On 21-11-22 15:02:27, Jonathan Cameron wrote:
> On Fri, 19 Nov 2021 16:02:31 -0800
> Ben Widawsky <ben.widawsky@xxxxxxxxx> wrote:
>
> > The original driver implementation used the doorbell timeout for the
> > Mailbox Interface Ready bit to piggy back off of, since the latter
> > doesn't have a defined timeout. This functionality, introduced in
> > 8adaf747c9f0 ("cxl/mem: Find device capabilities"), can now be improved
> > since a timeout has been defined with an ECN to the 2.0 spec.
> >
> > While devices implemented prior to the ECN could have an arbitrarily
> > long wait and still be within spec, the max ECN value (256s) is chosen
> > as the default for all devices. All vendors in the consortium agreed to
> > this amount and so it is reasonable to assume no devices made will
> > exceed this amount.
>
> Optimistic :)
>
Reasonable to assume is certainly not the same as "in reality". I can soften
this wording.
> >
> > Signed-off-by: Ben Widawsky <ben.widawsky@xxxxxxxxx>
> > ---
> > This patch did not exist in RFCv2
> > ---
> > drivers/cxl/pci.c | 29 +++++++++++++++++++++++++++++
> > 1 file changed, 29 insertions(+)
> >
> > diff --git a/drivers/cxl/pci.c b/drivers/cxl/pci.c
> > index 6c8d09fb3a17..2cef9fec8599 100644
> > --- a/drivers/cxl/pci.c
> > +++ b/drivers/cxl/pci.c
> > @@ -2,6 +2,7 @@
> > /* Copyright(c) 2020 Intel Corporation. All rights reserved. */
> > #include <linux/io-64-nonatomic-lo-hi.h>
> > #include <linux/module.h>
> > +#include <linux/delay.h>
> > #include <linux/sizes.h>
> > #include <linux/mutex.h>
> > #include <linux/list.h>
> > @@ -298,6 +299,34 @@ static int cxl_pci_mbox_send(struct cxl_dev_state *cxlds, struct cxl_mbox_cmd *c
> > static int cxl_pci_setup_mailbox(struct cxl_dev_state *cxlds)
> > {
> > const int cap = readl(cxlds->regs.mbox + CXLDEV_MBOX_CAPS_OFFSET);
> > + unsigned long timeout;
> > + u64 md_status;
> > + int rc;
> > +
> > + /*
> > + * CXL 2.0 ECN "Add Mailbox Ready Time" defines a capability field to
> > + * dictate how long to wait for the mailbox to become ready. For
> > + * simplicity, and to handle devices that might have been implemented
>
> I'm not keen on the 'for simplicity' argument here. If the device is advertising
> a lower value, then that is what we should use. It's fine to wait the max time
> if nothing is specified. It'll cost us a few lines of code at most unless
> I am missing something...
>
> Jonathan
>
Let me pose it a different way, if a device advertises 1s, but for whatever
takes 4s to come up, should we penalize it over the device advertising 256s? The
way this field is defined in the spec would [IMHO] lead vendors to simply put
the max field in there to game the driver, so why not start off with just
insisting they don't?
> > + * prior to the ECN, wait the max amount of time no matter what the
> > + * device says.
> > + */
> > + timeout = jiffies + 256 * HZ;
> > +
> > + rc = check_device_status(cxlds);
> > + if (rc)
> > + return rc;
> > +
> > + do {
> > + md_status = readq(cxlds->regs.memdev + CXLMDEV_STATUS_OFFSET);
> > + if (md_status & CXLMDEV_MBOX_IF_READY)
> > + break;
> > + if (msleep_interruptible(100))
> > + break;
> > + } while (!time_after(jiffies, timeout));
> > +
> > + /* It's assumed that once the interface is ready, it will remain ready. */
> > + if (!(md_status & CXLMDEV_MBOX_IF_READY))
> > + return -EIO;
> >
> > cxlds->mbox_send = cxl_pci_mbox_send;
> > cxlds->payload_size =
>
