On Mon, Dec 8, 2008 at 12:45, Hennerich, Michael wrote:
>From: Mike Frysinger [mailto:[EMAIL PROTECTED]
>>On Mon, Dec 8, 2008 at 05:16, Hennerich, Michael wrote:
>>>>comments welcome of course ... one question for sure is volatile
>>>>usage. we have no volatile markings on the structures that represent
>>>>DMA mmrs. this wasnt really a problem before because the functions
>>>>were extern and no function polled a register directly, but now that
>>>>they're dedicated, we need volatile marking. ive stuck it on
>>>>"volatile struct dma_register *regs", but i'm still not entirely
>>>>clear when it comes to pointers ...
>>>
>>> I think volatile usage in not complete considered evil.
>>>
>>> Check: linux-2.6.x/Documentation/volatile-considered-harmful.txt
>>
>>the arguments against volatile there do not apply to us here. the
>>volatile markings are for the MMR addresses. so we either use
>>volatile, or i switch all the functions to use bfin_read/bfin_write
>>macros ...
>
> Use volatile - the document explicitly notes that this is a legitimate
> usage. It actually tells you that exactly for this reason volatile was
> originally invented.
sorry, i should have been a little more specific. what i'm worried
about is how the volatile gets applied, not whether it's ok to use.
struct regs {
void *a;
unsigned long b;
};
volatile struct dma_register *regs;
now, we set "regs" to a base address in the MMR space and the offsets
in the structure are used to get the right DMA register for a specific
channel. what i want to make sure is that the volatile only applies
to the structure and not the pointer itself. i.e. i dont want "regs"
(the address 0xffc0????) being constantly reloaded as the pointer
storage is not volatile, just the memory the pointer is pointing to.
i think what i'm using is correct: "volatile struct dma_register
*regs". if i were to do "struct dma_register * volatile regs", then
i'd get the other case: the pointer storage itself is volatile, but
the memory it is pointing to is not.
-mike
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