Hello Alexey, thank you for this feedback!
On Sat, 2020-08-22 at 19:33 +1000, Alexey Kardashevskiy wrote:
> > +#define TCE_RPN_BITS 52 /* Bits 0-51 represent
> > RPN on TCE */
>
> Ditch this one and use MAX_PHYSMEM_BITS instead? I am pretty sure this
> is the actual limit.
I understand this MAX_PHYSMEM_BITS(51) comes from the maximum physical memory
addressable in the machine. IIUC, it means we can access physical address up to
(1ul << MAX_PHYSMEM_BITS).
This 52 comes from PAPR "Table 9. TCE Definition" which defines bits
0-51 as the RPN. By looking at code, I understand that it means we may input
any address < (1ul << 52) to TCE.
In practice, MAX_PHYSMEM_BITS should be enough as of today, because I suppose
we can't ever pass a physical page address over
(1ul << 51), and TCE accepts up to (1ul << 52).
But if we ever increase MAX_PHYSMEM_BITS, it doesn't necessarily means that
TCE_RPN_BITS will also be increased, so I think they are independent values.
Does it make sense? Please let me know if I am missing something.
>
>
> > +#define TCE_RPN_MASK(ps) ((1ul << (TCE_RPN_BITS - (ps))) - 1)
> > #define TCE_VALID 0x800 /* TCE valid */
> > #define TCE_ALLIO 0x400 /* TCE valid for all lpars */
> > #define TCE_PCI_WRITE 0x2 /* write from PCI
> > allowed */
> > diff --git a/arch/powerpc/platforms/pseries/iommu.c
> > b/arch/powerpc/platforms/pseries/iommu.c
> > index e4198700ed1a..8fe23b7dff3a 100644
> > --- a/arch/powerpc/platforms/pseries/iommu.c
> > +++ b/arch/powerpc/platforms/pseries/iommu.c
> > @@ -107,6 +107,9 @@ static int tce_build_pSeries(struct iommu_table *tbl,
> > long index,
> > u64 proto_tce;
> > __be64 *tcep;
> > u64 rpn;
> > + const unsigned long tceshift = tbl->it_page_shift;
> > + const unsigned long pagesize = IOMMU_PAGE_SIZE(tbl);
> > + const u64 rpn_mask = TCE_RPN_MASK(tceshift);
>
> Using IOMMU_PAGE_SIZE macro for the page size and not using
> IOMMU_PAGE_MASK for the mask - this incosistency makes my small brain
> explode :) I understand the history but maaaaan... Oh well, ok.
>
Yeah, it feels kind of weird after two IOMMU related consts. :)
But sure IOMMU_PAGE_MASK() would not be useful here :)
And this kind of let me thinking:
> > + rpn = __pa(uaddr) >> tceshift;
> > + *tcep = cpu_to_be64(proto_tce | (rpn & rpn_mask) << tceshift);
Why not:
rpn_mask = TCE_RPN_MASK(tceshift) << tceshift;
rpn = __pa(uaddr) & rpn_mask;
*tcep = cpu_to_be64(proto_tce | rpn)
I am usually afraid of changing stuff like this, but I think it's safe.
> Good, otherwise. Thanks,
Thank you for reviewing!
