L4Re: Allocating pinned memory and obtaining its physical address

[Jakub Jermář]

Hi Paul,

On 4/23/23 00:29, Paul Boddie wrote:
> Hello,
> 
> Sorry to be lazy and not dig too deeply into the documentation and code, but 
> once upon a time it was possible to use the l4re_ma functions to allocate 
> pinned memory. Then, one would use the dataspace protocol to obtain the 
> physical address of the memory via the capability involved in memory 
> allocation.
> 
> This exercise would be sufficient to get into a position to initialise 
> peripherals in a system-on-a-chip, setting registers with the physical memory 
> addresses they would need for things like DMA. The initialisation of things 
> like framebuffers would rely on this procedure.
> 
> I see that the "phys" operation in the dataspace protocol was discarded some 
> time ago, and now there is a "DMA space" abstraction instead:
> 
> https://l4re.org/doc/group__api__l4re__c__dma.html
> 
> Are there any examples of how I might obtain a physical address associated 
> with a block of memory using this new API?

The idea is that you have a dataspace and a DMA space. In order to
obtain a DMA address that you can then use to program your I/O device,
you need to map a portion of the dataspace into the DMA space. After use
you unmap it from the DMA space.

See for example the implementation of the Inout buffer in our nvme-driver:

https://github.com/kernkonzept/nvme-driver/blob/master/server/src/inout_buffer.h#L46

This API hides the differences between physical addresses and addresses
translated by IOMMU - you simply get a DMA address with which you work.
Whether you get a physical or IOMMU-virtual address depends on how the
DMA space was created (and also on the capabilities of the underlying
hardware).

Cheers,
Jakub

> 
> Thanks in advance for any guidance that might be offered!
> 
> Paul
> 
> 
> 
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