Date:	Thu, 6 Aug 2015 22:33:40 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Vignesh R <vigneshr@...com>, Michal Suchanek <hramrach@...il.com>,
	Mark Brown <broonie@...nel.org>,
	devicetree <devicetree@...r.kernel.org>,
	Brian Norris <computersforpeace@...il.com>,
	Tony Lindgren <tony@...mide.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-spi <linux-spi@...r.kernel.org>,
	Huang Shijie <b32955@...escale.com>,
	MTD Maling List <linux-mtd@...ts.infradead.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	David Woodhouse <dwmw2@...radead.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 1/5] spi: introduce flag for memory mapped read

On Thu, Aug 06, 2015 at 06:14:00PM +0200, Geert Uytterhoeven wrote:
> On Thu, Aug 6, 2015 at 3:51 PM, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
> > On Thu, Aug 06, 2015 at 05:55:23PM +0530, Vignesh R wrote:
> >> On the whole following are my requirements:
> >> 1. to be able to communicate with non -flash SPI devices via config port
> >> ( this functionality is supported by current driver, I dont want to
> >> break it). Or pump any spi_message on to SPI bus directly.
> >> 2. take advantage of memory mapped port in order to increase read
> >> throughput( and use dma in future) when the slave is a m25p80 type flash.
> >> 3. handle m25p80 as well as other slave on multiple chipselects.
> >>
> >> I just need to know whether the user that requested the transfer is
> >> m25p80 driver. If yes, ti-qspi driver can take advantage of memory
> >> mapped interface, else just use config port to access SPI bus directly.
> >
> > The problem with this approach is that it's an abomination.  It's adding
> > a SPI-user specific hack which is detected by a specific driver.  That's
> > really not sane - what happens when we have lots of these kinds of "I'm
> > an X SPI-user" with drivers detecting that?  It's not maintainable in the
> > long term.
> >
> > Yes, your requirements _today_ seem simple and easy, but you're only
> > thinking about today, not tomorrow when you've moved on and someone else
> > has to maintain the mess left behind (or delete it from mainline because
> > they're sick of dealing with a hack.)
> >
> >> The spi_message that is received in transfer_one_message() is too
> >> generic to imply the slave device that is on the other side of the wire.
> >> IMO, the read command does not imply that the slave is m25p80 flash
> >> (besides the read opcodes vary across vendors of m25p80 and across modes).
> >
> > I can see both sides of the argument.
> >
> > Mark is saying: if the SPI driver detects that the message to be transmitted
> > is a read command followed by the appropriate number of dummy bytes, and
> > then the data being read _and_ it's using quad-mode access, and the hardware
> > generates _exactly_ that in hardware using the memory mapped mode, there is
> > no reason _not_ to use the hardware to achieve that SPI transaction.  The
> > bus activity will be identical to what happens when the SPI controller is
> > used manually to achieve that bus sequence.
> >
> > You're saying: but the documentation says you can't use it for anything
> > except m25p80.  If you look at 24.5.4.1.2, it tells you what the SFI
> > generates on the bus, which is:
> >
> > 1. CS active
> > 2. Read command byte sent
> > 3. 1-4 address bytes sent
> > 4. 0-3 dummy bytes sent
> > 5. data bytes read from bus
> > 6. CS inactive
> >
> > So, Mark's point is "if we can detect a transaction which fits _that_
> > bus activity, there's no reason not to use this acceleration for the
> > transaction."
> >
> > What you're failing to counter with is: we don't have enough information
> > in the SPI driver to know how many dummy bytes there are between the
> > address bytes and the data read from the bus.
> 
> Irrespective of the dummy bytes.
> What if the spi device is not a FLASH ROM, but some other device,
> which receives a data packet that accidentally looks like an m25p80 READ
> command?

Well, for the most part it looks like it should still work, but there
could be a gotcha, but first, let's get rid of a myth there.

The QSPI is _not_ specific to the M25P80.  The manual says nothing
about being specific to that device.  What it says is that it's for
SPI NOR memory.  It will work with bus widths of 1, 2 or 4 data lines,
so it probably works with non-M25P80 SPI NOR devices too - and the fact
that the read and write commands are completely programmable suggests
that using it with SPI NOR devices which do not use the M25P80 read
command value is intended.


The SFI is a state machine based translator which sits behind the SPI
interface (look at the manual).  It sequence sthe SPI bus through a
series of standard SPI states which happen to be the states I detailed
above.

Now, the first byte of the SFI-generated SPI message can be programmed
to any 8 bit value.  So the first byte of the SPI message is totally
under software control.  The next one to four bytes which comprise the
"address" can be controlled to by deciding where in the memory map to
start reading from.  Hence, the value of those bytes is also totally
under software control.  The number of dummy bytes can be programmed
too.  So far so good.

So, if we know that we have a SPI message which says "send 0x01 0x20
0x30, send one dummy byte, read 32 bytes", if we program the SFI to
send a read command as 0x01, program an address length of 2 bytes
with one dummy byte, and then read the next 32 bytes at the appropriate
offset in the memory mapping to cause the next two bytes to be 0x20,
0x30, then what we end up with on the bus is:

	send 0x01, 0x20, 0x30
	send one dummy byte

That much is good, but now is the problem - how does the SFI know that
we're going to require to read 32 bytes?  I think the answer to that
is that it doesn't know, so it probably just reads the number of bytes
which the access on the SoC bus is asking for, which makes it
indeterminant from a software point of view to control how many bytes
will be read without provoking another "send 0x01, next address, dummy
byte" sequence.

So, I'm now on the side of not parsing commands in the SPI driver, and
back on the idea that this needs to be handled in some other manner
which doesn't involve polluting the SPI core with flag-hacks.

-- 
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