> Date: Mon, 15 Feb 2021 01:19:29 +0100
> From: Patrick Wildt <patr...@blueri.se>
>
> Am Mon, Feb 15, 2021 at 09:55:56AM +1000 schrieb David Gwynne:
> >
> >
> > > On 15 Feb 2021, at 07:54, Mark Kettenis <mark.kette...@xs4all.nl> wrote:
> > >
> > > One of the aspects of device access is whether CPU writes to a device
> > > are posted or non-posted. For non-posted writes, the CPU will wait
> > > for the device to acknowledge that the write has performed. If the
> > > device sits on a bus far away, this can take a while and slow things
> > > down. The alternative are so-called posted writes. The CPU will
> > > "post" the write to the bus without waiting for an acknowledgement.
> > > The CPU may receive an asynchronous notifaction at a later time that
> > > the write didn't succeed or a failing write may be dropped without
> > > further botification. On most architectures whether writes are posted
> > > or not is a property of the bus between the CPU and the device. For
> > > example, memory mapped I/O on the PCI bus is always posted and there
> > > is nothing the CPU can do about it.
> > >
> > > On the ARM architecture though we can indicate to the CPU whether
> > > writes to a certain address range should be posted or not. This is
> > > done by specifying certain memory attributes in the mappings used by
> > > the MMU. The OpenBSD kernel always specifies device access as
> > > non-posted. On all ARM implementations we have seen so far this seems
> > > to work even for writes to devices connected to a PCIe bus. There
> > > might be a penalty though, so I need to investigate this a bit
> > > further.
> > >
> > > However, on Apple's M1 SoC, this isn't the case. Non-posted writes to
> > > a bus that uses posted writes fail and vice-versa. So in order to use
> > > the PCIe bus on these SoCs we need to specify the right memory
> > > attributes. The diff below implements this by introducing a new
> > > BUS_SPACE_MAP_POSTED flag. At this point I don't expect generic
> > > drivers to use this flag yet. So there is no need to add it for other
> > > architectures. But I don't rule out we may have to use this flag in
> > > sys/dev/fdt sometime in the future. That is why I posted this to a
> > > wider audience.
> >
> > You don't want to (ab)use one of the existing flags? If I squint
> > and read kind of quickly I could imagine this is kind of like
> > write combining, like what BUS_SPACE_MAP_PREFETCHABLE can do on
> > pci busses.
>
> BUS_SPACE_MAP_PREFETCHABLE should be "normal uncached" memory on arm64,
> which is different to device memory. That said I have a device where
> amdgpu(4) doesn't behave if it's "normal uncached", and I'm not sure if
> it's the HW's fault or if there's some barrier missing. Still, I would
> not use BUS_SPACE_MAP_PREFETCHABLE for nGnRnE vs nGnRE.
>
> More info on device vs normal is here:
>
> https://developer.arm.com/documentation/102376/0100/Normal-memory
> https://developer.arm.com/documentation/102376/0100/Device-memory
BUS_SPACE_MAP_PREFETCHABLE is used for parts of the address space that
are "side-effect free". That means that multiple writes may be
combined into one and reads might actually fetch more data than you
asked for. Typical use is a frambuffer or some other device memory
that is accessed across a PCI bus. In most cases that is not what you
want to access device registers where a read or a write triggers some
action in the device.
Posted writes still happen as issued (and in principle in the same
order as issued). But they may complete at a later time after the CPU
has executed many more instructions. The traditional way to make sure
posted writes on a PCI bus have completed is to read something back
from the device.
So as Patrick said, it's a different thing.
> > If this does leak into fdt, would it just be a nop on other archs
> > that use those drivers?
Most likely, yes. All other architectures that I know have don't
require the CPU to do someting different for posted and non-posted
writes.
> > >
> > > ok?
