Re: [PATCH 1/2] PCI: Avoid pointless capability searches

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

On Fri, 7 Feb 2025, Bjorn Helgaas wrote:

> From: Bjorn Helgaas <bhelgaas@google.com>
> 
> Many of the save/restore functions in the pci_save_state() and
> pci_restore_state() paths depend on both a PCI capability of the device and
> a pci_cap_saved_state structure to hold the configuration data, and they
> skip the operation if either is missing.
> 
> Look for the pci_cap_saved_state first so if we don't have one, we can skip
> searching for the device capability, which requires several slow config
> space accesses.
> 
> Remove some error messages if the pci_cap_saved_state is not found so we
> don't complain about having no saved state for a capability the device
> doesn't have.  We have already warned in pci_allocate_cap_save_buffers() if
> the capability is present but we were unable to allocate a buffer.
> 
> Other than the message change, no functional change intended.
> 
> Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
> ---
>  drivers/pci/pci.c       | 27 ++++++++++++++-------------
>  drivers/pci/pcie/aspm.c | 15 ++++++++-------
>  drivers/pci/vc.c        | 22 +++++++++++-----------
>  3 files changed, 33 insertions(+), 31 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 869d204a70a3..503376bf7e75 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -1686,10 +1686,8 @@ static int pci_save_pcie_state(struct pci_dev *dev)
>  		return 0;
>  
>  	save_state = pci_find_saved_cap(dev, PCI_CAP_ID_EXP);
> -	if (!save_state) {
> -		pci_err(dev, "buffer not found in %s\n", __func__);
> +	if (!save_state)
>  		return -ENOMEM;
> -	}
>  
>  	cap = (u16 *)&save_state->cap.data[0];
>  	pcie_capability_read_word(dev, PCI_EXP_DEVCTL, &cap[i++]);
> @@ -1742,19 +1740,17 @@ static void pci_restore_pcie_state(struct pci_dev *dev)
>  
>  static int pci_save_pcix_state(struct pci_dev *dev)
>  {
> -	int pos;
>  	struct pci_cap_saved_state *save_state;
> +	u8 pos;
> +
> +	save_state = pci_find_saved_cap(dev, PCI_CAP_ID_PCIX);
> +	if (!save_state)
> +		return -ENOMEM;
>  
>  	pos = pci_find_capability(dev, PCI_CAP_ID_PCIX);
>  	if (!pos)
>  		return 0;
>  
> -	save_state = pci_find_saved_cap(dev, PCI_CAP_ID_PCIX);
> -	if (!save_state) {
> -		pci_err(dev, "buffer not found in %s\n", __func__);
> -		return -ENOMEM;
> -	}
> -
>  	pci_read_config_word(dev, pos + PCI_X_CMD,
>  			     (u16 *)save_state->cap.data);
>  
> @@ -1763,14 +1759,19 @@ static int pci_save_pcix_state(struct pci_dev *dev)
>  
>  static void pci_restore_pcix_state(struct pci_dev *dev)
>  {
> -	int i = 0, pos;
>  	struct pci_cap_saved_state *save_state;
> +	u8 pos;
> +	int i = 0;
>  	u16 *cap;
>  
>  	save_state = pci_find_saved_cap(dev, PCI_CAP_ID_PCIX);
> -	pos = pci_find_capability(dev, PCI_CAP_ID_PCIX);
> -	if (!save_state || !pos)
> +	if (!save_state)
>  		return;
> +
> +	pos = pci_find_capability(dev, PCI_CAP_ID_PCIX);
> +	if (!pos)
> +		return;
> +
>  	cap = (u16 *)&save_state->cap.data[0];
>  
>  	pci_write_config_word(dev, pos + PCI_X_CMD, cap[i++]);
> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> index e0bc90597dca..007e4a082e6f 100644
> --- a/drivers/pci/pcie/aspm.c
> +++ b/drivers/pci/pcie/aspm.c
> @@ -35,16 +35,14 @@ void pci_save_ltr_state(struct pci_dev *dev)
>  	if (!pci_is_pcie(dev))
>  		return;
>  
> +	save_state = pci_find_saved_ext_cap(dev, PCI_EXT_CAP_ID_LTR);
> +	if (!save_state)
> +		return;
> +
>  	ltr = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_LTR);
>  	if (!ltr)
>  		return;
>  
> -	save_state = pci_find_saved_ext_cap(dev, PCI_EXT_CAP_ID_LTR);
> -	if (!save_state) {
> -		pci_err(dev, "no suspend buffer for LTR; ASPM issues possible after resume\n");
> -		return;
> -	}
> -
>  	/* Some broken devices only support dword access to LTR */
>  	cap = &save_state->cap.data[0];
>  	pci_read_config_dword(dev, ltr + PCI_LTR_MAX_SNOOP_LAT, cap);
> @@ -57,8 +55,11 @@ void pci_restore_ltr_state(struct pci_dev *dev)
>  	u32 *cap;
>  
>  	save_state = pci_find_saved_ext_cap(dev, PCI_EXT_CAP_ID_LTR);
> +	if (!save_state)
> +		return;
> +
>  	ltr = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_LTR);
> -	if (!save_state || !ltr)
> +	if (!ltr)
>  		return;
>  
>  	/* Some broken devices only support dword access to LTR */
> diff --git a/drivers/pci/vc.c b/drivers/pci/vc.c
> index a4ff7f5f66dd..c39f3be518d4 100644
> --- a/drivers/pci/vc.c
> +++ b/drivers/pci/vc.c
> @@ -355,20 +355,17 @@ int pci_save_vc_state(struct pci_dev *dev)
>  	int i;
>  
>  	for (i = 0; i < ARRAY_SIZE(vc_caps); i++) {
> -		int pos, ret;
>  		struct pci_cap_saved_state *save_state;
> +		int pos, ret;
> +
> +		save_state = pci_find_saved_ext_cap(dev, vc_caps[i].id);
> +		if (!save_state)
> +			return -ENOMEM;
>  
>  		pos = pci_find_ext_capability(dev, vc_caps[i].id);
>  		if (!pos)
>  			continue;
>  
> -		save_state = pci_find_saved_ext_cap(dev, vc_caps[i].id);
> -		if (!save_state) {
> -			pci_err(dev, "%s buffer not found in %s\n",
> -				vc_caps[i].name, __func__);
> -			return -ENOMEM;
> -		}

I think this order change will cause a functional change because 
pci_allocate_vc_save_buffers() only allocated for those capabilities that 
are exist for dev. Thus, the loop will prematurely exit.

> -
>  		ret = pci_vc_do_save_buffer(dev, pos, save_state, true);
>  		if (ret) {
>  			pci_err(dev, "%s save unsuccessful %s\n",
> @@ -392,12 +389,15 @@ void pci_restore_vc_state(struct pci_dev *dev)
>  	int i;
>  
>  	for (i = 0; i < ARRAY_SIZE(vc_caps); i++) {
> -		int pos;
>  		struct pci_cap_saved_state *save_state;
> +		int pos;
> +
> +		save_state = pci_find_saved_ext_cap(dev, vc_caps[i].id);
> +		if (!save_state)
> +			continue;
>  
>  		pos = pci_find_ext_capability(dev, vc_caps[i].id);
> -		save_state = pci_find_saved_ext_cap(dev, vc_caps[i].id);
> -		if (!save_state || !pos)
> +		if (!pos)
>  			continue;
>  
>  		pci_vc_do_save_buffer(dev, pos, save_state, false);
> 

-- 
 i.