linux-kernel - Re: [PATCH] PCI: dw-rockchip: Configure max payload size on host init

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Message-ID: <vc6z6pzgk2sjgkghnwfzlmj3dblrvpkfnsmnlb2nnkj2leju7t@uxy3kthse7g6>
Date: Mon, 21 Apr 2025 20:23:18 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Hans Zhang <18255117159@....com>
Cc: Niklas Cassel <cassel@...nel.org>, Bjorn Helgaas <helgaas@...nel.org>, 
	Shawn Lin <shawn.lin@...k-chips.com>, lpieralisi@...nel.org, kw@...ux.com, bhelgaas@...gle.com, 
	heiko@...ech.de, robh@...nel.org, jingoohan1@...il.com, 
	thomas.richard@...tlin.com, linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH] PCI: dw-rockchip: Configure max payload size on host init

On Sat, Apr 19, 2025 at 01:21:54AM +0800, Hans Zhang wrote:
> 
> 
> On 2025/4/18 22:55, Niklas Cassel wrote:
> > 
> > 
> > On 18 April 2025 14:33:08 CEST, Hans Zhang <18255117159@....com> wrote:
> > > 
> > > Dear Bjorn,
> > > 
> > > Thanks your for reply. Niklas and I attempted to modify the relevant logic in drivers/pci/probe.c and found that there was a lot of code judging the global variable pcie_bus_config. At present, there is no good method. I will keep trying.
> > > 
> > > I wonder if you have any good suggestions? It seems that the code logic regarding pcie_bus_config is a little complicated and cannot be modified for the time being?
> > 
> > 
> > Hans,
> > 
> > If:
> > 
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index 364fa2a514f8..2e1c92fdd577 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -2983,6 +2983,13 @@ void pcie_bus_configure_settings(struct pci_bus *bus)
> >           if (!pci_is_pcie(bus->self))
> >                   return;
> >    +       /*
> > +        * Start off with DevCtl.MPS == DevCap.MPS, unless PCIE_BUS_TUNE_OFF.
> > +        * This might get overriden by a MPS strategy below.
> > +        */
> > +       if (pcie_bus_config != PCIE_BUS_TUNE_OFF)
> > +               smpss = pcie_get_mps(bus->self);
> > +
> 
> Dear Niklas,
> 
> Thank you very much for your reply and thoughts.
> 
> pcie_get_mps: Returns maximum payload size in bytes
> 
> I guess you want to obtain the DevCap MPS. But the purpose of the smpss
> variable is to save the DevCtl MPS.
> 
> >           /*
> >            * FIXME - Peer to peer DMA is possible, though the endpoint would need
> >            * to be aware of the MPS of the destination.  To work around this,
> > 
> > 
> > 
> > does not work, can't you modify the code slightly so that it works?
> > 
> > I haven't tried myself, but considering that it works when walking the bus, it seems that it should be possible to get something working.
> > 
> 
> 
> After making the following modifications, my test shows that it is normal.
> If the consideration is not comprehensive. Could Bjorn and Niklas please
> review my revisions?
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 364fa2a514f8..5b54f1b0a91d 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2951,8 +2951,7 @@ static int pcie_bus_configure_set(struct pci_dev *dev,
> void *data)
>         if (!pci_is_pcie(dev))
>                 return 0;
> 
> -       if (pcie_bus_config == PCIE_BUS_TUNE_OFF ||
> -           pcie_bus_config == PCIE_BUS_DEFAULT)
> +       if (pcie_bus_config == PCIE_BUS_TUNE_OFF)
>                 return 0;
> 
>         mps = 128 << *(u8 *)data;
> @@ -2991,7 +2990,8 @@ void pcie_bus_configure_settings(struct pci_bus *bus)
>         if (pcie_bus_config == PCIE_BUS_PEER2PEER)
>                 smpss = 0;
> 
> -       if (pcie_bus_config == PCIE_BUS_SAFE) {
> +       if ((pcie_bus_config == PCIE_BUS_SAFE) ||
> +           (pcie_bus_config != PCIE_BUS_TUNE_OFF)) {
>                 smpss = bus->self->pcie_mpss;
> 
>                 pcie_find_smpss(bus->self, &smpss);
> 
> 

As I replied to Niklas, I'd like to limit the changes to platforms having
controller drivers. So here is my proposal:

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 364fa2a514f8..d32a0f90beb1 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -3228,6 +3228,17 @@ int pci_host_probe(struct pci_host_bridge *bridge)
         */
        pci_assign_unassigned_root_bus_resources(bus);
 
+       if (pcie_bus_config != PCIE_BUS_TUNE_OFF) {
+               /* Configure root ports MPS to be MPSS by default */
+               for_each_pci_bridge(dev, bus) {
+                       if (pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)
+                               continue;
+
+                       pcie_write_mps(dev, 128 << dev->pcie_mpss);
+                       pcie_write_mrrs(dev);
+               }
+       }
+
        list_for_each_entry(child, &bus->children, node)
                pcie_bus_configure_settings(child);

- Mani

-- 
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