CODEWITHSUNDEEP
linux-kernellinux-device-driverembedded-linux
Yonatan Amir
Yonatan Amir

Reputation: 85

How to access physical address during interrupt handler linux

I wrote an interrupt handler in linux. Part of the handler logic I need to access physical address. I used iormap function but then I fell into KDB during handler time. I started to debug it and i saw the below code which finally called by ioremap kernel code

What should I do? Is there any other way instead of map the region before? If i will need to map it before it means that i will probably need to map and cache a lot of unused area. BTW what are the limits for ioremap?

Upvotes: 1

Views: 462

Answers (1)

wkz
wkz

Reputation: 2263

Setting up a new memory mapping is an expensive operation, which typically requires calls to potentially blocking functions (e.g. grabbing locks). So your strategy has two problems:

  1. Calling a blocking function is not possible in your context (there is no kernel thread associated with your interrupt handler, so there is no way for the kernel to resume it if it had to be put to sleep).

  2. Setting up/tearing down a mapping per IRQ would be a bad idea performance-wise (even if we ignore the fact that it can't be done).

Typically, you would setup any mappings you need in a driver's probe() function (or in the module's init() if it's more of a singleton thing). This mapping is then kept in some private device data structure, which is passed as the last argument to some variant of request_irq(), so that the kernel then passes it back as the second argument to the IRQ handler.

Not sure what you mean by "need to map and cache a lot of unused area".

Depending on your particular system, you may end up consuming an entry in your CPU's MMU, or you may just re-use a broader mapping that was setup by whoever wrote the BSP. That's just the cost of doing business on a virtual memory system.

Caching is typically not enabled on I/O memory because of the many side-effects of both reads and writes. For the odd cases when you need it, you have to use ioremap_cached().

Upvotes: 4

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