Re: About PHYS_TO_DMAP

On 4/14/23 04:31, Dmitry Salychev wrote:
> 
> Hi,
> 
> I'm struggling to understand which KVA will be returned by PHYS_TO_DMAP
> on arm64. For example, if I'll create a DMA tag this way:
> 
> 	bus_dma_tag_create(
> 	    bus_get_dma_tag(dev),
> 	    sc->buf_align, 0,		/* alignment, boundary */
> 	    DMAP_MAX_PHYSADDR,		/* low restricted addr */
> 	    DMAP_MIN_PHYSADDR,		/* high restricted addr */

I think you are confused about the purpose of lowaddr and highaddr. They 
specify the window of bus-space addresses that are *inaccessible* *to 
the device* for DMA. It does not prevent you from providing a buffer 
backed by physical memory within this range, but in this case bounce 
pages will be used as an intermediate to perform the DMA transaction.

Most commonly, if the device can only address a 32-bit range, then the 
values lowaddr=BUS_SPACE_MAXADDR_32BIT and highaddr=BUS_SPACE_MAXADDR 
will be given. If the entire bus range is accessible to the device for 
DMA, a zero-sized window will be given: lowaddr=BUS_SPACE_MAXADDR, 
highaddr=BUS_SPACE_MAXADDR.

This is all described in adequate detail in busdma(9), but it is still 
not easily understood without a lot of code study, in my experience.

> 	    NULL, NULL,			/* filter, filterarg */
> 	    BUF_SIZE, 1,		/* maxsize, nsegments */
> 	    BUF_SIZE, 0,		/* maxsegsize, flags */
> 	    NULL, NULL,			/* lockfunc, lockarg */
> 	    &dmat);
> 
> in order to restrict any physical addresses but a window defined by
> DMAP_MIN_PHYSADDR and DMAP_MAX_PHYSADDR. Later on when I'll be
> mapping my mbuf (BUF_SIZE) with
> 
> 	bus_dmamap_load_mbuf_sg(dmat, dmap,
> 	    m, &segs, &nsegs, BUS_DMA_NOWAIT);
> 
> I expect that
> 
> 	m->m_data == PHYS_TO_DMAP(segs[0].ds_addr)

Why do you expect or need this to be the case?

busdma is not responsible for setting or modifying m_data, it comes from 
wherever you allocated the mbuf. Calling bus_dmamap_load_mbuf_sg() will 
prepare a DMA mapping where the m_data buffer is used as the 
source/destination for the DMA transaction, but busdma does not allocate 
the buffer itself.

> 
> but it isn't true. Could somebody explain what exactly is returned by
> PHYS_TO_DMAP in this case and whether it's possible to translate
> physical address to KVA as fast as possible (O(1) ideally).
>

PHYS_TO_DMAP is always a linear calculation of: physaddr + DMAP_MIN_ADDRESS.

I do not think PHYS_TO_DMAP is in use at all in this example, or 
anywhere within busdma really.

> Regards,
> Dmitry
> 
> --
> Open source software/hardware enthusiast
> hackaday.io/dsl | github.com/mcusim | patreon.com/salychev
>