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

[Anurag Kumar Vulisha <anurag.kumar.vulisha@xilinx.com>]

+ Michal

> -----Original Message-----
> From: Anurag Kumar Vulisha
> Sent: Friday, February 26, 2016 7:18 PM
> To: 'Rob Herring'
> 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
>
> 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:
> > > Hi Arnd,
> > >
> > > > -----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;
> > > > ijc+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:
> > > > > >
> > > > > > 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.
> >
> > 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


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