Re: [PATCH v9 5/6] cache: Add L2 cache management for Andes AX45MP RISC-V core

[Lad, Prabhakar]

Hi Arnd,

Thank you for the review.


On Wed, Jun 14, 2023 at 2:11 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Wed, Jun 14, 2023, at 12:47, Prabhakar wrote:
>
> > +static void ax45mp_dma_cache_inv(phys_addr_t paddr, unsigned long size)
> > +{
> > +     unsigned long start = (unsigned long)phys_to_virt(paddr);
> > +     char cache_buf[2][AX45MP_CACHE_LINE_SIZE];
> > +     unsigned long end = start + size;
> > +     unsigned long old_start = start;
> > +     unsigned long old_end = end;
> > +     unsigned long line_size;
> > +     unsigned long flags;
> > +
> > +     if (unlikely(start == end))
> > +             return;
> > +
> > +     line_size = ax45mp_priv.ax45mp_cache_line_size;
> > +
> > +     memset(&cache_buf, 0x0, sizeof(cache_buf));
> > +     start = start & (~(line_size - 1));
> > +     end = ((end + line_size - 1) & (~(line_size - 1)));
> > +
> > +     local_irq_save(flags);
> > +     if (unlikely(start != old_start))
> > +             memcpy(&cache_buf[0][0], (void *)start, line_size);
> > +
> > +     if (unlikely(end != old_end))
> > +             memcpy(&cache_buf[1][0], (void *)(old_end & (~(line_size - 1))),
> > line_size);
> > +
> > +     ax45mp_cpu_dcache_inval_range(start, end, line_size);
> > +
> > +     if (unlikely(start != old_start))
> > +             memcpy((void *)start, &cache_buf[0][0], (old_start & (line_size -
> > 1)));
> > +
> > +     local_irq_restore(flags);
> > +}
>
> I'm not quite sure what this does, maybe some comments would help.
>
> This looks like a novel method of preserving partial cache lines
> at the beginning (but not the end?) of an unaligned area.
>
I missed this earlier, (I removed the preserving of cache line from
then end and left the beginning part. Samuel pointed to earlier on
these patches if the cache-line-size is 64 we dont need to do this).
Preserving cache lines is not needed at all,  Ive verified this and
just doing a _inval() is sufficient. I'll update this and send a new
version.

Cheers,
Prabhakar

> As far as I can tell, most dma_mapping implementations don't
> even try to do that at all, but the ones that do take a different
> approach by calling _wback_inv() on partial cache lines instead
> of calling _inv().
>
> I'd say this does not belong into the low-level operations
> here, especially since the ZICBOM variant doesn't do this either.
>

>       Arnd