Re: [PATCH 1/2] mmc: renesas_sdhi: fix swiotlb buffer is full

Geert Uytterhoeven
2017-11-03T14:01:12Z

Hi Konrad,

On Fri, Nov 3, 2017 at 2:23 PM, Konrad Rzeszutek Wilk <konrad-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:
> ..snip..
>> > >
>> > > Anyway, I made v2 patches by using swiotlb related definitions. Would you check it?
>> >
>> > Did I miss that email? As in was I cc-ed?
>>
>> This was my fault. When I submitted v2 patches, I didn't include your email and iommu mailing list...
>
> No problem.
>>
>> > > https://patchwork.kernel.org/patch/10018879/
>> >
>> > Why not use IO_TLB_SEGSIZE << IO_TLB_SHIFT or alternatively
>> > swiotlb_max_segment? See 5584f1b1d73e9
>>
>> I already made such a patch as v2 and it was merged into mmc.git / fixes branch.
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/ulfh/mmc.git/commit/?h=fixes&id=e90e8da72ad694a16a4ffa6e5adae3610208f73b
>
> What happens if the user has swiotlb=4096 on the command line (meaning
> less than the default value)? Your max value will be incorrect. Could you use
> swiotlb_max_segment?

No, as that's the total size of the swiotlb buffer, not the maximum size of
a single segment.

Quoting an earlier part of the thread:

On Thu, Oct 19, 2017 at 1:39 PM, Yoshihiro Shimoda
<yoshihiro.shimoda.uh-zM6kxYcvzFBBDgjK7y7TUQ@public.gmane.org> wrote:
>> >> iommu: Is there a better (generic) way to handle this?
>> >
>> > Yes. See 7453c549f5f6485c0d79cad7844870dcc7d1b34d, aka swiotlb_max_segment
>>
>> Thanks for the pointer!
>>
>> While I agree this can be used to avoid the swiotlb buffer full issue,
>> I believe it is a suboptimal solution if the device actually uses an IOMMU.
>> It limits the mapping size if CONFIG_SWIOTLB=y, which is always the
>> case for arm/arm64 these days.
>
> I'm afraid but I misunderstood this API's spec when I read it at first.
> After I tried to use it, I found the API cannot be used for a workaround because
> this API returns total size of swiotlb.
>
> For example:
> - The swiotlb_max_segment() returns 64M bytes from the API when a default setting.
> - In this case, the maximum size per a map is 256k bytes.
> - The swiotlb_max_segment() returns 128M bytes from the API when I added swiotlb=65536
> into the kernel parameter on arm64.
> - In this case, the maximum size per a map is still 256k bytes because
> the swiotlb has hardcoded the size by the following code:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/swiotlb.c?h=v4.14-rc5#n254

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert-Td1EMuHUCqxL1ZNQvxDV9g@public.gmane.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