RE: [RFC PATCH] drivers: ata: Read Rx water mark value from device-tree

[Anurag Kumar Vulisha]

Hi Arnd,

> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd@xxxxxxxx]
> Sent: Tuesday, February 23, 2016 3:51 PM
> To: Anurag Kumar Vulisha
> Cc: robh+dt@xxxxxxxxxx; pawel.moll@xxxxxxx; mark.rutland@xxxxxxx;
> ijc+devicetree@xxxxxxxxxxxxxx; galak@xxxxxxxxxxxxxx; tj@xxxxxxxxxx;
> devicetree@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-
> ide@xxxxxxxxxxxxxxx; 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:
> > >
> > > 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.
> > 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.
> >
> 
> 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.

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.

Thanks,
Anurag Kumar V