CONFIG_ARM_DMA_MEM_BUFFERABLE and readl/writel weirdness

[Saravana Kannan]

On 03/02/2011 12:23 AM, Arnd Bergmann wrote:
> On Wednesday 02 March 2011 02:23:15 Saravana Kannan wrote:
>> There are so many other drivers that don't use or care about DMA and
>> might still want to ensure some ordering constraints between their
>> readl(s)/writel(s). They can't depend on readl/writel taking care of it
>> for them since their code could be used in a kernel configuration that
>> doesn't enable this config.
>
> What exactly are the ordering constraints, do you need the full
> dmb() for readl() and dsb() for writel(), or just a compiler barrier?

I wasn't referring to a compiler barrier. I'm referring to one of the 
real barrier instructions.

> I think we need the barrier() even for the relaxed variant, but
> that is fairly lightweight. What would be a reason to have more?

Please see my response to Russell in other other email. I think it 
should answer your question by clarifying my point.

>> Firstly, I don't know how many people noticed this and realize they
>> can't depend on readl/writel to take care of the mb()s for them. Seems
>> like an unnecessary encouragement to make mistakes when it didn't need
>> to be so.
>>
>> Secondly, even if they realize they have to take care of it, they will
>> have to continue using mb()s in to force ordering between their
>> reads/writes. So, are we depending on the compiler to optimize these
>> extra mb() out in the case where the config is enabled? I'm not sure it
>> will be able to optimize out the extra mb()s in all cases.
>
> The compiler certainly won't merge multiple inline assembly statements,
> but multiple barrier() statements don't make it worse than just one.
> barrier() just forces accessing variables from memory that could otherwise
> be kept in registers.

Yes, I was aware of how compiler barriers work and how multiple 
consecutive compiler barriers are the same as one compiler barrier. The 
email was about the real barrier instructions.

> The other problems we still need to fix are the complete absence of
> barriers in inb()/outb() style accessors, which are meant to be stricter
> than readl()/writel(), and the fact that we rely on undefined behavior
> in gcc regarding the atomicity of multi-byte accesses, as we recently
> found out when a new gcc turned a readl into byte accesses.

I haven't gotten that far :-) I really appreciate response!

Thanks,
Saravana
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