Re: [PATCH 5/6] MIPS: Loongson: Introduce and use cpu_has_coherent_cache feature

[James Hogan <james.hogan@imgtec.com>]

[-- Attachment #1: Type: text/plain, Size: 2540 bytes --]

Hi Huacai,

On Wed, Jan 27, 2016 at 12:58:42PM +0800, Huacai Chen wrote:
> "cache coherency" here means the coherency across cores, not ic/dc
> coherency, could you please suggest a suitable name?

Data cache coherency across cores is pretty much a requirement for SMP.
It looks more like for various reasons you can skip the cache management
functions, e.g.

> >> @@ -503,6 +509,9 @@ static void r4k_flush_cache_range(struct vm_area_struct *vma,
> >>  {
> >>       int exec = vma->vm_flags & VM_EXEC;
> >>
> >> +     if (cpu_has_coherent_cache)
> >> +             return;

This seems to suggest:
1) Your dcaches don't alias.
2) Your icache is coherent with your dcache, otherwise you would need to
   flush the icache here so that mprotect RW->RX makes code executable
   without risk of stale lines existing in icache.

So, is that the case?

If so, it would seem better to ensure that cpu_has_dc_aliases evaluates
to false, and add a similar one for icache coherency, hence my original
suggestion.


> >> +
> >>       if (cpu_has_dc_aliases || (exec && !cpu_has_ic_fills_f_dc))
> >>               r4k_on_each_cpu(local_r4k_flush_cache_range, vma);

(Note, this cpu_has_ic_fills_f_dc check is wrong, it shouldn't prevent
icache flush, see http://patchwork.linux-mips.org/patch/12179/)

Cheers
James

> >>  }
> >> @@ -627,6 +636,9 @@ static void r4k_flush_cache_page(struct vm_area_struct *vma,
> >>  {
> >>       struct flush_cache_page_args args;
> >>
> >> +     if (cpu_has_coherent_cache)
> >> +             return;
> >> +
> >>       args.vma = vma;
> >>       args.addr = addr;
> >>       args.pfn = pfn;
> >> @@ -636,11 +648,17 @@ static void r4k_flush_cache_page(struct vm_area_struct *vma,
> >>
> >>  static inline void local_r4k_flush_data_cache_page(void * addr)
> >>  {
> >> +     if (cpu_has_coherent_cache)
> >> +             return;
> >> +
> >>       r4k_blast_dcache_page((unsigned long) addr);
> >>  }
> >>
> >>  static void r4k_flush_data_cache_page(unsigned long addr)
> >>  {
> >> +     if (cpu_has_coherent_cache)
> >> +             return;
> >> +
> >>       if (in_atomic())
> >>               local_r4k_flush_data_cache_page((void *)addr);
> >>       else
> >> @@ -825,6 +843,9 @@ static void local_r4k_flush_cache_sigtramp(void * arg)
> >>
> >>  static void r4k_flush_cache_sigtramp(unsigned long addr)
> >>  {
> >> +     if (cpu_has_coherent_cache)
> >> +             return;
> >> +
> >>       r4k_on_each_cpu(local_r4k_flush_cache_sigtramp, (void *) addr);
> >>  }
> >>
> >> --
> >> 2.4.6
> >>
> >>
> >>
> >>
> >>

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 819 bytes --]