From 0e95c85341b7b5be34f999b6023e3df4d03f4977 Mon Sep 17 00:00:00 2001 From: Mauro Carvalho Chehab Date: Sun, 14 May 2017 14:45:35 -0300 Subject: [PATCH] io_ordering.txt: standardize document format Each text file under Documentation follows a different format. Some doesn't even have titles! Change its representation to follow the adopted standard, using ReST markups for it to be parseable by Sphinx: - Add a title; - mark literal-blocks as such. Signed-off-by: Mauro Carvalho Chehab Signed-off-by: Jonathan Corbet --- Documentation/io_ordering.txt | 62 +++++++++++++++++++---------------- 1 file changed, 33 insertions(+), 29 deletions(-) diff --git a/Documentation/io_ordering.txt b/Documentation/io_ordering.txt index 9faae6f26d32..2ab303ce9a0d 100644 --- a/Documentation/io_ordering.txt +++ b/Documentation/io_ordering.txt @@ -1,3 +1,7 @@ +============================================== +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 @@ -8,39 +12,39 @@ 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: +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) - ... + ... + 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: +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) + ... + 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