> > >
> > >
> > > Index: arch/arm64/arm64/locore.S
> > > ===================================================================
> > > RCS file: /cvs/src/sys/arch/arm64/arm64/locore.S,v
> > > retrieving revision 1.32
> > > diff -u -p -r1.32 locore.S
> > > --- arch/arm64/arm64/locore.S 19 Oct 2020 17:57:40 -0000 1.32
> > > +++ arch/arm64/arm64/locore.S 14 Feb 2021 21:28:26 -0000
> > > @@ -233,9 +233,10 @@ switch_mmu_kernel:
> > > mair:
> > > /* Device | Normal (no cache, write-back, write-through) */
> > > .quad MAIR_ATTR(0x00, 0) | \
> > > - MAIR_ATTR(0x44, 1) | \
> > > - MAIR_ATTR(0xff, 2) | \
> > > - MAIR_ATTR(0x88, 3)
> > > + MAIR_ATTR(0x04, 1) | \
> > > + MAIR_ATTR(0x44, 2) | \
> > > + MAIR_ATTR(0xff, 3) | \
> > > + MAIR_ATTR(0x88, 4)
> > > tcr:
> > > .quad (TCR_T1SZ(64 - VIRT_BITS) | TCR_T0SZ(64 - 48) | \
> > > TCR_AS | TCR_TG1_4K | TCR_CACHE_ATTRS | TCR_SMP_ATTRS)
> > > Index: arch/arm64/arm64/locore0.S
> > > ===================================================================
> > > RCS file: /cvs/src/sys/arch/arm64/arm64/locore0.S,v
> > > retrieving revision 1.5
> > > diff -u -p -r1.5 locore0.S
> > > --- arch/arm64/arm64/locore0.S 28 May 2019 20:32:30 -0000 1.5
> > > +++ arch/arm64/arm64/locore0.S 14 Feb 2021 21:28:26 -0000
> > > @@ -34,8 +34,8 @@
> > > #include <machine/pte.h>
> > >
> > > #define DEVICE_MEM 0
> > > -#define NORMAL_UNCACHED 1
> > > -#define NORMAL_MEM 2
> > > +#define NORMAL_UNCACHED 2
> > > +#define NORMAL_MEM 3
> > >
> > > /*
> > > * We assume:
> > > Index: arch/arm64/arm64/machdep.c
> > > ===================================================================
> > > RCS file: /cvs/src/sys/arch/arm64/arm64/machdep.c,v
> > > retrieving revision 1.57
> > > diff -u -p -r1.57 machdep.c
> > > --- arch/arm64/arm64/machdep.c 11 Feb 2021 23:55:48 -0000 1.57
> > > +++ arch/arm64/arm64/machdep.c 14 Feb 2021 21:28:27 -0000
> > > @@ -1188,7 +1188,7 @@ pmap_bootstrap_bs_map(bus_space_tag_t t,
> > >
> > > for (pa = startpa; pa < endpa; pa += PAGE_SIZE, va += PAGE_SIZE)
> > > pmap_kenter_cache(va, pa, PROT_READ | PROT_WRITE,
> > > - PMAP_CACHE_DEV);
> > > + PMAP_CACHE_DEV_NGNRNE);
> > >
> > > virtual_avail = va;
> > >
> > > Index: arch/arm64/arm64/pmap.c
> > > ===================================================================
> > > RCS file: /cvs/src/sys/arch/arm64/arm64/pmap.c,v
> > > retrieving revision 1.70
> > > diff -u -p -r1.70 pmap.c
> > > --- arch/arm64/arm64/pmap.c 25 Jan 2021 19:37:17 -0000 1.70
> > > +++ arch/arm64/arm64/pmap.c 14 Feb 2021 21:28:28 -0000
> > > @@ -472,7 +472,7 @@ pmap_enter(pmap_t pm, vaddr_t va, paddr_
> > > if (pa & PMAP_NOCACHE)
> > > cache = PMAP_CACHE_CI;
> > > if (pa & PMAP_DEVICE)
> > > - cache = PMAP_CACHE_DEV;
> > > + cache = PMAP_CACHE_DEV_NGNRNE;
> > > pg = PHYS_TO_VM_PAGE(pa);
> > >
> > > pmap_lock(pm);
> > > @@ -648,7 +648,7 @@ _pmap_kenter_pa(vaddr_t va, paddr_t pa,
> > > pmap_pte_insert(pted);
> > >
> > > ttlb_flush(pm, va & ~PAGE_MASK);
> > > - if (cache == PMAP_CACHE_CI || cache == PMAP_CACHE_DEV)
> > > + if (cache == PMAP_CACHE_CI || cache == PMAP_CACHE_DEV_NGNRNE)
> > > cpu_idcache_wbinv_range(va & ~PAGE_MASK, PAGE_SIZE);
> > > }
> > >
> > > @@ -735,7 +735,9 @@ pmap_fill_pte(pmap_t pm, vaddr_t va, pad
> > > break;
> > > case PMAP_CACHE_CI:
> > > break;
> > > - case PMAP_CACHE_DEV:
> > > + case PMAP_CACHE_DEV_NGNRNE:
> > > + break;
> > > + case PMAP_CACHE_DEV_NGNRE:
> > > break;
> > > default:
> > > panic("pmap_fill_pte:invalid cache mode");
> > > @@ -1637,8 +1639,12 @@ pmap_pte_update(struct pte_desc *pted, u
> > > attr |= ATTR_IDX(PTE_ATTR_CI);
> > > attr |= ATTR_SH(SH_INNER);
> > > break;
> > > - case PMAP_CACHE_DEV:
> > > - attr |= ATTR_IDX(PTE_ATTR_DEV);
> > > + case PMAP_CACHE_DEV_NGNRNE:
> > > + attr |= ATTR_IDX(PTE_ATTR_DEV_NGNRNE);
> > > + attr |= ATTR_SH(SH_INNER);
> > > + break;
> > > + case PMAP_CACHE_DEV_NGNRE:
> > > + attr |= ATTR_IDX(PTE_ATTR_DEV_NGNRE);
> > > attr |= ATTR_SH(SH_INNER);
> > > break;
> > > default:
> > > Index: arch/arm64/dev/arm64_bus_space.c
> > > ===================================================================
> > > RCS file: /cvs/src/sys/arch/arm64/dev/arm64_bus_space.c,v
> > > retrieving revision 1.7
> > > diff -u -p -r1.7 arm64_bus_space.c
> > > --- arch/arm64/dev/arm64_bus_space.c 20 Aug 2018 19:38:07 -0000
> > > 1.7
> > > +++ arch/arm64/dev/arm64_bus_space.c 14 Feb 2021 21:28:29 -0000
> > > @@ -191,8 +191,12 @@ generic_space_map(bus_space_tag_t t, bus
> > > {
> > > u_long startpa, endpa, pa;
> > > vaddr_t va;
> > > - int cache = flags & BUS_SPACE_MAP_CACHEABLE ?
