Re: [PATCH 19/22] platform/x86/intel/pmc/ssram: Refactor memory barrier for reentrant probe

[Ilpo Järvinen]

On Thu, 12 Mar 2026, David E. Box wrote:

> From: Xi Pardee <xi.pardee@xxxxxxxxxxxxxxx>
> 
> Switch the readiness synchronization from a global device_probed flag to
> per-index devid publication.

It would be nice if you could expand the synchronization explanation a 
bit. It's mostly in the comments below but it wouldn't hurt to add it here 
as well.

> This is required because a subsequent patch
> makes probe reentrant, so a single global flag can no longer reliably
> signal completion.
> 
> Signed-off-by: Xi Pardee <xi.pardee@xxxxxxxxxxxxxxx>
> Signed-off-by: David E. Box <david.e.box@xxxxxxxxxxxxxxx>
> ---
>  .../platform/x86/intel/pmc/ssram_telemetry.c  | 34 ++++++++-----------
>  1 file changed, 14 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/platform/x86/intel/pmc/ssram_telemetry.c b/drivers/platform/x86/intel/pmc/ssram_telemetry.c
> index 7db98037c521..246efdcf6950 100644
> --- a/drivers/platform/x86/intel/pmc/ssram_telemetry.c
> +++ b/drivers/platform/x86/intel/pmc/ssram_telemetry.c
> @@ -37,7 +37,6 @@ static const struct ssram_type pci_main = {
>  };
>  
>  static struct pmc_ssram_telemetry pmc_ssram_telems[3];
> -static bool device_probed;
>  
>  static inline u64 get_base(void __iomem *addr, u32 offset)
>  {
> @@ -52,8 +51,13 @@ static void pmc_ssram_get_devid_pwrmbase(void __iomem *ssram, unsigned int pmc_i
>  	pwrm_base = get_base(ssram, SSRAM_PWRM_OFFSET);
>  	devid = readw(ssram + SSRAM_DEVID_OFFSET);
>  
> -	pmc_ssram_telems[pmc_idx].devid = devid;
>  	pmc_ssram_telems[pmc_idx].base_addr = pwrm_base;
> +	/*
> +	 * Memory barrier is used to ensure the correct write order between base_addr
> +	 * and devid.
> +	 */
> +	smp_wmb();
> +	pmc_ssram_telems[pmc_idx].devid = devid;
>  }
>  
>  static int
> @@ -151,32 +155,28 @@ static int pmc_ssram_telemetry_pci_init(struct pci_dev *pcidev)
>   * * 0           - Success
>   * * -EAGAIN     - Probe function has not finished yet. Try again.
>   * * -EINVAL     - Invalid pmc_idx
> - * * -ENODEV     - PMC device is not available
>   */
>  int pmc_ssram_telemetry_get_pmc_info(unsigned int pmc_idx,
>  				     struct pmc_ssram_telemetry *pmc_ssram_telemetry)
>  {
> +	if (pmc_idx >= MAX_NUM_PMC)
> +		return -EINVAL;
> +
>  	/*
>  	 * PMCs are discovered in probe function. If this function is called before
> -	 * probe function complete, the result would be invalid. Use device_probed
> -	 * variable to avoid this case. Return -EAGAIN to inform the consumer to call
> +	 * probe function complete, the result would be invalid. Use devid to avoid
> +	 * this case. Return -EAGAIN to inform the consumer to call
>  	 * again later.
>  	 */
> -	if (!device_probed)
> +	if (!pmc_ssram_telems[pmc_idx].devid)
>  		return -EAGAIN;
>  
> +	pmc_ssram_telemetry->devid = pmc_ssram_telems[pmc_idx].devid;
>  	/*
>  	 * Memory barrier is used to ensure the correct read order between
> -	 * device_probed variable and PMC info.
> +	 * devid variable and base_addr.
>  	 */
>  	smp_rmb();
> -	if (pmc_idx >= MAX_NUM_PMC)
> -		return -EINVAL;
> -
> -	if (!pmc_ssram_telems[pmc_idx].devid)
> -		return -ENODEV;
> -
> -	pmc_ssram_telemetry->devid = pmc_ssram_telems[pmc_idx].devid;
>  	pmc_ssram_telemetry->base_addr = pmc_ssram_telems[pmc_idx].base_addr;
>  	return 0;
>  }
> @@ -209,12 +209,6 @@ static int pmc_ssram_telemetry_probe(struct pci_dev *pcidev, const struct pci_de
>  		ret = -EINVAL;
>  
>  probe_finish:

This label becomes pointless after the removal and the gotoers should just 
return directly.

> -	/*
> -	 * Memory barrier is used to ensure the correct write order between PMC info
> -	 * and device_probed variable.
> -	 */
> -	smp_wmb();
> -	device_probed = true;
>  	return ret;

-- 
 i.