Re: [PATCH] PCI: Add PCI_EXT_CAP_ID_PL_64GT define

[Jonathan Cameron]

On Thu,  8 Aug 2024 10:12:39 +0800
412574090@xxxxxxx wrote:

> > On Tue, Aug 06, 2024 at 10:27:46AM +0800, 412574090@xxxxxxx wrote:  
> > > From: weiyufeng <weiyufeng@xxxxxxxxxx>
> > > 
> > > PCIe r6.0, sec 7.7.7.1, defines a new 64.0 GT/s PCIe Extended Capability
> > > ID,Add the define for PCI_EXT_CAP_ID_PL_64GT for drivers that will want
> > > this whilst doing Gen6 accesses.
> > > 
> > > Signed-off-by: weiyufeng <weiyufeng@xxxxxxxxxx>
> > > ---
> > >  include/uapi/linux/pci_regs.h | 1 +
> > >  1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/include/uapi/linux/pci_regs.h b/include/uapi/linux/pci_regs.h
> > > index 94c00996e633..cc875534dae1 100644
> > > --- a/include/uapi/linux/pci_regs.h
> > > +++ b/include/uapi/linux/pci_regs.h
> > > @@ -741,6 +741,7 @@
> > >  #define PCI_EXT_CAP_ID_DLF	0x25	/* Data Link Feature */
> > >  #define PCI_EXT_CAP_ID_PL_16GT	0x26	/* Physical Layer 16.0 GT/s */
> > >  #define PCI_EXT_CAP_ID_PL_32GT  0x2A    /* Physical Layer 32.0 GT/s */
> > > +#define PCI_EXT_CAP_ID_PL_64GT  0x31    /* Physical Layer 64.0 GT/s */  
> >
> > It probably makes sense to add this (with the corrections noted by
> > Ilpo), but I *would* like to see where it's used.
> >
> > I asked a similar question at
> > https://lore.kernel.org/all/20230531095713.293229-1-ben.dooks@xxxxxxxxxxxxxxx/
> > when we added PCI_EXT_CAP_ID_PL_32GT, but never got a specific
> > response.  I don't really want to end up with drivers doing their own
> > thing if it's something that could be done in the PCI core and shared.
> >  
> PCI_EXT_CAP_ID_PL_32GT and PCI_EXT_CAP_ID_PL_64GT have not used now,but 
> PCI_EXT_CAP_ID_PL_16GT have usage example,in drivers/pci/controller/dwc/pcie-tegra194.c
> function config_gen3_gen4_eq_presets():
> 
> offset = dw_pcie_find_ext_capability(pci,
> 				     PCI_EXT_CAP_ID_PL_16GT) +
> 		PCI_PL_16GT_LE_CTRL;
> 
> PCI_EXT_CAP_ID_PL_32GT and PCI_EXT_CAP_ID_PL_64GT could be used while need to
> get this similar attribute。

I'll bite.  In PCI_EXTE_CAP_ID_PL_32GT PCIe 6.1 which I happen to have
open has some writeable fields in the control register.  So
kind of fair enough a driver might write them.  In my view we should
probably have waited for such a use to turn up.

The Physical Layer 64.0 GT/s Extended Capability control register is
entirely reserved. So as of now, I don't see a use for this capability
until the PCIe spec adds something.

> 
> > >  #define PCI_EXT_CAP_ID_DOE	0x2E	/* Data Object Exchange */
> > >  #define PCI_EXT_CAP_ID_MAX	PCI_EXT_CAP_ID_DOE
> > >  
> > > -- 
> > > 2.25.1
> > >   
> --
> Thanks,
> 
> weiyufeng
> 
>