Re: [PATCH] ARM: bcm2835: Use 0x4 prefix for DMA bus addresses to SDRAM.

[Noralf TrÃnnes]

Den 05.05.2015 02:07, skrev Eric Anholt:
> Noralf TrÃnnes <noralf@xxxxxxxxxxx> writes:
>
> > Den 04.05.2015 21:33, skrev Eric Anholt:
> > > There exists a tiny MMU, configurable only by the VC (running the
> > > closed firmware), which maps from the ARM's physical addresses to bus
> > > addresses.  These bus addresses determine the caching behavior in the
> > > VC's L1/L2 (note: separate from the ARM's L1/L2) according to the top
> > > 2 bits.  The bits in the bus address mean:
> > >
> > >   From the VideoCore processor:
> > > 0x0... L1 and L2 cache allocating and coherent
> > > 0x4... L1 non-allocating, but coherent. L2 allocating and coherent
> > > 0x8... L1 non-allocating, but coherent. L2 non-allocating, but coherent
> > > 0xc... SDRAM alias. Cache is bypassed. Not L1 or L2 allocating or coherent
> > >
> > >   From the GPU peripherals (note: all peripherals bypass the L1
> > > cache. The ARM will see this view once through the VC MMU):
> > > 0x0... Do not use
> > > 0x4... L1 non-allocating, and incoherent. L2 allocating and coherent.
> > > 0x8... L1 non-allocating, and incoherent. L2 non-allocating, but coherent
> > > 0xc... SDRAM alias. Cache is bypassed. Not L1 or L2 allocating or coherent
> > >
> > > The 2835 firmware always configures the MMU to turn ARM physical
> > > addresses with 0x0 top bits to 0x4, meaning present in L2 but
> > > incoherent with L1.  However, any bus addresses we were generating in
> > > the kernel to be passed to a device had 0x0 bits.  That would be a
> > > reserved (possibly totally incoherent) value if sent to a GPU
> > > peripheral like USB, or L1 allocating if sent to the VC (like a
> > > firmware property request).  By setting dma-ranges, all of the devices
> > > below it get a dev->dma_pfn_offset, so that dma_alloc_coherent() and
> > > friends return addresses with 0x4 bits and avoid cache incoherency.
> > >
> > > This matches the behavior in the downstream 2708 kernel (see
> > > BUS_OFFSET in arch/arm/mach-bcm2708/include/mach/memory.h).
> > >
> > > Signed-off-by: Eric Anholt <eric@xxxxxxxxxx>
> > > Cc: popcornmix@xxxxxxxxx
> > > ---
> > >    arch/arm/boot/dts/bcm2835.dtsi | 1 +
> > >    1 file changed, 1 insertion(+)
> > >
> > > diff --git a/arch/arm/boot/dts/bcm2835.dtsi b/arch/arm/boot/dts/bcm2835.dtsi
> > > index 5734650..2df1b5c 100644
> > > --- a/arch/arm/boot/dts/bcm2835.dtsi
> > > +++ b/arch/arm/boot/dts/bcm2835.dtsi
> > > @@ -15,6 +15,7 @@
> > >    		#address-cells = <1>;
> > >    		#size-cells = <1>;
> > >    		ranges = <0x7e000000 0x20000000 0x02000000>;
> > > +		dma-ranges = <0x40000000 0x00000000 0x1f000000>;
> > >    
> > >    		timer@7e003000 {
> > >    			compatible = "brcm,bcm2835-system-timer";
> > This was quite a coincidence. I discovered the need for 'dma-ranges'
> > yesterday while trying to get the downstream bcm2708_fb driver to
> > work with ARCH_BCM2835. The driver is using the mailbox to get info
> > about the framebuffer from the firmware. When it failed I discovered
> > that the bus address was wrong.
> >
> > What I don't understand, is that mmc and spi works fine with a "wrong"
> > bus address. It's only the framebuffer driver and the vchiq driver
> > when using mailbox that fails.
> >
> > Tested-by: Noralf TrÃnnes <noralf@xxxxxxxxxxx>
> Yeah, it was the mailbox driver I've been trying to merge, on pi2, that
> made me get this patch together.  I'm suspicious that 0x0 works the same
> as 0x4 for GPU peripherals (mmc, spi, vc4) on pi1, though I've had
> occasional instability (something like 3 events per ~5000 tests) that I
> sure hope is due to this.

Since you mention Pi2:
Dom Cobley made me aware that 0xC is used on MACH_BCM2709.
The macros in arch/arm/mach-bcm270X/include/mach/memory.h are identical,
but arch/arm/mach-bcm2709/Kconfig has BCM2708_NOL2CACHE as default (as 
opposed to 2708/Kconfig).
This changes the _REAL_BUS_OFFSET macro:

#ifdef CONFIG_BCM2708_NOL2CACHE
 #define _REAL_BUS_OFFSET UL(0xC0000000)   /* don't use L1 or L2 caches */
#else
 #define _REAL_BUS_OFFSET UL(0x40000000)   /* use L2 cache */
#endif


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