Subject: Re: [PATCH 00/11] of: Fix DMA configuration for non-DT masters

From: Nicolas Saenz Julienne <[email-masked]>
Date: Thu, 26 Sep 2019 12:44:53 +0200

> > > > Robin, have you looked into supporting multiple dma-ranges? It's the
> > > > next thing
> > > > we need for BCM STB's PCIe. I'll have a go at it myself if nothing is in
> > > > the
> > > > works already.
> > > 
> > > Multiple dma-ranges as far as configuring inbound windows should work
> > > already other than the bug when there's any parent translation. But if
> > > you mean supporting multiple DMA offsets and masks per device in the
> > > DMA API, there's nothing in the works yet.

Sorry, I meant supporting multiple DMA offsets[1]. I think I could still make
it with a single DMA mask though.

> > There's also the in-between step of making of_dma_get_range() return a
> > size based on all the dma-ranges entries rather than only the first one
> > - otherwise, something like [1] can lead to pretty unworkable default
> > masks. We implemented that when doing acpi_dma_get_range(), it's just
> > that the OF counterpart never caught up.
> 
> Right. I suppose we assume any holes in the ranges are addressable by
> the device but won't get used for other reasons (such as no memory
> there). However, to be correct, the range of the dma offset plus mask
> would need to be within the min start and max end addresses. IOW,
> while we need to round up (0xa_8000_0000 - 0x2c1c_0000) to the next
> power of 2, the 'correct' thing to do is round down.

IIUC I also have this issue on my list. The RPi4 PCIe block has an integration
bug that only allows DMA to the lower 3GB. With dma-ranges of size 0xc000_0000
you get a 32bit DMA mask wich is not what you need. So far I faked it in the
device-tree but I guess it be better to add an extra check in
of_dma_configure(), decrease the mask and print some kind of warning stating
that DMA addressing is suboptimal.

Regards,
Nicolas

[1] https://lkml.org/lkml/2018/9/19/641

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