Re: [Qemu-devel] [PATCH] add function DMA_set_return and delete bh_schedule in dma.c

[Zhi Hui Li]

On 2012年04月17日 16:46, Paolo Bonzini wrote:
> Il 16/04/2012 14:14, Stefan Hajnoczi ha scritto:
> > Why are you removing the rearm behavior?  I'm pretty sure other ISA
> > devices rely on this otherwise the code wouldn't exist.
>
> Li is correct in that the code can be simplified a lot, but indeed his
> axe went a bit too far. :)
>
> With the exception of the floppy all other DMA devices do not reenter
> DMA_run, and do not need the idle bottom half hack because DMA is done
> once per interrupt.  And now that the floppy is asynchronous it doesn't
> need the idle bottom half hack, either.
>
> So DMA_run can call channel_run from a while loop like this:
>
>     while ((0 == (d->mask&  mask))&&  (0 != (d->status&  (mask<<  4)))) {
>         // pseudo-code, channel_running and channel_is_asynchronous
>         // would be fields in dma_regs
>         if (channel_running[ichan]) {
>             assert(channel_is_asynchronous[ichan]);
>             break;
>         }
>
>         channel_running[ichan]++;
>         channel_run (icont, ichan);
>
>         // channel_is_asynchronous would be set by a new function
>         // DMA_register_channel_async.
>         if (channel_is_asynchronous[ichan]) {
>              break;
>         }
>         channel_running[ichan]--;
>     }
>
> and DMA_set_return needs to do a tail call to "complete" the while loop:
>
>     assert(channel_is_asynchronous[ichan]);
>     assert(channel_running[ichan] == 1);
>     channel_running[ichan]--;
>     if ((0 == (d->mask&  mask))&&  (0 != (d->status&  (mask<<  4)))) {
>         channel_run (icont, ichan);
>     }
>
> Paolo

Thank you very much for giving me suggestions, I have tried the method, 
it works well. But I also have another idea, I modify fdc.c by adding a 
flag to avoid re-enter, I have tested it. I will send a patch later. If 
you have any comments, please give me your opinion. thanks.