linux-kernel - Re: [PATCH v2 01/21] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section

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Date:   Fri, 12 Apr 2019 08:12:26 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Will Deacon <will.deacon@....com>, linux-arch@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        Ingo Molnar <mingo@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, Tony Luck <tony.luck@...el.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Akira Yokosawa <akiyks@...il.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v2 01/21] docs/memory-barriers.txt: Rewrite "KERNEL I/O
 BARRIER EFFECTS" section

On Fri, 2019-04-05 at 14:59 +0100, Will Deacon wrote:
> +     1. All readX() and writeX() accesses to the same peripheral are ordered
> +        with respect to each other. For example, this ensures that MMIO register
> +       writes by the CPU to a particular device will arrive in program order.

Minor nit... I would have said "All readX() and writeX() accesses _from
the same CPU_ to the same peripheral... and then s/the CPU/this CPU.

> -     Accesses to this space may be fully synchronous (as on i386), but
> -     intermediary bridges (such as the PCI host bridge) may not fully honour
> -     that.
> +     2. A writeX() by the CPU to the peripheral will first wait for the
> +        completion of all prior CPU writes to memory. For example, this ensures
> +        that writes by the CPU to an outbound DMA buffer allocated by
> +        dma_alloc_coherent() will be visible to a DMA engine when the CPU writes
> +        to its MMIO control register to trigger the transfer.

Similarily "the CPU" -> "a CPU"
>  
> -     They are guaranteed to be fully ordered with respect to each other.
> +     3. A readX() by the CPU from the peripheral will complete before any
> +       subsequent CPU reads from memory can begin. For example, this ensures
> +       that reads by the CPU from an incoming DMA buffer allocated by
> +       dma_alloc_coherent() will not see stale data after reading from the DMA
> +       engine's MMIO status register to establish that the DMA transfer has
> +       completed.
>  
> -     They are not guaranteed to be fully ordered with respect to other types of
> -     memory and I/O operation.
> +     4. A readX() by the CPU from the peripheral will complete before any
> +       subsequent delay() loop can begin execution. For example, this ensures
> +       that two MMIO register writes by the CPU to a peripheral will arrive at
> +       least 1us apart if the first write is immediately read back with readX()
> +       and udelay(1) is called prior to the second writeX().
>  
> - (*) readX(), writeX():
> +     __iomem pointers obtained with non-default attributes (e.g. those returned
> +     by ioremap_wc()) are unlikely to provide many of these guarantees.

So we give up on defining _wc semantics ? :-) Fair enough, it's a
mess...

 .../...

> +All of these accessors assume that the underlying peripheral is little-endian,
> +and will therefore perform byte-swapping operations on big-endian architectures.

This is not true of readsX/writesX, those will perform native accesses and are
intrinsically endian neutral.

> +Composing I/O ordering barriers with SMP ordering barriers and LOCK/UNLOCK
> +operations is a dangerous sport which may require the use of mmiowb(). See the
> +subsection "Acquires vs I/O accesses" for more information.

Cheers,
Ben.