Message-ID: <alpine.DEB.2.20.1706290748230.4589@mgerlach-VirtualBox>
Date:   Thu, 29 Jun 2017 08:03:46 -0700 (PDT)
From:   matthew.gerlach@...ux.intel.com
To:     Marek Vasut <marek.vasut@...il.com>
cc:     Rob Herring <robh@...nel.org>, vndao@...era.com,
        dwmw2@...radead.org, computersforpeace@...il.com,
        boris.brezillon@...e-electrons.com, richard@....at,
        cyrille.pitchen@...ev4u.fr, 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 Thu, 29 Jun 2017, Marek Vasut wrote:

> On 06/29/2017 01:09 AM, Rob Herring wrote:
>> On Tue, Jun 27, 2017 at 08:57:14AM -0700, 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.
>>>
>>>>
>>>>> 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.
>>
>> Given the comment that it is reversing bits in each byte, that seems
>> fairly Altera specific. I'd be more in favor of a generic property if it
>> was flipping all the bits in a word (for any size word).
>
> Actually, I'd prefer to fix up the FPGA bitstream in software and then
> write the fixed up bitstream into the flash. That way there's no need
> for any such DT property and other FPGA vendors (ie. xilinx) do it that
> way already.
>
> -- 
> Best regards,
> Marek Vasut
>

While I totally understand Marek's point of view regarding bit flipping,
I think it is instructive to explore the other point of view.  For me the 
issue of flipping the bits in user space versus kernel space came down to 
laziness, which along with impatience and hubris are considered by some to 
be good traits for a software engineer.  I looked around for an existing user 
space to perform the function, but I did not find an existing tool, but I 
suppose I could look more closely at Xilinx's tools.  I do not want to 
create my own tool, and then it was pointed out to me that the necessary 
bit flipping functions already exist in the kernel.

Even if we decided that performing the bit flipping in the kernel makes 
sense, my original binding proposal is clearly inadequate.  As Rob pointed 
such a binding should be more generic to support flipping the bits in any word 
size (i.e. 8, 16, 32, 64 ...).  Additionally, Marek has pointed out that a 
portion of the flash could requiring flipping, and another portion of the 
same flash would not require flipping.  Therefore, a bit flipping binding 
is not a property of the flash controller, but rather it is a property of 
a particular flash partition.

Matthew Gerlach

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