Hi Arnd,

On Tue, 29 Sep 2015, Arnd Bergmann wrote:

On Tuesday 29 September 2015 13:11:55 Peter Griffin wrote:

quoted

Hi Arnd,

On Tue, 29 Sep 2015, Arnd Bergmann wrote:

quoted

On Tuesday 29 September 2015 11:04:40 Peter Griffin wrote:

quoted

"The hardware is identical, and different firmware is used to apply
   it in different ways."

Which is the case with fdma. By encoding the "way you wish to apply it" into the
compatible string, it causes problems if you want to change for example fdma0
to do some other function other than audio.

You then require a DT update, (when the hardware hasn't changed, just the
firmware) which is the same problem as using the filename directly in DT.

Therefore I believe it is important that the DT binding does *not* encode the
way the hardware is to be applied into the binding in *any* way, and defers this
 decision to the driver.
That is the rationale / reasoning behind choosing the fdma instance number.

Assuming you agree with my arguments above, then the choice becomes between 
having a fdma instance DT property, or having lots of compatibles where the only
difference is the appending of the instance number. I think out of the two I prefer
my original approach.

Any thoughts from the DT folks?

To me both approaches sound wrong: basing the firmware name on the instance
number requires that each instance is always used in the same way, which
is not guaranteed to be the case,

Does it? I didn't think it did.

Using the instance number as a DT property defers the decision over what firmware to
load to the driver, which can choose whatever firmware name it wishes.

e.g. in v4.3 it could load xyz.elf, in v4.4 it could choose abc.elf. The DT will remain
unchanged, but the use of that fdma instance has changed.

We currently only have one firmware for each instance with the "use" compiled into it.
If in the future we had two firmwares with different "uses" for the same instance some extra
logic would be required in the driver to make a decision on which firmware to load.

Ok, I probably need some more background about what the firmware on this
device does, and what it could do with a different firmware. Could you
elaborate?

So the fdma hw is a dma engine based around a xp70 slim core. It supports: -
* block memory move between 2 system locations
* paced transfer from system memory to paced peripheral
* paced transfer from a paced peripheral to system memory

I believe each firmware for each instance supports those 3 things.

However the xp70 also has some ancilary HW to facilitate Start Code Detection.
It is this feature which I believe would require a different firmware if we wanted to make
use of it. Looking at the functional spec each xp70 also
has 16 gp output signals which we could also control from the firmware. Whether
these are actually connected to anything useful inside the SoC I don't know.

quoted

quoted

and you correctly describe the problem with
using the compatible string for the firmware name if the driver for the FDMA
does not actually care what firmware is being used here.

Whatever code makes the decision as to how the FDMA is used should also
decide on the name of the firmware file.

The code which makes this decision currently is the st_fdma.c driver. However it does
need to know which fdma controller it is operating on to make this decision correctly.

Apart from passing the fdma instance number in DT, how else can we determine which
controller we are?

I guess we could infer it by having a table in the driver containing the base addresses
of the controllers for a given SoC, and match that against what DT passes us in the
reg property. But that seems ugly, and is encoding the same information in two
different places.

I'm open to suggestions if there is a better way to do this.

Using the address would be the same thing, that doesn't change the
fundamental logic. Can you explain why it matters which instance
a firmware is used on for this driver?

The reason we care, is that each instance has its own firmware file.

I just did a hexdump on the 3 different firmwares and compared them. Although the majority
of the binary is the same, there are various bytes which change at several different offsets
in the firmware file depending on the instance.

I don't have a xp70 toolchain or know enough about the cpu architecture to analyze what exactly
the firmware is doing at these locations.

regards,

Peter.