[PATCH 1/2] ARM: dts: rockchip: reserve unusable memory region on rk3288

[heiko@sntech.de (Heiko Stübner)]

Hi Doug,

Am Samstag, 1. August 2015, 14:32:32 schrieb Doug Anderson:
> On Sat, Aug 1, 2015 at 5:35 AM, Heiko St?bner <heiko@sntech.de> wrote:
> > The rk3288 has problems accessing the memory region 0xfe000000~0xff000000.
> > So block off the affected region to prevent its use.
> > 
> > Tested on 4GB Veyron Minnie and 2GB Veyron Jerry devices.
> > 
> > Signed-off-by: Heiko Stuebner <heiko@sntech.de>
> > ---
> > 
> >  arch/arm/boot/dts/rk3288.dtsi | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/arch/arm/boot/dts/rk3288.dtsi b/arch/arm/boot/dts/rk3288.dtsi
> > index 2db91c9..19766af 100644
> > --- a/arch/arm/boot/dts/rk3288.dtsi
> > +++ b/arch/arm/boot/dts/rk3288.dtsi
> > @@ -169,6 +169,16 @@
> > 
> >                 };
> >         
> >         };
> > 
> > +       reserved-memory {
> > +               #address-cells = <1>;
> > +               #size-cells = <1>;
> > +               ranges;
> > +
> > +               unusable at fe000000 {
> > +                       reg = <0xfe000000 0x1000000>;
> > +               };
> > +       };
> > +
> 
> I don't object to just reserving this memory, but I do object a little
> to this implementation.
> 
> It's 16 MB we're talking about here, which is pretty small compared to
> the 4G of memory that you must have when this is a problem.  However,
> at some point we might want to try to actually use this 16 MB.
> 
> The memory is actually _not_ unusable, it's only unusable for DMA.  In
> theory the kernel is supposed to have a way to mark memory as unusable
> for DMA which would then allow us to use this memory for non-DMA
> purposes...

Ok, the patch message in the chromeos-patch didn't mention dma, but I could've 
thought of that myself, sorry.


> Nobody ever managed to figure this out in the Chrome OS tree (it was
> 16 megs so we just reserved it and never got back around to it), but
> when folks were looking at it they looked at things like ZONE_DMA,
> dma_set_mask_and_coherent, etc.
> 
> In any case, to leave the door open for people to fix this in the
> future, it seems prudent to fix this in code like
> <https://chromium-review.googlesource.com/#/c/245107/> rather than to
> put the limit in DTS.


The dt code when creating the platform-devices assumes a 32bit dma mask and 
expects the drivers to set the correct dma_masks (in of_dma_configure). But 
from the original description this looks more like a limitation of the rk3288, 
not the individual dwmmc, dwc2, etc.

So from my cursory glance, ZONE_DMA seems to be the best solution. As you said 
that this was investigated too, do you know if they encountered any obstacles 
that resulted in not adjusting the dma zone?


@Jeffy: do you know how widespread this is? I.e. are socs like the rk3128 or 
possibly more importantly the rk3368 also affected by this in 4GB 
configurations?


If the rk3368 does it too, this might get "interesting". In one of the mails 
about ZONE_DMA from september 2014, Arnd mentioned that there was a patch 
about getting the zone size from devicetree floating around, but it doesn't 
look like this landed.


Heiko