Re: [PATCH v7 6/7] PCI: Bring out the pcie link speed to MBps logic to new function

[Bjorn Helgaas <helgaas@kernel.org>]

Mention the new interface name in the subject and in the commit log.

s/pcie/PCIe/

The subject says "to MBps", but the commit log says "to frequency".

On Fri, Feb 23, 2024 at 08:18:03PM +0530, Krishna chaitanya chundru wrote:
> Bring the switch case in pcie_link_speed_mbps to new function to
> the header file so that it can be used in other places like
> in controller driver.

s/pcie_link_speed_mbps/pcie_link_speed_mbps()/ to identify it as a
function.

> Create a new macro to convert from MBps to frequency.

Include the new macro name here.

I think pcie_link_speed_mbps() returns Mb/s (mega*bits* per second),
not MB/s (mega*bytes* per second).

> Signed-off-by: Krishna chaitanya chundru <quic_krichai@quicinc.com>
> ---
>  drivers/pci/pci.c | 19 +------------------
>  drivers/pci/pci.h | 24 ++++++++++++++++++++++++
>  2 files changed, 25 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index d8f11a078924..b441ab862a8d 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -6309,24 +6309,7 @@ int pcie_link_speed_mbps(struct pci_dev *pdev)
>  	if (err)
>  		return err;
>  
> -	switch (to_pcie_link_speed(lnksta)) {
> -	case PCIE_SPEED_2_5GT:
> -		return 2500;
> -	case PCIE_SPEED_5_0GT:
> -		return 5000;
> -	case PCIE_SPEED_8_0GT:
> -		return 8000;
> -	case PCIE_SPEED_16_0GT:
> -		return 16000;
> -	case PCIE_SPEED_32_0GT:
> -		return 32000;
> -	case PCIE_SPEED_64_0GT:
> -		return 64000;
> -	default:
> -		break;
> -	}
> -
> -	return -EINVAL;
> +	return pcie_link_speed_to_mbps(to_pcie_link_speed(lnksta));
>  }
>  EXPORT_SYMBOL(pcie_link_speed_mbps);
>  
> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> index 2336a8d1edab..82e715ebe383 100644
> --- a/drivers/pci/pci.h
> +++ b/drivers/pci/pci.h
> @@ -282,6 +282,30 @@ void pci_bus_put(struct pci_bus *bus);
>  	 (speed) == PCIE_SPEED_2_5GT  ?  2500*8/10 : \
>  	 0)
>  
> +static inline int pcie_link_speed_to_mbps(enum pci_bus_speed speed)
> +{
> +	switch (speed) {
> +	case PCIE_SPEED_2_5GT:
> +		return 2500;
> +	case PCIE_SPEED_5_0GT:
> +		return 5000;
> +	case PCIE_SPEED_8_0GT:
> +		return 8000;
> +	case PCIE_SPEED_16_0GT:
> +		return 16000;
> +	case PCIE_SPEED_32_0GT:
> +		return 32000;
> +	case PCIE_SPEED_64_0GT:
> +		return 64000;
> +	default:
> +		break;
> +	}
> +
> +	return -EINVAL;
> +}
> +
> +#define PCIE_MBS2FREQ(speed) (pcie_link_speed_to_mbps(speed) * 1000)

I feel like I might have asked some of this before; if so, my
apologies and maybe a comment would be useful here to save answering
again.

The MBS2FREQ name suggests that "speed" is Mb/s, but it's not; it's an
enum pci_bus_speed just like PCIE_SPEED2MBS_ENC() takes.

When PCI SIG defines a new data rate, PCIE_MBS2FREQ() will do
something completely wrong when pcie_link_speed_to_mbps() returns
-EINVAL.  I think it would be better to do this in a way that we can
warn about the unknown speed and fall back to some reasonable default
instead of whatever (-EINVAL * 1000) works out to.

PCIE_MBS2FREQ() looks an awful lot like PCIE_SPEED2MBS_ENC(), except
that it doesn't adjust for the encoding overhead and it multiplies by
1000.  I don't know what that result means.  The name suggests a
frequency?

  pcie_link_speed_to_mbps(PCIE_SPEED_2_5GT) == 2500 Mbit/s (raw data rate)
  PCIE_SPEED2MBS_ENC(PCIE_SPEED_2_5GT) == 2000 Mbit/s or 2 Gbit/s (effective data rate)
  PCIE_MBS2FREQ(PCIE_SPEED_2_5GT) == 2500000 (? 2.5M of something)

I don't really know how OPP works, but it looks like maybe
PCIE_MBS2FREQ() is a shim that depends on how the OPP tables in DT are
encoded?  I'm surprised that the DT OPP tables aren't encoded with
either the raw data rate or the effective data rate directly instead
of what looks like the raw data rate / 1000.

Is this a standard OPP encoding that will apply to other drivers?  If
so, it would be helpful to point to where that encoding is defined.
If not, PCIE_MBS2FREQ() should probably be defined in pcie-qcom.c.

Bjorn