On Fri, Nov 19, 2021 at 4:03 PM Ben Widawsky <ben.widawsky@xxxxxxxxx> wrote:
>
> The expectation is that the mailbox interface ready bit is the first
> step in access through the mailbox interface. Therefore, waiting for the
> doorbell busy bit to be clear would imply that the mailbox interface is
> ready. The original driver implementation used the doorbell timeout for
> the Mailbox Interface Ready bit to piggyback off of, since the latter
> doesn't have a defined timeout (introduced in 8adaf747c9f0 ("cxl/mem:
> Find device capabilities"), a timeout has since been defined with an ECN
> to the 2.0 spec). With the current driver waiting for mailbox interface
> ready as a part of probe() it's no longer necessary to use the
> piggyback.
>
> With the piggybacking no longer necessary it doesn't make sense to check
> doorbell status when acquiring the mailbox. It will be checked during
> the normal mailbox exchange protocol.
>
> Signed-off-by: Ben Widawsky <ben.widawsky@xxxxxxxxx>
> ---
> This patch did not exist in RFCv2
> ---
> drivers/cxl/pci.c | 25 ++++++-------------------
> 1 file changed, 6 insertions(+), 19 deletions(-)
>
> diff --git a/drivers/cxl/pci.c b/drivers/cxl/pci.c
> index 2cef9fec8599..869b4fc18e27 100644
> --- a/drivers/cxl/pci.c
> +++ b/drivers/cxl/pci.c
> @@ -221,27 +221,14 @@ static int cxl_pci_mbox_get(struct cxl_dev_state *cxlds)
>
> /*
> * XXX: There is some amount of ambiguity in the 2.0 version of the spec
> - * around the mailbox interface ready (8.2.8.5.1.1). The purpose of the
> + * around the mailbox interface ready (8.2.8.5.1.1). The purpose of the
> * bit is to allow firmware running on the device to notify the driver
> - * that it's ready to receive commands. It is unclear if the bit needs
> - * to be read for each transaction mailbox, ie. the firmware can switch
> - * it on and off as needed. Second, there is no defined timeout for
> - * mailbox ready, like there is for the doorbell interface.
> - *
> - * Assumptions:
> - * 1. The firmware might toggle the Mailbox Interface Ready bit, check
> - * it for every command.
> - *
> - * 2. If the doorbell is clear, the firmware should have first set the
> - * Mailbox Interface Ready bit. Therefore, waiting for the doorbell
> - * to be ready is sufficient.
> + * that it's ready to receive commands. The spec does not clearly define
> + * under what conditions the bit may get set or cleared. As of the 2.0
> + * base specification there was no defined timeout for mailbox ready,
> + * like there is for the doorbell interface. This was fixed with an ECN,
> + * but it's possible early devices implemented this before the ECN.
Can we just drop comment block altogether? Outside of
cxl_pci_setup_mailbox() the only time the mailbox status should be
checked is after a doorbell timeout after submitting a command.
> */
> - rc = cxl_pci_mbox_wait_for_doorbell(cxlds);
> - if (rc) {
> - dev_warn(dev, "Mailbox interface not ready\n");
> - goto out;
> - }
> -
> md_status = readq(cxlds->regs.memdev + CXLMDEV_STATUS_OFFSET);
> if (!(md_status & CXLMDEV_MBOX_IF_READY && CXLMDEV_READY(md_status))) {
> dev_err(dev, "mbox: reported doorbell ready, but not mbox ready\n");
This error message is obsolete since nothing is pre-checking the
mailbox anymore, and per above I see no problem waiting to check the
status until after the mailbox has failed to respond after a timeout.
