linux-kernel - Re: [PATCH] dma-mapping: Relax warnings for per-device areas

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Date:   Fri, 6 Jul 2018 22:54:50 +0200
From:   Fredrik Noring <noring@...rew.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org,
        "Maciej W. Rozycki" <macro@...ux-mips.org>, JuergenUrban@....de
Subject: Re: [PATCH] dma-mapping: Relax warnings for per-device areas

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