Re: [PATCH v4 7/7] soc: renesas: Add L2 cache management for RZ/Five SoC

[Geert Uytterhoeven]

Hi Prabhakar,

On Sat, Nov 26, 2022 at 10:10 PM Lad, Prabhakar
<prabhakar.csengg@xxxxxxxxx> wrote:
> On Fri, Nov 25, 2022 at 7:43 PM Samuel Holland <samuel@xxxxxxxxxxxx> wrote:
> > On 11/24/22 11:22, Prabhakar wrote:
> > > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
> > >
> > > On the AX45MP core, cache coherency is a specification option so it may
> > > not be supported. In this case DMA will fail. As a workaround, firstly we
> > > allocate a global dma coherent pool from which DMA allocations are taken
> > > and marked as non-cacheable + bufferable using the PMA region as specified
> > > in the device tree. Synchronization callbacks are implemented to
> > > synchronize when doing DMA transactions.
> > >
> > > The Andes AX45MP core has a Programmable Physical Memory Attributes (PMA)
> > > block that allows dynamic adjustment of memory attributes in the runtime.
> > > It contains a configurable amount of PMA entries implemented as CSR
> > > registers to control the attributes of memory locations in interest.
> > >
> > > Below are the memory attributes supported:
> > > * Device, Non-bufferable
> > > * Device, bufferable
> > > * Memory, Non-cacheable, Non-bufferable
> > > * Memory, Non-cacheable, Bufferable
> > > * Memory, Write-back, No-allocate
> > > * Memory, Write-back, Read-allocate
> > > * Memory, Write-back, Write-allocate
> > > * Memory, Write-back, Read and Write-allocate
> > >
> > > This patch adds support to configure the memory attributes of the memory
> > > regions as passed from the l2 cache node and exposes the cache management
> > > ops.
> >
> > Forgive my ignorance, but why do you need both a DMA pool and explicit
> > cache maintenance? Wouldn't the purpose of marking a memory region as
> > permanently non-cacheable be to avoid cache maintenance? And likewise,
> > if you are doing cache maintenance anyway, why does it matter if/how the
> > memory is cacheable?
> >
> "Memory, Non-cacheable, Bufferable" raises an AXI signal for
> transactions hence needing SW implementation for cache maintenance.
>
> > > More info about PMA (section 10.3):
> > > Link: http://www.andestech.com/wp-content/uploads/AX45MP-1C-Rev.-5.0.0-Datasheet.pdf
> > >
> > > Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>

> > > +static int ax45mp_configure_pma_regions(struct device_node *np)
> > > +{
> > > +     const char *propname = "andestech,pma-regions";
> > > +     u32 start, size, flags;
> > > +     unsigned int entry_id;
> > > +     unsigned int i;
> > > +     int count;
> > > +     int ret;
> > > +
> > > +     count = of_property_count_elems_of_size(np, propname, sizeof(u32) * 3);
> > > +     if (count < 0)
> > > +             return count;
> > > +
> > > +     if (count > AX45MP_MAX_PMA_REGIONS)
> > > +             return -EINVAL;
> > > +
> > > +     for (i = 0, entry_id = 0 ; entry_id < count ; i += 3, entry_id++) {
> > > +             of_property_read_u32_index(np, propname, i, &start);
> > > +             of_property_read_u32_index(np, propname, i + 1, &size);
> > > +             of_property_read_u32_index(np, propname, i + 2, &flags);
> > > +             ret = ax45mp_sbi_set_pma(start, size, flags, entry_id);
> > > +             if (!ret)
> > > +                     pr_err("Failed to setup PMA region 0x%x - 0x%x flags: 0x%x",
> > > +                            start, start + size, flags);
> > > +     }
> > > +
> > > +     return 0;
> > > +}
> >
> > If firmware support is required to set up these PMA regions, why is
> > Linux doing this at all? The firmware has access to the devicetree as
> > well. It can set this up before entering S-mode, and then you don't need
> > to expose this capability via an SBI extension. In fact, firmware could
> > generate the reserved-memory node based on these regions at runtime (or
> > vice versa).
> >
> That's a good point. I'll do some research on this and get back.
>
> Btw are there any existing examples where the firmware adds DT nodes?

/memory, reserved-memory, optee on ARM, RPC status on R-Car Gen3/4, ...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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