Re: [PATCH 04/23] cxl/pci: Implement Interface Ready Timeout

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Fri, 19 Nov 2021 16:02:31 -0800
Ben Widawsky <ben.widawsky@intel.com> 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 :)

> 
> Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> ---
> 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

> +	 * 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 =