Hi Joel,

On Tue, Dec 07, 2021 at 02:46:03AM +0000, Joel Stanley wrote:

On Tue, 7 Dec 2021 at 01:23, Gabriel L. Somlo [off-list ref] wrote:

quoted

quoted

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[...]
+
+       ret = dma_set_mask(&pdev->dev, DMA_BIT_MASK(32));

Is this going to be true on all platforms? How do we handle those
where it's not true?

I'll need to do a bit of digging here, unless anyone has ideas ready
to go...

I'm not an expert either, so let's consult the docs:

Documentation/core-api/dma-api-howto.rst

This suggests we should be using dma_set_mask_and_coherent?

But we're setting the mask to 32, which is the default, so perhaps we
don't need this call at all?

(I was thinking of the microwatt soc, which is a 64 bit soc but the
peripherals are on a 32 bit bus, and some of the devices are behind a
smaller bus again. But I think we're ok, as the DMA wishbone is
32-bit).

 
So I did a bit of digging, and as it turns out the LiteX DMA base
registers are 64-bit wide, which I think means that they can
essentially do `xlen` bits of DMA addressing, at least when used
as part of a LiteX SoC (no idea what additional quirks occur if/when
LiteSDCard, or any other 64-bit-DMA-capable LiteX IP block would be
used as a standalone component in a different system).

Does this mean that, depending on maybe CONFIG_ARCH_DMA_ADDR_T_64BIT
or something similar, we should actually set DMA_BIT_MASK(64)? Maybe
something like:

#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
	ret = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64));
	if (ret)
		goto err;
#endif

Leave it to the default 32 unless we're on a 64-bit-DMA capable
system, in which case it's safe to assume we need the above setting?

What do you think, does that make sense?

Thanks,
--Gabriel