Forcing PIO mode instead of DMA via DT property

[Aggrwal Poonam-B10812]

> -----Original Message-----
> From: devicetree-discuss [mailto:devicetree-discuss-
> bounces+poonam.aggrwal=freescale.com at lists.ozlabs.org] On Behalf Of Marek
> Vasut
> Sent: Monday, July 23, 2012 9:01 AM
> To: Aggrwal Poonam-B10812
> Cc: pavel at denx.de; devicetree-discuss at lists.ozlabs.org; Wolfgang Denk
> Subject: Re: Forcing PIO mode instead of DMA via DT property
> 
> Dear Aggrwal Poonam-B10812,
> 
> > > -----Original Message-----
> > > From: devicetree-discuss [mailto:devicetree-discuss-
> > > bounces+poonam.aggrwal=freescale.com at lists.ozlabs.org] On Behalf Of
> > > bounces+Marek
> > > Vasut
> > > Sent: Monday, July 23, 2012 2:58 AM
> > > To: devicetree-discuss at lists.ozlabs.org
> > > Cc: pavel at denx.de; Wolfgang Denk
> > > Subject: Forcing PIO mode instead of DMA via DT property
> > >
> > > Hi guys,
> > >
> > > I'm trying to implement a driver that can do both DMA and PIO, and
> > > it would be nice if the user was able to select the mode (on a
> > > per-bus
> > > basis) using the DT.
> > > The PIO mode can reduce the overhead in some cases and therefore be
> > > better choice than the DMA (for example when most transfers move
> > > only very few data, or when board-specific hardware properties kick
> in).
> > >
> > > I was thinking about using some "manf,use-pio" DT property, but I
> > > haven't found any such example yet, so I wonder if this is a good
> idea.
> >
> > Not sure, but as far as I understand device tree is to publish the
> > hardware capabilities, not the user choice/configuration.
> 
> This might actually be one of the things I've been missing about DT, so
> thanks for stating that clearly :-)
> 
> > One option would be to
> > put a compile time flag in the driver, which prefers PIO than DMA. I
> > believe by default DMA is the first preference.
> 
> Certainly, but please consider you might want to use PIO only on selected
> instances of the driver (aka. selected busses, I wasn't probably clear
> enough, this is a bus driver), not degrade performance on all of them.
> Possibly due to a specific hardware wiring of the particular system.
Okay, if the choice to use PIO is for a specific controller(bus) because of hardware reasons, may be device tree could be a place.
But if it is simply that the user wants PIO mode for reasons like "less bytes to transfer" then device tree may not be the right place.

Regards
Poonam

> 
> > Regards
> > Poonam
> 
> Best regards,
> Marek Vasut
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