CC linux-kernel
2010/9/18 Wolfgang Mües <wolfgang@xxxxxxxxxxxxx>:
> Hi,
Thanks for your reply.
>
> Am Freitag, 17. September 2010 16:59:25 schrieb Ming Lei:
>> The warning only means dma_free_coherent is called with irq disabled.
>> It is not very difficult to fix the warning in usb hcd, but I need to know
>> why the WARN_ON below is added into dma_free_coherent.
>>
>> WARN_ON(irqs_disabled());
>>
>> Anyone know the reason?
>
> Hmm... Coherent buffers are ment to be kept for the whole time a driver
Documentation/DMA_API.txt
Consistent memory is memory for which a write by either the
device or the processor can immediately be read by the processor
or device without having to worry about caching effects.
IMO, it is not necessary for a coherent buffer to be kept for
the whole time a driver operates on a device. Also, coherent buffer
is very limited in some arch(such as ARM), maybe we should use it
more dynamically, instead of keeping it for long time.
> operates on a device. If such a buffer is freed in an interrupt context (== a
irqs_disabled() doesn't mean the buffer is freed in an interrupt context.
> momentary action), this is most likely a bug in the driver. At least it is a
> misuse of the concept of coherent buffers.
If it is only a misuse, seems WARN_ON can be removed.
Sorry, I still don't get the real reason why WARN_ON(irqs_disabled()) is
added in dma_free_coherent.
--
Lei Ming
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