Re: Better MI memory commands

Daniel Jacobowitz
2010-07-09T18:59:17Z

On Fri, Jul 09, 2010 at 10:54:07PM +0400, Vladimir Prus wrote:
> On Wednesday 07 July 2010 20:29:55 Daniel Jacobowitz wrote:
>
> > > - if (xfer == 0)
> > > - return xfered;
> > > - if (xfer < 0)
> > > - {
> > > - remaining = half;
> > > - }
> > > - else
> > > - {
> > > - /* We have successfully read the first half. So, the
> > > - error must be in the second half. Adjust start and
> > > - remaining to point at the second half. */
> > > - xfered += xfer;
> > > - start += xfer;
> > > - remaining -= xfer;
> > > - }
> > > - half = remaining/2;
> > > + xfree (buffer);
> > > + xfered += to_read;
> > > }
> >
> > Why do we skip to_read bytes if we succeed at reading zero bytes?
> > For that matter, what does a return value of zero mean? It seems like
> > this would mean the same as -1.
>
> I am not really sure. Per documentation of target_read:
>
> Return the number of bytes actually transfered, or -1 if the
> transfer is not supported or otherwise fails. Return of a positive
> value less than LEN indicates that no further transfer is possible.
>
> So, value of 0 seems to mean 'there are no more bytes that that, honest',
> and that we probably don't need to try further. Documentation for
> to_xfer_partial seem to give such meaning to return of 0.

I'd suggest treating 0 and -1 the same, for memory.

> On the
> other hand, it's not clear what return value of <LEN might mean,
> and whether we should try to read remaining chunk. What would you suggest?

A return of less than LEN from xfer_partial doesn't mean anything; you
just retry. A return of less than LEN from target_read, though, is
supposed to mean that there is no point in retrying; the next byte is
inaccessible or does not exist.

It doesn't look like memory reads (unlike other partial transfers)
implement that; usually they just fail. But we can treat it that way
anyway. So <LEN means we got some number of bytes, and then we should
see what happens after those successful bytes.

--
Daniel Jacobowitz