> > > - PMAP_CACHE_WB : PMAP_CACHE_DEV;
> > > + int cache = PMAP_CACHE_DEV_NGNRNE;
> > > +
> > > + if (flags & BUS_SPACE_MAP_CACHEABLE)
> > > + cache = PMAP_CACHE_WB;
> > > + if (flags & BUS_SPACE_MAP_POSTED)
> > > + cache = PMAP_CACHE_DEV_NGNRE;
> > >
> > > startpa = trunc_page(offs);
> > > endpa = round_page(offs + size);
> > > Index: arch/arm64/include/bus.h
> > > ===================================================================
> > > RCS file: /cvs/src/sys/arch/arm64/include/bus.h,v
> > > retrieving revision 1.7
> > > diff -u -p -r1.7 bus.h
> > > --- arch/arm64/include/bus.h 13 Apr 2020 21:34:54 -0000 1.7
> > > +++ arch/arm64/include/bus.h 14 Feb 2021 21:28:29 -0000
> > > @@ -130,7 +130,7 @@ struct bus_space {
> > > (*(t)->_space_subregion)((t), (h), (o), (s), (p))
> > >
> > > #define BUS_SPACE_MAP_CACHEABLE 0x01
> > > -#define BUS_SPACE_MAP_KSEG0 0x02
> > > +#define BUS_SPACE_MAP_POSTED 0x02
> > > #define BUS_SPACE_MAP_LINEAR 0x04
> > > #define BUS_SPACE_MAP_PREFETCHABLE 0x08
> > >
> > > Index: arch/arm64/include/pmap.h
> > > ===================================================================
> > > RCS file: /cvs/src/sys/arch/arm64/include/pmap.h,v
> > > retrieving revision 1.14
> > > diff -u -p -r1.14 pmap.h
> > > --- arch/arm64/include/pmap.h 21 Oct 2020 21:53:47 -0000 1.14
> > > +++ arch/arm64/include/pmap.h 14 Feb 2021 21:28:29 -0000
> > > @@ -42,7 +42,8 @@
> > > #define PMAP_CACHE_CI (PMAP_MD0) /* cache
> > > inhibit */
> > > #define PMAP_CACHE_WT (PMAP_MD1) /* writethru */
> > > #define PMAP_CACHE_WB (PMAP_MD1|PMAP_MD0) /* writeback */
> > > -#define PMAP_CACHE_DEV (PMAP_MD2) /* device
> > > mapping */
> > > +#define PMAP_CACHE_DEV_NGNRNE (PMAP_MD2) /* device
> > > nGnRnE */
> > > +#define PMAP_CACHE_DEV_NGNRE (PMAP_MD2|PMAP_MD0) /* device nGnRE
> > > */
> > > #define PMAP_CACHE_BITS (PMAP_MD0|PMAP_MD1|PMAP_MD2)
> > >
> > > #define PTED_VA_MANAGED_M (PMAP_MD3)
> > > Index: arch/arm64/include/pte.h
> > > ===================================================================
> > > RCS file: /cvs/src/sys/arch/arm64/include/pte.h,v
> > > retrieving revision 1.5
> > > diff -u -p -r1.5 pte.h
> > > --- arch/arm64/include/pte.h 13 Apr 2017 23:29:02 -0000 1.5
> > > +++ arch/arm64/include/pte.h 14 Feb 2021 21:28:30 -0000
> > > @@ -53,11 +53,11 @@
> > > #define ATTR_IDX(x) ((x) << 2)
> > > #define ATTR_IDX_MASK (7 << 2)
> > >
> > > -#define PTE_ATTR_DEV 0
> > > -#define PTE_ATTR_CI 1
> > > -#define PTE_ATTR_WB 2
> > > -#define PTE_ATTR_WT 3
> > > -
> > > +#define PTE_ATTR_DEV_NGNRNE 0
> > > +#define PTE_ATTR_DEV_NGNRE 1
> > > +#define PTE_ATTR_CI 2
> > > +#define PTE_ATTR_WB 3
> > > +#define PTE_ATTR_WT 4
> > >
> > > #define SH_INNER 3
> > > #define SH_OUTER 2
> > >
> >
>
