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

[Konrad Rzeszutek Wilk <konrad-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

On Fri, Nov 03, 2017 at 03:01:12PM +0100, Geert Uytterhoeven wrote:
> 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.


Aaah, then you are all fine! Thanks for the quote!
> 
> 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