On Tuesday 16 June 2015 at 18:11:56 Torsten Fleischer wrote:
> On Monday 15 June 2015 at 17:17:51 Cyrille Pitchen wrote:
[...]
> > @@ -365,16 +371,13 @@ static int atc_get_bytes_left(struct dma_chan *chan,
> > dma_cookie_t cookie) }
> >
> > /*
> >
> > - * For the last descriptor in the chain we can calculate
> > + * For the current descriptor in the chain we can calculate
> >
> > * the remaining bytes using the channel's register.
> >
> > - * Note that the transfer width of the first and last
> > - * descriptor may differ.
> >
> > */
> >
> > - if (!desc->lli.dscr)
> > - ret = atc_calc_bytes_left_from_reg(ret, atchan, desc);
> > + ret = atc_calc_bytes_left_from_reg(ret, atchan);
>
> I see two issues here.
>
> First this always returns the number if remaining bytes of the current child
> transfer and not of the entire hardware linked list transfer.
>
Oops, I was wrong with the first issue. Please ignore this.
Sorry for that.
> Finally, for the calculation of the remaining bytes of a hardware linked
> list the value of DSCR is used to get the current child transfer.
> But regarding to a running DMA transfer, reading DSCR and CTRLA is not
> atomic. If you read CTRLA after DSCR then the value may not refer to the
> child transfer as read from DSCR, but to a subsequent one.
> Its only safe for the first transfer where CTRLA is read before DSCR and for
> the last transfer where CTRLA is read after DSCR.
>
> > } else {
> >
> > /* single transfer */
> >
> > - ret = atc_calc_bytes_left_from_reg(ret, atchan, desc_first);
> > + ret = atc_calc_bytes_left_from_reg(ret, atchan);
> >
> > }
> >
> > return ret;
> >
[...]
Regards,
Torsten
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