linux-kernel - Re: [PATCH + an old question] firewire: ohci: use memory barriers to order descriptor updates

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Message-ID: <4C4FF3B3.1050800@s5r6.in-berlin.de>
Date:	Wed, 28 Jul 2010 11:09:07 +0200
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Clemens Ladisch <clemens@...isch.de>
CC:	linux-kernel@...r.kernel.org, linux1394-devel@...ts.sourceforge.net
Subject: Re: [PATCH + an old question] firewire: ohci: use memory barriers
 to order descriptor updates

Clemens Ladisch wrote:
> Stefan Richter wrote:
>> We need:
>> 2. a write memory barrier between branch_address update and wake-up of
>>    the DMA unit by MMIO register write.
>>
>> Barrier 2 is implicit in writel() on most machines --- or at least I
>> think it is.  See this from arch/x86/include/asm/io.h:
>>
>>     #define build_mmio_write(name, size, type, reg, barrier) \
>>     static inline void name(type val, volatile void __iomem *addr) \
>>     { asm volatile("mov" size " %0,%1": :reg (val), \
>>     "m" (*(volatile type __force *)addr) barrier); }
>>
>>     build_mmio_write(writel, "l", unsigned int, "r", :"memory")
>>
>> Does this order the mmio write relative to previous memory writes?
> 
> This asm barrier prevents the compiler from reordering.
> 
> The main purpose of writel() and friends is to access the address space
> where memory-mapped I/O ranges reside; there are architectures where the
> normal memory access commands cannot be used.  This does not necessarily
> imply anything about reordering semantics.
> 
> However, PCI address ranges are marked by the device's config registers
> as either cacheable or not, and the kernel heeds this when mapping these
> ranges.  Registers are, of course, marked as uncacheable.

But the memory to which we wrote before that is cachable (though
cache-coherent, allocated by dma_alloc_coherent).  This memory write
should not cross the mmio register write.

Documentation/DocBook/deviceiobook.tmpl mentions that mmio reads to a
device are ordered WRT to DMA transactions that the device issued before
that mmio read.  But no mentions of mmio write to a device vs. preceding
memory accesses by the CPU.

Well, a quick look how some hopefully well-written drivers use and don't
use wmb() indicates that I don't need to worry.  Perhaps it is time to
look for a PCI book.
-- 
Stefan Richter
-=====-==-=- -=== ===--
http://arcgraph.de/sr/
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