https://github.com/torvalds/linux
Revision ca122fe376fc43f7565e3e56e6777d06a433a4cc authored by Linus Torvalds on 25 November 2017, 18:21:54 UTC, committed by Linus Torvalds on 25 November 2017, 18:21:54 UTC
Pull ARC updates from Vineet Gupta: - more changes for HS48 cores: supporting MMUv5, detecting new micro-arch gizmos - axs10x platform wiring up reset driver merged in this cycle - ARC perf driver optimizations * tag 'arc-4.15-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/vgupta/arc: ARC: perf: avoid vmalloc backed mmap ARCv2: perf: optimize given that num counters <= 32 ARCv2: perf: tweak overflow interrupt ARC: [plat-axs10x] DTS: Add reset controller node to manage ethernet reset ARCv2: boot log: updates for HS48: dual-issue, ECC, Loop Buffer ARCv2: Accomodate HS48 MMUv5 by relaxing MMU ver checking ARC: [plat-axs10x] auto-select AXS101 or AXS103 given the ISA config
Tip revision: ca122fe376fc43f7565e3e56e6777d06a433a4cc authored by Linus Torvalds on 25 November 2017, 18:21:54 UTC
Merge tag 'arc-4.15-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/vgupta/arc
Merge tag 'arc-4.15-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/vgupta/arc
Tip revision: ca122fe
io_ordering.txt
==============================================
Ordering I/O writes to memory-mapped addresses
==============================================
On some platforms, so-called memory-mapped I/O is weakly ordered. On such
platforms, driver writers are responsible for ensuring that I/O writes to
memory-mapped addresses on their device arrive in the order intended. This is
typically done by reading a 'safe' device or bridge register, causing the I/O
chipset to flush pending writes to the device before any reads are posted. A
driver would usually use this technique immediately prior to the exit of a
critical section of code protected by spinlocks. This would ensure that
subsequent writes to I/O space arrived only after all prior writes (much like a
memory barrier op, mb(), only with respect to I/O).
A more concrete example from a hypothetical device driver::
...
CPU A: spin_lock_irqsave(&dev_lock, flags)
CPU A: val = readl(my_status);
CPU A: ...
CPU A: writel(newval, ring_ptr);
CPU A: spin_unlock_irqrestore(&dev_lock, flags)
...
CPU B: spin_lock_irqsave(&dev_lock, flags)
CPU B: val = readl(my_status);
CPU B: ...
CPU B: writel(newval2, ring_ptr);
CPU B: spin_unlock_irqrestore(&dev_lock, flags)
...
In the case above, the device may receive newval2 before it receives newval,
which could cause problems. Fixing it is easy enough though::
...
CPU A: spin_lock_irqsave(&dev_lock, flags)
CPU A: val = readl(my_status);
CPU A: ...
CPU A: writel(newval, ring_ptr);
CPU A: (void)readl(safe_register); /* maybe a config register? */
CPU A: spin_unlock_irqrestore(&dev_lock, flags)
...
CPU B: spin_lock_irqsave(&dev_lock, flags)
CPU B: val = readl(my_status);
CPU B: ...
CPU B: writel(newval2, ring_ptr);
CPU B: (void)readl(safe_register); /* maybe a config register? */
CPU B: spin_unlock_irqrestore(&dev_lock, flags)
Here, the reads from safe_register will cause the I/O chipset to flush any
pending writes before actually posting the read to the chipset, preventing
possible data corruption.
![swh spinner](/static/img/swh-spinner.gif)
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