Re: [PATCH] swiotlb: set IO TLB segment size via cmdline

[Roman Skakun]

Hi Jan,

Thanks for the answer.

>> From: Roman Skakun <roman_skakun@xxxxxxxx>
>>
>> It is possible when default IO TLB size is not
>> enough to fit a long buffers as described here [1].
>>
>> This patch makes a way to set this parameter
>> using cmdline instead of recompiling a kernel.
>>
>> [1] https://www.xilinx.com/support/answers/72694.html
>
>  I'm not convinced the swiotlb use describe there falls under "intended
>  use" - mapping a 1280x720 framebuffer in a single chunk?

I had the same issue while mapping DMA chuck ~4MB for gem fb when
using xen vdispl.
I got the next log:
[ 142.030421] rcar-fcp fea2f000.fcp: swiotlb buffer is full (sz:
3686400 bytes), total 32768 (slots), used 32 (slots)

It happened when I tried to map bounce buffer, which has a large size.
The default size if 128(IO_TLB_SEGSIZE) * 2048(IO_TLB_SHIFT) = 262144
bytes, but we requested 3686400 bytes.
When I change IO_TLB_SEGSIZE to 2048. (2048(IO_TLB_SEGSIZE)  *
2048(IO_TLB_SHIFT) = 4194304bytes).
It makes possible to retrieve a bounce buffer for requested size.
After changing this value, the problem is gone.

>  the bottom of this page is also confusing, as following "Then we can
>  confirm the modified swiotlb size in the boot log:" there is a log
>  fragment showing the same original size of 64Mb.

I suspect, this is a mistake in the article.
According to https://elixir.bootlin.com/linux/v5.14.4/source/kernel/dma/swiotlb.c#L214
and
https://elixir.bootlin.com/linux/v5.15-rc1/source/kernel/dma/swiotlb.c#L182
The IO_TLB_SEGSIZE is not used to calculate total size of swiotlb area.
This explains why we have the same total size before and after changing of
TLB segment size.

>  In order to be sure to catch all uses like this one (including ones
>  which make it upstream in parallel to yours), I think you will want
>  to rename the original IO_TLB_SEGSIZE to e.g. IO_TLB_DEFAULT_SEGSIZE.

I don't understand your point. Can you clarify this?

>> +     /* get max IO TLB segment size */
>> +     if (isdigit(*str)) {
>> +             tmp = simple_strtoul(str, &str, 0);
>> +             if (tmp)
>> +                     io_tlb_seg_size = ALIGN(tmp, IO_TLB_SEGSIZE);
>
> From all I can tell io_tlb_seg_size wants to be a power of 2. Merely
> aligning to a multiple of IO_TLB_SEGSIZE isn't going to be enough.

Yes, right, thanks!

Cheers,
Roman.

вт, 14 сент. 2021 г. в 18:29, Jan Beulich <jbeulich@xxxxxxxx>:
>
> On 14.09.2021 17:10, Roman Skakun wrote:
> > From: Roman Skakun <roman_skakun@xxxxxxxx>
> >
> > It is possible when default IO TLB size is not
> > enough to fit a long buffers as described here [1].
> >
> > This patch makes a way to set this parameter
> > using cmdline instead of recompiling a kernel.
> >
> > [1] https://www.xilinx.com/support/answers/72694.html
>
> I'm not convinced the swiotlb use describe there falls under "intended
> use" - mapping a 1280x720 framebuffer in a single chunk? (As an aside,
> the bottom of this page is also confusing, as following "Then we can
> confirm the modified swiotlb size in the boot log:" there is a log
> fragment showing the same original size of 64Mb.
>
> > --- a/arch/mips/cavium-octeon/dma-octeon.c
> > +++ b/arch/mips/cavium-octeon/dma-octeon.c
> > @@ -237,7 +237,7 @@ void __init plat_swiotlb_setup(void)
> >               swiotlbsize = 64 * (1<<20);
> >  #endif
> >       swiotlb_nslabs = swiotlbsize >> IO_TLB_SHIFT;
> > -     swiotlb_nslabs = ALIGN(swiotlb_nslabs, IO_TLB_SEGSIZE);
> > +     swiotlb_nslabs = ALIGN(swiotlb_nslabs, swiotlb_io_seg_size());
>
> In order to be sure to catch all uses like this one (including ones
> which make it upstream in parallel to yours), I think you will want
> to rename the original IO_TLB_SEGSIZE to e.g. IO_TLB_DEFAULT_SEGSIZE.
>
> > @@ -81,15 +86,30 @@ static unsigned int max_segment;
> >  static unsigned long default_nslabs = IO_TLB_DEFAULT_SIZE >> IO_TLB_SHIFT;
> >
> >  static int __init
> > -setup_io_tlb_npages(char *str)
> > +setup_io_tlb_params(char *str)
> >  {
> > +     unsigned long tmp;
> > +
> >       if (isdigit(*str)) {
> > -             /* avoid tail segment of size < IO_TLB_SEGSIZE */
> > -             default_nslabs =
> > -                     ALIGN(simple_strtoul(str, &str, 0), IO_TLB_SEGSIZE);
> > +             default_nslabs = simple_strtoul(str, &str, 0);
> >       }
> >       if (*str == ',')
> >               ++str;
> > +
> > +     /* get max IO TLB segment size */
> > +     if (isdigit(*str)) {
> > +             tmp = simple_strtoul(str, &str, 0);
> > +             if (tmp)
> > +                     io_tlb_seg_size = ALIGN(tmp, IO_TLB_SEGSIZE);
>
> From all I can tell io_tlb_seg_size wants to be a power of 2. Merely
> aligning to a multiple of IO_TLB_SEGSIZE isn't going to be enough.
>
> Jan
>


-- 
Best Regards, Roman.