>>>>>>>> +          controls the window.
>>>>>>>> +- window-size : The size of the window which must be an even power
>>>>>>>> of 2.
>>>>>>>> +- read-bit-reverse : A boolean indicating the data read from the
>>>>>>>> flash should
>>>>>>>> +             be bit reversed on a byte by byte basis before being
>>>>>>>> +             delivered to the MTD layer.
>>>>>>>> +- write-bit-reverse : A boolean indicating the data written to the
>>>>>>>> flash should
>>>>>>>> +              be bit reversed on a byte by byte basis.
>>>>>>>
>>>>>>> Is there ever a usecase where you need to set just one of these
>>>>>>> props ?
>>>>>>> Also, they're altera specific, so altr, prefix should be added.
>>>>>>
>>>>>> In general, I think if bit reversal is required, it would be
>>>>>> required in
>>>>>> both directions.  However, anything is possible when using FPGAs.  So
>>>>>> I thought separate booleans would be future proofing the bindings.
>>>>>
>>>>> Maybe we should drop this whole thing and add it when this is actually
>>>>> required.
>>>>>
>>>>> Are there any users of this in the wild currently ?
>>>>>
>>>>> What is the purpose of doing this per-byte bit reverse instead of
>>>>> storing th bits in the original order ?
>>>>
>>>> Hi Marek,
>>>>
>>>> Yes, there is hardware that has been in the wild for years that needs
>>>> this bit reversal.  The specific use case is when a flash chip is
>>>> connected to
>>>> a FPGA, and the contents of the flash is used to configure the FPGA on
>>>> power up.  In this use case, there is no processor involved with
>>>> configuring the FPGA.  I am most familiar with this feature/bug with
>>>> Altera FPGAs, but I believe this issue exists with other programmable
>>>> devices.
>>>
>>> So the EPCQ/EPCS flash stores the bitstream in reverse or something ?
>>> What are you storing in that flash except for the bitstream, filesystem?
>>> Feel free to go into details, I believe it'd be useful to know exactly
>>> what the problem is you're trying to solve here.
>>
>> Hi Marek,
>>
>> I am trying to write an MTD/spi-nor driver for version 2 of the
>> Altera Quadspi contoller.  This controller is soft IP that is deployed
>> in a FPGA.  As such, this component/driver can be used in wide range of
>> use cases.  The controller could be used to update EPCQ/EPCS flash
>> stores containing bit streams, but this component could be used for
>> flash for filesystems or any non-volatile data store.  My hope is that
>> all possible use cases should be covered by this driver.
>
> How does this particular case where you have to reverse the bits look like ?

The use case for reversing the bits involves a processor updating 
EPCQ/EPCS flash whose contents are read by the FPGA on power up.  The 
processor and Altera Quadspi component, inside the configured FPGA, access 
the bits in one way serially, but the hardware that accesses the flash 
during power accesses the bits in the opposite way serially.

>
>>>>>> Thinking about this binding more, I wonder if the binding name(s)
>>>>>> should be (read|write)-bit8-reverse to indicate reversings the bits
>>>>>> in a byte as opposed to reversing the bits in a 32 bit word?
>>>>>>
>>>>>> I don't think bit reversal is specific to Altera/Intel components.
>>>>>> I see
>>>>>> a nand driver performing bit reversal, and I think I've recently seen
>>>>>> other FPGA based drivers requiring bit reversal.
>>>>>
>>>>> $ git grep bit.reverse Documentation/devicetree/ | wc -l
>>>>> 0
>>>>>
>>>>> So we don't have such a generic binding . It's up to Rob (I guess) to
>>>>> decide whether this is SoC specific and should've altr, prefix or not.
>>>>> IMO it is.
>>>>
>>>> I agree there is no generic binding at this time, and I look forward
>>>> to any input from Rob and anyone else on this issue.  I think it is
>>>> worth pointing out that this really isn't an issue of an SoC, but rather
>>>> it is an
>>>> issue of how data in the flash chip is accessed.I think what makes
>>>> this issue
>>>> "weird" is that we have different hardware accessing the data in the
>>>> flash with a different perspective.  The FPGA looks at the data from one
>>>> perspective on power up, and a processor trying to update the flash has
>>>> a different perspective.
>>>
>>> Another thing I'd ask here is, is that bit-reverse a hardware property
>>> or is that some software configuration thing ?
>>
>> I would say the bit reversal is a property of the FPGA that is reading
>> the flash at power up.
>
> So it's not a property of the block, but rather of the bus somewhere ?

You are correct, it is not a property of the Altera Quadspi component, but
a property of the fpga and external hardware that access the flash on 
power up.

Thanks again,
Matthew Gerlach

>
> -- 
> Best regards,
> Marek Vasut
>

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