Re: [PATCH 2/2] swiotlb: Add swiotlb=nobounce debug option

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Robin,

On Tue, Nov 1, 2016 at 12:46 PM, Robin Murphy <robin.murphy@arm.com> wrote:
>>>> To aid debugging and catch devices not supporting DMA to memory outside
>>>> the 32-bit address space, add a kernel command line option
>>>> "swiotlb=nobounce", which disables the use of bounce buffers.
>>>> If specified, trying to map memory that cannot be used with DMA will
>>>> fail, and a warning will be printed (rate-limited).
>>>
>>> This rationale seems questionable - how useful is non-deterministic
>>> behaviour for debugging really? What you end up with is DMA sometimes
>>> working or sometimes not depending on whether allocations happen to
>>> naturally fall below 4GB or not. In my experience, that in itself can be
>>> a pain in the arse to debug.
>>
>> It immediately triggered for me, though:
>>
>>     rcar-dmac e7300000.dma-controller: Cannot do DMA to address
>> 0x000000067a9b7000
>>     ravb e6800000.ethernet: Cannot do DMA to address 0x000000067aa07780
>>
>>> Most of the things you might then do to make things more deterministic
>>> again (like making the default DMA mask tiny or hacking out all the
>>> system's 32-bit addressable RAM) are also generally sufficient to make
>>> DMA fail earlier and make this option moot anyway. What's the specific
>>> use case motivating this?
>>
>> My use case is finding which drivers and DMA engines do not support 64-bit
>> memory. There's more info in my series "[PATCH/RFC 0/5] arm64: r8a7796: 64-bit
>> Memory and Ethernet Prototype"
>> (https://www.mail-archive.com/linux-renesas-soc@vger.kernel.org/msg08393.html)
>
> Thanks for the context. I've done very similar things in the past, and
> my first instinct would be to change the default DMA mask in
> of_dma_configure() to something which can't reach RAM (e.g. <30 bits),
> then instrument dma_set_mask() to catch cleverer drivers. That's a
> straightforward way to get 100% coverage - the problem with simply
> disabling bounce buffering is that whilst statistically it almost
> certainly will catch >95% of cases, there will always be some that it
> won't; if some driver only ever does a single dma_alloc_coherent() early
> enough that allocations are still fairly deterministic, and always
> happens to get a 32-bit address on that platform, it's likely to slip
> through the net.
>
> I'm not against the idea of SWIOTLB growing a runtime-disable option,
> I'm just not sure what situation it's actually the best solution for.

If I set the DMA mask to a small value, DMA is never used, and SWIOTLB
always falls back to bounce buffers (and DMAing from the small pool)?

That's the inverse of what I want to achieve: I want to avoid using the
bounce feature, to make sure the DMA engine is always used with whatever
kind of memory.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds