Hi Rob,

-----Original Message-----
From: Rob Herring [mailto:robh@kernel.org]
Sent: Wednesday, February 24, 2016 1:00 AM
To: Anurag Kumar Vulisha
Cc: Arnd Bergmann; pawel.moll@arm.com; mark.rutland@arm.com;
ijc+devicetree@hellion.org.uk; galak@codeaurora.org; tj@kernel.org;
devicetree@vger.kernel.org; linux-kernel@vger.kernel.org; linux-
ide@vger.kernel.org; Anirudha Sarangi; Srikanth Vemula; Punnaiah Choudary
Kalluri
Subject: Re: [RFC PATCH] drivers: ata: Read Rx water mark value from device-
tree

On Tue, Feb 23, 2016 at 03:29:55PM +0000, Anurag Kumar Vulisha wrote:

quoted

Hi Arnd,

quoted

-----Original Message-----
From: Arnd Bergmann [mailto:arnd@arndb.de]
Sent: Tuesday, February 23, 2016 3:51 PM
To: Anurag Kumar Vulisha
Cc: robh+dt@kernel.org; pawel.moll@arm.com; mark.rutland@arm.com;
ijc+devicetree@hellion.org.uk; galak@codeaurora.org; tj@kernel.org;
devicetree@vger.kernel.org; linux-kernel@vger.kernel.org; linux-
ide@vger.kernel.org; Anirudha Sarangi; Srikanth Vemula; Punnaiah
Choudary Kalluri
Subject: Re: [RFC PATCH] drivers: ata: Read Rx water mark value from
device- tree

On Tuesday 23 February 2016 05:58:32 Anurag Kumar Vulisha wrote:

quoted

quoted

I don't know what is appropriate because I have no idea what
Rxwatermark is good for. Can you try describing why we can't
just set it to the correct value for everyone automatically?

This RX watermark level sets the minimum number of free locations
within the RX FIFO .When the rx fifo level crosses the programmed
watermark level ,sata controller  will transmit HOLDS to the
device asking it

to wait. This happens when dma reads the rx fifo data slower than
the device is sending the data. Note that it can take some time for
the HOLDs to get to the other end and in the time there must be
enough room in the FIFO to absorb all data that could arrive from the

device.

quoted

quoted

quoted

Currently we are using 0x40 for this value, which works fine with
all hardware designs  we are currently having. But hoping that
this value may vary for future silicon versions, I wanted to make
this as a configurable

value. So for this reason I thought of moving it either to
device-tree or making it as a module_param() property.

quoted

Ok, so if this depends on the silicon version, your initial approach
would be better than the module_param.

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.

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.

Since we are  currently having one value in common for all the
hardware and also changing the rx water mark does not require any
changes other than vendor specific PTC register update , I think it
would be better  to  use device tree property for that rx watermark value.

Doing this makes the updating of rx watermark value easy, if any changes
required.

quoted

In future, if any silicon version rx water mark value doesn't work
with the current versions, then  I will do as you said by maintaining
the table in device driver. But at present  I feel that single rx
watermark property in device tree would be enough, since it works with all

the hardware versions we have.

If you currently have no reason to modify it now, then add it later when you
actually have a use case.

This property may vary from board to board . It also depends on the phy reference clock
frequency which can be changed based on use case. So moving this property to device
tree will make it easy for the  user  to  configure  this property based on the requirement.

Thanks,
Anuarg Kumar V