Date:   Wed, 28 Jun 2017 18:09:28 -0500
From:   Rob Herring <robh@...nel.org>
To:     matthew.gerlach@...ux.intel.com
Cc:     Marek Vasut <marek.vasut@...il.com>, 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 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).

Rob

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