On 21-02-01 14:23:47, David Rientjes wrote:
> On Mon, 1 Feb 2021, Ben Widawsky wrote:
>
> > > > > > +static int cxl_mem_setup_mailbox(struct cxl_mem *cxlm)
> > > > > > +{
> > > > > > + const int cap = cxl_read_mbox_reg32(cxlm,
> > > > > > CXLDEV_MB_CAPS_OFFSET);
> > > > > > +
> > > > > > + cxlm->mbox.payload_size =
> > > > > > + 1 << CXL_GET_FIELD(cap, CXLDEV_MB_CAP_PAYLOAD_SIZE);
> > > > > > +
> > > > > > + /* 8.2.8.4.3 */
> > > > > > + if (cxlm->mbox.payload_size < 256) {
> > > > > > + dev_err(&cxlm->pdev->dev, "Mailbox is too small (%zub)",
> > > > > > + cxlm->mbox.payload_size);
> > > > > > + return -ENXIO;
> > > > > > + }
> > > > >
> > > > > Any reason not to check cxlm->mbox.payload_size > (1 << 20) as well
> > > > > and
> > > > > return ENXIO if true?
> > > >
> > > > If some crazy vendor wanted to ship a mailbox larger than 1M, why
> > > > should the
> > > > driver not allow it?
> > > >
> > >
> > > Because the spec disallows it :)
> >
> > I don't see it being the driver's responsibility to enforce spec correctness
> > though. In certain cases, I need to use the spec, like I have to pick /some/
> > mailbox timeout. For other cases...
> >
> > I'm not too familiar with what other similar drivers may or may not do in
> > situations like this. The current 256 limit is mostly a reflection of that
> > being
> > too small to even support advertised mandatory commands. So things can't
> > work in
> > that scenario, but things can work if they have a larger register size (so
> > long
> > as the BAR advertises enough space).
> >
>
> I don't think things can work above 1MB, either, though. Section
> 8.2.8.4.5 specifies 20 bits to define the payload length, if this is
> larger than cxlm->mbox.payload_size it would venture into the reserved
> bits of the command register.
>
> So is the idea to allow cxl_mem_setup_mailbox() to succeed with a payload
> size > 1MB and then only check 20 bits for the command register?
So it's probably a spec bug, but actually the payload size is 21 bits... I'll
check if that was a mistake.
