PERFORCE change 152649 for review - busdma problem

[Alexander Motin]

Hans Petter Selasky wrote:
> The thing is, you cannot specify how many bytes are on the first page which 
> can take a full physical address. It is assumed that after the page offset 
> wraps to zero a new page begins.
> 
> Example:
> 
> xxx: means data
> 
> Virtual memory:
>   PG0  PG1
> | xxx|xxx |
> 
> In busdma bounce buffer:
> 
>   PG0  PG1
> |xxx |xxx |
> 
> You see there is a hole just before PG1, and it is impossible to tell the USB 
> hardware to skip this hole.

I have seen very alike organization at SD host controller. There is SDMA 
mode, to which most existing controllers limited. There is just initial 
address and total data size can be specified. But on every page boundary 
controller stops transfer and allows software to load the address of the 
next data page. This technique is ugly and new specification provides 
new ADMA and ADMA2 modes which allow start address and length to be 
specified for every data segment.

Now within my sdhci driver I have implemented page bouncing by myself, I 
am preallocating one aligned 4K page and reloading it with new data on 
each page boundary hit. There are many different data alignment 
bugs/limitations in that controllers, so such unperfect DMA operation 
also allows me to manage most of them. Linux sdhci driver instead just 
don't use this page boundary interrupt, operating with only one 
continuous memory block, completely disabling DMA on buggy controllers.

If I understand correctly present busdma API (may be not), it does not 
allow you to be sure that both of segment boundaries will be aligned to 
the page boundaries. You are only specifying maximum number of segments 
and maximum segment size, so busdma, if it wish, may theoretically give 
you several small segments instead of single complete page. So if you 
are not ready to receive uneven sized segments, you probably should not 
specify that you want to receive more then 1 segment.

May be it would be good to add some flag to the bus_dma_tag_create() 
which would signaled that hardware is able to handle segment boundaries 
only at page boundaries. It would also allow us to properly combine 
alignment and page boundary requirements. But it is surely not easy task.

Correct me please if I am wrong.

-- 
Alexander Motin