On 2.3.2016 10:11, Arnd Bergmann wrote:

On Wednesday 02 March 2016 09:05:49 Michal Simek wrote:

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On 2.3.2016 06:53, Anurag Kumar Vulisha wrote:

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I would probably make this dependent on the compatible string
instead, and have a table in the device driver that uses a
specific value for each variant of the device, but either way should be

fine.

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Having a separate property is most appropriate if for each
hardware revision there is exactly one ideal value, while a table
in the driver makes more sense if this takes a bit of tuning and
the driver might choose to optimize it differently based on other
constraints, such as its own interrupt handler implementation.

that 0x40 is value choose based on testing that it is not causing any
visible problem and this is used as default value in the driver
(PTC_RX_WM_VAL - ahci_ceva.c)

Values which you can setup are in range 0x0 - 0x7f (7bits). It means
hardware fifo size is probably 0x80.

And this dt/module parameter is IMHO just sw setting setup by user.
It means I am not quite sure that this is DT parameter because it is
just SW setting.
I expect this range will be valid for all silicon revisions.
If happen that any silicon revision can't setup certain level because of
HW bug we can handle it via DT parameter or specific compatible string.
But setting up watermark SW level via DT doesn't look correct to me.

Please let me know what you think.

Ok, thanks for the background. I think we should just leave it to be
set by the driver then. Please make sure that each SoC specific .dtsi
file has a unique "compatible" string for the device though, so that
the driver can later override it based on the specific variant if
that ends up being necessary for performance or bug-avoidance.

No problem with default value in driver. Something has to be setup.
Reset value based on reg spec I was checking is 0x20. Based on our
testing we saw some issues that's why 0x40 was setup as default value.
There is a need to be able to configure this value for example for
testing different values that's why I think module parameter should be
the right way to go.

If this should be DT parameters there should be different ceva IP which
allows different fifo size and different watermark level to be setup by
user.

What do you think? Does it sound reasonable.

Thanks,
Michal