Messages in this thread | | | Date | Fri, 6 Jul 2018 22:54:50 +0200 | From | Fredrik Noring <> | Subject | Re: [PATCH] dma-mapping: Relax warnings for per-device areas |
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Hi Robin,
> > Does dma_set_coherent_mask want a device object representing the IOP? Such > > a thing is currently not implemented, but can certainly be done. > > Nope, just the same OHCI device as the dma_declare_coherent_memory() call.
Ah... and then some kind of dma_ops structure is needed to avoid -EIO? Possibly using set_dma_ops()?
By the way, the DMA hardware supports executing device->{memory,FIFO}, {memory,FIFO}->device and FIFO<->memory simultaneously, with hardware stall control, between different devices where memory or FIFOs act as intermediate buffers. Memory can also be a source or a destination.
That might be a way to have the OHCI efficiently transfer data from/to main memory. I suppose it would be two simultaneously linked DMA transfers such as
OHCI <-> SIF <-> main memory
or even three linked DMA transfers as in
OHCI <-> IOP memory <-> SIF <-> main memory
where SIF is the IOP DMA interface, which is a bidirectional hardware FIFO.
Does the kernel DMA subsystem support simultaneously linked DMA transfers? In addition, the DMA hardware also supports scatter-gather (chaining), so memory need not be physically continuous.
> > As you noted, the kernel cannot and must not allocate any kind of normal > > memory for this device. Typical DMA addresses 0-0x200000 are mapped to > > 0x1c000000-0x1c200000 and is memory managed exclusively by the IOP. What > > would be a suitable mask for that? > > For the sake of accuracy, I guess maybe DMA_BIT_MASK(20) since that's what > the OHCI's effective addressing capability is, even if it does happen to be > to remote IOP RAM.
Isn't it 21 for 2 MiB? Hmm... I'm considering raising that to 8 MiB since there are such devices too.
> Alternatively, there is perhaps some degree of argument > for deliberately picking a nonzero but useless value like 1, although it > looks like the MIPS allocator (at least the dma-default one) never actually > checks whether the page it gets is within range of the device's coherent > mask, which it probably should do.
Perhaps Maciej knows more about the details of the MIPS allocator?
Fredrik
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